[V5] VECT: Add SELECT_VL support

Message ID 20230608020541.2548506-1-juzhe.zhong@rivai.ai
State Unresolved
Headers
Series [V5] VECT: Add SELECT_VL support |

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Commit Message

juzhe.zhong@rivai.ai June 8, 2023, 2:05 a.m. UTC
  From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>

Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
Co-authored-by: Richard Biener <rguenther@suse.de>

This patch address comments from Richard && Richi and rebase to trunk.

This patch is adding SELECT_VL middle-end support
allow target have target dependent optimization in case of
length calculation.

This patch is inspired by RVV ISA and LLVM:
https://reviews.llvm.org/D99750

The SELECT_VL is same behavior as LLVM "get_vector_length" with
these following properties:

1. Only apply on single-rgroup.
2. non SLP.
3. adjust loop control IV.
4. adjust data reference IV.
5. allow non-vf elements processing in non-final iteration

Code:
   # void vvaddint32(size_t n, const int*x, const int*y, int*z)
    # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }

Take RVV codegen for example:

Before this patch:
vvaddint32:
        ble     a0,zero,.L6
        csrr    a4,vlenb
        srli    a6,a4,2
.L4:
        mv      a5,a0
        bleu    a0,a6,.L3
        mv      a5,a6
.L3:
        vsetvli zero,a5,e32,m1,ta,ma
        vle32.v v2,0(a1)
        vle32.v v1,0(a2)
        vsetvli a7,zero,e32,m1,ta,ma
        sub     a0,a0,a5
        vadd.vv v1,v1,v2
        vsetvli zero,a5,e32,m1,ta,ma
        vse32.v v1,0(a3)
        add     a2,a2,a4
        add     a3,a3,a4
        add     a1,a1,a4
        bne     a0,zero,.L4
.L6:
        ret

After this patch:

vvaddint32:
    vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
    vle32.v v0, (a1)         # Get first vector
      sub a0, a0, t0         # Decrement number done
      slli t0, t0, 2         # Multiply number done by 4 bytes
      add a1, a1, t0         # Bump pointer
    vle32.v v1, (a2)         # Get second vector
      add a2, a2, t0         # Bump pointer
    vadd.vv v2, v0, v1       # Sum vectors
    vse32.v v2, (a3)         # Store result
      add a3, a3, t0         # Bump pointer
      bnez a0, vvaddint32    # Loop back
      ret                    # Finished

gcc/ChangeLog:

        * doc/md.texi: Add SELECT_VL support.
        * internal-fn.def (SELECT_VL): Ditto.
        * optabs.def (OPTAB_D): Ditto.
        * tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
        * tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
        * tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
        (vectorizable_store): Ditto.
        (vectorizable_load): Ditto.
        * tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.

Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
Co-authored-by: Richard Biener <rguenther@suse.de>

---
 gcc/doc/md.texi             | 22 ++++++++++
 gcc/internal-fn.def         |  1 +
 gcc/optabs.def              |  1 +
 gcc/tree-vect-loop-manip.cc | 32 ++++++++++----
 gcc/tree-vect-loop.cc       | 72 +++++++++++++++++++++++++++++++
 gcc/tree-vect-stmts.cc      | 86 ++++++++++++++++++++++++++++++++-----
 gcc/tree-vectorizer.h       |  6 +++
 7 files changed, 201 insertions(+), 19 deletions(-)
  

Comments

juzhe.zhong@rivai.ai June 8, 2023, 7:35 a.m. UTC | #1
Bootstrap && Regression PASSed
Ok for trunk ?



juzhe.zhong@rivai.ai
 
From: juzhe.zhong
Date: 2023-06-08 10:05
To: gcc-patches
CC: richard.sandiford; rguenther; Ju-Zhe Zhong
Subject: [PATCH V5] VECT: Add SELECT_VL support
From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
 
Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
Co-authored-by: Richard Biener <rguenther@suse.de>
 
This patch address comments from Richard && Richi and rebase to trunk.
 
This patch is adding SELECT_VL middle-end support
allow target have target dependent optimization in case of
length calculation.
 
This patch is inspired by RVV ISA and LLVM:
https://reviews.llvm.org/D99750
 
The SELECT_VL is same behavior as LLVM "get_vector_length" with
these following properties:
 
1. Only apply on single-rgroup.
2. non SLP.
3. adjust loop control IV.
4. adjust data reference IV.
5. allow non-vf elements processing in non-final iteration
 
Code:
   # void vvaddint32(size_t n, const int*x, const int*y, int*z)
    # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }
 
Take RVV codegen for example:
 
Before this patch:
vvaddint32:
        ble     a0,zero,.L6
        csrr    a4,vlenb
        srli    a6,a4,2
.L4:
        mv      a5,a0
        bleu    a0,a6,.L3
        mv      a5,a6
.L3:
        vsetvli zero,a5,e32,m1,ta,ma
        vle32.v v2,0(a1)
        vle32.v v1,0(a2)
        vsetvli a7,zero,e32,m1,ta,ma
        sub     a0,a0,a5
        vadd.vv v1,v1,v2
        vsetvli zero,a5,e32,m1,ta,ma
        vse32.v v1,0(a3)
        add     a2,a2,a4
        add     a3,a3,a4
        add     a1,a1,a4
        bne     a0,zero,.L4
.L6:
        ret
 
After this patch:
 
vvaddint32:
    vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
    vle32.v v0, (a1)         # Get first vector
      sub a0, a0, t0         # Decrement number done
      slli t0, t0, 2         # Multiply number done by 4 bytes
      add a1, a1, t0         # Bump pointer
    vle32.v v1, (a2)         # Get second vector
      add a2, a2, t0         # Bump pointer
    vadd.vv v2, v0, v1       # Sum vectors
    vse32.v v2, (a3)         # Store result
      add a3, a3, t0         # Bump pointer
      bnez a0, vvaddint32    # Loop back
      ret                    # Finished
 
gcc/ChangeLog:
 
        * doc/md.texi: Add SELECT_VL support.
        * internal-fn.def (SELECT_VL): Ditto.
        * optabs.def (OPTAB_D): Ditto.
        * tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
        * tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
        * tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
        (vectorizable_store): Ditto.
        (vectorizable_load): Ditto.
        * tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.
 
Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
Co-authored-by: Richard Biener <rguenther@suse.de>
 
---
gcc/doc/md.texi             | 22 ++++++++++
gcc/internal-fn.def         |  1 +
gcc/optabs.def              |  1 +
gcc/tree-vect-loop-manip.cc | 32 ++++++++++----
gcc/tree-vect-loop.cc       | 72 +++++++++++++++++++++++++++++++
gcc/tree-vect-stmts.cc      | 86 ++++++++++++++++++++++++++++++++-----
gcc/tree-vectorizer.h       |  6 +++
7 files changed, 201 insertions(+), 19 deletions(-)
 
diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 6a435eb4461..95f7fe1f802 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -4974,6 +4974,28 @@ for (i = 1; i < operand3; i++)
   operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
@end smallexample
+@cindex @code{select_vl@var{m}} instruction pattern
+@item @code{select_vl@var{m}}
+Set operand 0 to the number of scalar iterations that should be handled
+by one iteration of a vector loop.  Operand 1 is the total number of
+scalar iterations that the loop needs to process and operand 2 is a
+maximum bound on the result (also known as the maximum ``vectorization
+factor'').
+
+The maximum value of operand 0 is given by:
+@smallexample
+operand0 = MIN (operand1, operand2)
+@end smallexample
+However, targets might choose a lower value than this, based on
+target-specific criteria.  Each iteration of the vector loop might
+therefore process a different number of scalar iterations, which in turn
+means that induction variables will have a variable step.  Because of
+this, it is generally not useful to define this instruction if it will
+always calculate the maximum value.
+
+This optab is only useful on targets that implement @samp{len_load_@var{m}}
+and/or @samp{len_store_@var{m}}.
+
@cindex @code{check_raw_ptrs@var{m}} instruction pattern
@item @samp{check_raw_ptrs@var{m}}
Check whether, given two pointers @var{a} and @var{b} and a length @var{len},
diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
index 3ac9d82aace..5d638de6d06 100644
--- a/gcc/internal-fn.def
+++ b/gcc/internal-fn.def
@@ -177,6 +177,7 @@ DEF_INTERNAL_OPTAB_FN (VEC_SET, 0, vec_set, vec_set)
DEF_INTERNAL_OPTAB_FN (LEN_STORE, 0, len_store, len_store)
DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
+DEF_INTERNAL_OPTAB_FN (SELECT_VL, ECF_CONST | ECF_NOTHROW, select_vl, binary)
DEF_INTERNAL_OPTAB_FN (CHECK_RAW_PTRS, ECF_CONST | ECF_NOTHROW,
       check_raw_ptrs, check_ptrs)
DEF_INTERNAL_OPTAB_FN (CHECK_WAR_PTRS, ECF_CONST | ECF_NOTHROW,
diff --git a/gcc/optabs.def b/gcc/optabs.def
index 6c064ff4993..f31b69c5d85 100644
--- a/gcc/optabs.def
+++ b/gcc/optabs.def
@@ -488,3 +488,4 @@ OPTAB_DC (vec_series_optab, "vec_series$a", VEC_SERIES)
OPTAB_D (vec_shl_insert_optab, "vec_shl_insert_$a")
OPTAB_D (len_load_optab, "len_load_$a")
OPTAB_D (len_store_optab, "len_store_$a")
+OPTAB_D (select_vl_optab, "select_vl$a")
diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 3f735945e67..1c8100c1a1c 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -534,7 +534,7 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
   _10 = (unsigned long) count_12(D);
   ...
   # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
-    _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
+    _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
   ...
   vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
   ...
@@ -549,15 +549,28 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
       tree step = rgc->controls.length () == 1 ? rgc->controls[0]
       : make_ssa_name (iv_type);
       /* Create decrement IV.  */
-      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
- &incr_gsi, insert_after, &index_before_incr,
- &index_after_incr);
-      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
-     index_before_incr,
-     nitems_step));
+      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
+ {
+   create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
+      insert_after, &index_before_incr, &index_after_incr);
+   tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
+    index_before_incr, nitems_step);
+   gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
+ }
+      else
+ {
+   create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
+      &incr_gsi, insert_after, &index_before_incr,
+      &index_after_incr);
+   gimple_seq_add_stmt (header_seq,
+        gimple_build_assign (step, MIN_EXPR,
+     index_before_incr,
+     nitems_step));
+ }
       *iv_step = step;
       *compare_step = nitems_step;
-      return index_before_incr;
+      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
+        : index_before_incr;
     }
   /* Create increment IV.  */
@@ -888,7 +901,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
   /* Get a boolean result that tells us whether to iterate.  */
   edge exit_edge = single_exit (loop);
   gcond *cond_stmt;
-  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
+  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
+      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
     {
       gcc_assert (compare_step);
       tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 5b7a0da0034..ace9e759f5b 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -974,6 +974,7 @@ _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
     can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
     using_partial_vectors_p (false),
     using_decrementing_iv_p (false),
+    using_select_vl_p (false),
     epil_using_partial_vectors_p (false),
     partial_load_store_bias (0),
     peeling_for_gaps (false),
@@ -2737,6 +2738,77 @@ start_over:
LOOP_VINFO_VECT_FACTOR (loop_vinfo))))
     LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo) = true;
+  /* If a loop uses length controls and has a decrementing loop control IV,
+     we will normally pass that IV through a MIN_EXPR to calcaluate the
+     basis for the length controls.  E.g. in a loop that processes one
+     element per scalar iteration, the number of elements would be
+     MIN_EXPR <N, VF>, where N is the number of scalar iterations left.
+
+     This MIN_EXPR approach allows us to use pointer IVs with an invariant
+     step, since only the final iteration of the vector loop can have
+     inactive lanes.
+
+     However, some targets have a dedicated instruction for calculating the
+     preferred length, given the total number of elements that still need to
+     be processed.  This is encapsulated in the SELECT_VL internal function.
+
+     If the target supports SELECT_VL, we can use it instead of MIN_EXPR
+     to determine the basis for the length controls.  However, unlike the
+     MIN_EXPR calculation, the SELECT_VL calculation can decide to make
+     lanes inactive in any iteration of the vector loop, not just the last
+     iteration.  This SELECT_VL approach therefore requires us to use pointer
+     IVs with variable steps.
+
+     Once we've decided how many elements should be processed by one
+     iteration of the vector loop, we need to populate the rgroup controls.
+     If a loop has multiple rgroups, we need to make sure that those rgroups
+     "line up" (that is, they must be consistent about which elements are
+     active and which aren't).  This is done by vect_adjust_loop_lens_control.
+
+     In principle, it would be possible to use vect_adjust_loop_lens_control
+     on either the result of a MIN_EXPR or the result of a SELECT_VL.
+     However:
+
+     (1) In practice, it only makes sense to use SELECT_VL when a vector
+ operation will be controlled directly by the result.  It is not
+ worth using SELECT_VL if it would only be the input to other
+ calculations.
+
+     (2) If we use SELECT_VL for an rgroup that has N controls, each associated
+ pointer IV will need N updates by a variable amount (N-1 updates
+ within the iteration and 1 update to move to the next iteration).
+
+     Because of this, we prefer to use the MIN_EXPR approach whenever there
+     is more than one length control.
+
+     In addition, SELECT_VL always operates to a granularity of 1 unit.
+     If we wanted to use it to control an SLP operation on N consecutive
+     elements, we would need to make the SELECT_VL inputs measure scalar
+     iterations (rather than elements) and then multiply the SELECT_VL
+     result by N.  But using SELECT_VL this way is inefficient because
+     of (1) above.
+
+     2. We don't apply SELECT_VL on single-rgroup when both (1) and (2) are
+ satisfied:
+
+     (1). LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo) is true.
+     (2). LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant () is true.
+
+     Since SELECT_VL (variable step) will make SCEV analysis failed and then
+     we will fail to gain benefits of following unroll optimizations. We prefer
+     using the MIN_EXPR approach in this situation.  */
+  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
+    {
+      tree iv_type = LOOP_VINFO_RGROUP_IV_TYPE (loop_vinfo);
+      if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
+   OPTIMIZE_FOR_SPEED)
+   && LOOP_VINFO_LENS (loop_vinfo).length () == 1
+   && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
+   && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
+       || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
+ LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
+    }
+
   /* If we're vectorizing an epilogue loop, the vectorized loop either needs
      to be able to handle fewer than VF scalars, or needs to have a lower VF
      than the main loop.  */
diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index c7e4e71d3c5..34e4ad3c922 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -3148,6 +3148,52 @@ vect_get_data_ptr_increment (vec_info *vinfo,
   return iv_step;
}
+/* Prepare the pointer IVs which needs to be updated by a variable amount.
+   Such variable amount is the outcome of .SELECT_VL. In this case, we can
+   allow each iteration process the flexible number of elements as long as
+   the number <= vf elments.
+
+   Return data reference according to SELECT_VL.
+   If new statements are needed, insert them before GSI.  */
+
+static tree
+vect_get_loop_variant_data_ptr_increment (
+  vec_info *vinfo, tree aggr_type, gimple_stmt_iterator *gsi,
+  vec_loop_lens *loop_lens, dr_vec_info *dr_info,
+  vect_memory_access_type memory_access_type)
+{
+  loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo);
+  tree step = vect_dr_behavior (vinfo, dr_info)->step;
+
+  /* TODO: We don't support gather/scatter or load_lanes/store_lanes for pointer
+     IVs are updated by variable amount but we will support them in the future.
+   */
+  gcc_assert (memory_access_type != VMAT_GATHER_SCATTER
+       && memory_access_type != VMAT_LOAD_STORE_LANES);
+
+  /* When we support SELECT_VL pattern, we dynamic adjust
+     the memory address by .SELECT_VL result.
+
+     The result of .SELECT_VL is the number of elements to
+     be processed of each iteration. So the memory address
+     adjustment operation should be:
+
+     addr = addr + .SELECT_VL (ARG..) * step;
+  */
+  tree loop_len
+    = vect_get_loop_len (loop_vinfo, gsi, loop_lens, 1, aggr_type, 0, 0);
+  tree len_type = TREE_TYPE (loop_len);
+  /* Since the outcome of .SELECT_VL is element size, we should adjust
+     it into bytesize so that it can be used in address pointer variable
+     amount IVs adjustment.  */
+  tree tmp = fold_build2 (MULT_EXPR, len_type, loop_len,
+   wide_int_to_tree (len_type, wi::to_widest (step)));
+  tree bump = make_temp_ssa_name (len_type, NULL, "ivtmp");
+  gassign *assign = gimple_build_assign (bump, tmp);
+  gsi_insert_before (gsi, assign, GSI_SAME_STMT);
+  return bump;
+}
+
/* Check and perform vectorization of BUILT_IN_BSWAP{16,32,64,128}.  */
static bool
@@ -8655,11 +8701,21 @@ vectorizable_store (vec_info *vinfo,
slp_node, &gs_info, &dataref_ptr,
&vec_offsets);
  else
-     dataref_ptr
-       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
-   simd_lane_access_p ? loop : NULL,
-   offset, &dummy, gsi, &ptr_incr,
-   simd_lane_access_p, bump);
+     {
+       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
+   && memory_access_type != VMAT_INVARIANT)
+ bump = vect_get_loop_variant_data_ptr_increment (
+   vinfo, aggr_type, gsi, loop_lens, dr_info,
+   memory_access_type);
+
+       if (j != 0)
+ gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
+       dataref_ptr
+ = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
+     simd_lane_access_p ? loop : NULL,
+     offset, &dummy, gsi, &ptr_incr,
+     simd_lane_access_p, bump);
+     }
}
       else
{
@@ -10046,11 +10102,21 @@ vectorizable_load (vec_info *vinfo,
   &vec_offsets);
    }
  else
-     dataref_ptr
-       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
-   at_loop,
-   offset, &dummy, gsi, &ptr_incr,
-   simd_lane_access_p, bump);
+     {
+       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
+   && memory_access_type != VMAT_INVARIANT)
+ bump = vect_get_loop_variant_data_ptr_increment (
+   vinfo, aggr_type, gsi, loop_lens, dr_info,
+   memory_access_type);
+
+       if (j != 0)
+ gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
+       dataref_ptr
+ = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
+     at_loop, offset, &dummy, gsi,
+     &ptr_incr, simd_lane_access_p,
+     bump);
+     }
  if (mask)
    vec_mask = vec_masks[0];
}
diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 1e44f8542d7..af25d20bd7e 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -825,6 +825,11 @@ public:
(b) can iterate more than once.  */
   bool using_decrementing_iv_p;
+  /* True if we've decided to use output of select_vl to adjust IV of
+     both loop control and data reference pointer. This is only true
+     for single-rgroup control.  */
+  bool using_select_vl_p;
+
   /* True if we've decided to use partially-populated vectors for the
      epilogue of loop.  */
   bool epil_using_partial_vectors_p;
@@ -898,6 +903,7 @@ public:
#define LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P(L) (L)->can_use_partial_vectors_p
#define LOOP_VINFO_USING_PARTIAL_VECTORS_P(L) (L)->using_partial_vectors_p
#define LOOP_VINFO_USING_DECREMENTING_IV_P(L) (L)->using_decrementing_iv_p
+#define LOOP_VINFO_USING_SELECT_VL_P(L) (L)->using_select_vl_p
#define LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P(L)                             \
   (L)->epil_using_partial_vectors_p
#define LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS(L) (L)->partial_load_store_bias
-- 
2.36.3
  
Richard Biener June 9, 2023, 8:13 a.m. UTC | #2
On Thu, 8 Jun 2023, juzhe.zhong@rivai.ai wrote:

> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
> 
> Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> This patch address comments from Richard && Richi and rebase to trunk.
> 
> This patch is adding SELECT_VL middle-end support
> allow target have target dependent optimization in case of
> length calculation.
> 
> This patch is inspired by RVV ISA and LLVM:
> https://reviews.llvm.org/D99750
> 
> The SELECT_VL is same behavior as LLVM "get_vector_length" with
> these following properties:
> 
> 1. Only apply on single-rgroup.
> 2. non SLP.
> 3. adjust loop control IV.
> 4. adjust data reference IV.
> 5. allow non-vf elements processing in non-final iteration
> 
> Code:
>    # void vvaddint32(size_t n, const int*x, const int*y, int*z)
>     # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }
> 
> Take RVV codegen for example:
> 
> Before this patch:
> vvaddint32:
>         ble     a0,zero,.L6
>         csrr    a4,vlenb
>         srli    a6,a4,2
> .L4:
>         mv      a5,a0
>         bleu    a0,a6,.L3
>         mv      a5,a6
> .L3:
>         vsetvli zero,a5,e32,m1,ta,ma
>         vle32.v v2,0(a1)
>         vle32.v v1,0(a2)
>         vsetvli a7,zero,e32,m1,ta,ma
>         sub     a0,a0,a5
>         vadd.vv v1,v1,v2
>         vsetvli zero,a5,e32,m1,ta,ma
>         vse32.v v1,0(a3)
>         add     a2,a2,a4
>         add     a3,a3,a4
>         add     a1,a1,a4
>         bne     a0,zero,.L4
> .L6:
>         ret
> 
> After this patch:
> 
> vvaddint32:
>     vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
>     vle32.v v0, (a1)         # Get first vector
>       sub a0, a0, t0         # Decrement number done
>       slli t0, t0, 2         # Multiply number done by 4 bytes
>       add a1, a1, t0         # Bump pointer
>     vle32.v v1, (a2)         # Get second vector
>       add a2, a2, t0         # Bump pointer
>     vadd.vv v2, v0, v1       # Sum vectors
>     vse32.v v2, (a3)         # Store result
>       add a3, a3, t0         # Bump pointer
>       bnez a0, vvaddint32    # Loop back
>       ret                    # Finished
> 
> gcc/ChangeLog:
> 
>         * doc/md.texi: Add SELECT_VL support.
>         * internal-fn.def (SELECT_VL): Ditto.
>         * optabs.def (OPTAB_D): Ditto.
>         * tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
>         * tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
>         * tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
>         (vectorizable_store): Ditto.
>         (vectorizable_load): Ditto.
>         * tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.
>
> Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> ---
>  gcc/doc/md.texi             | 22 ++++++++++
>  gcc/internal-fn.def         |  1 +
>  gcc/optabs.def              |  1 +
>  gcc/tree-vect-loop-manip.cc | 32 ++++++++++----
>  gcc/tree-vect-loop.cc       | 72 +++++++++++++++++++++++++++++++
>  gcc/tree-vect-stmts.cc      | 86 ++++++++++++++++++++++++++++++++-----
>  gcc/tree-vectorizer.h       |  6 +++
>  7 files changed, 201 insertions(+), 19 deletions(-)
> 
> diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
> index 6a435eb4461..95f7fe1f802 100644
> --- a/gcc/doc/md.texi
> +++ b/gcc/doc/md.texi
> @@ -4974,6 +4974,28 @@ for (i = 1; i < operand3; i++)
>    operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
>  @end smallexample
>  
> +@cindex @code{select_vl@var{m}} instruction pattern
> +@item @code{select_vl@var{m}}
> +Set operand 0 to the number of scalar iterations that should be handled
> +by one iteration of a vector loop.  Operand 1 is the total number of
> +scalar iterations that the loop needs to process and operand 2 is a
> +maximum bound on the result (also known as the maximum ``vectorization
> +factor'').
> +
> +The maximum value of operand 0 is given by:
> +@smallexample
> +operand0 = MIN (operand1, operand2)
> +@end smallexample
> +However, targets might choose a lower value than this, based on
> +target-specific criteria.  Each iteration of the vector loop might
> +therefore process a different number of scalar iterations, which in turn
> +means that induction variables will have a variable step.  Because of
> +this, it is generally not useful to define this instruction if it will
> +always calculate the maximum value.
> +
> +This optab is only useful on targets that implement @samp{len_load_@var{m}}
> +and/or @samp{len_store_@var{m}}.
> +
>  @cindex @code{check_raw_ptrs@var{m}} instruction pattern
>  @item @samp{check_raw_ptrs@var{m}}
>  Check whether, given two pointers @var{a} and @var{b} and a length @var{len},
> diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
> index 3ac9d82aace..5d638de6d06 100644
> --- a/gcc/internal-fn.def
> +++ b/gcc/internal-fn.def
> @@ -177,6 +177,7 @@ DEF_INTERNAL_OPTAB_FN (VEC_SET, 0, vec_set, vec_set)
>  DEF_INTERNAL_OPTAB_FN (LEN_STORE, 0, len_store, len_store)
>  
>  DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
> +DEF_INTERNAL_OPTAB_FN (SELECT_VL, ECF_CONST | ECF_NOTHROW, select_vl, binary)
>  DEF_INTERNAL_OPTAB_FN (CHECK_RAW_PTRS, ECF_CONST | ECF_NOTHROW,
>  		       check_raw_ptrs, check_ptrs)
>  DEF_INTERNAL_OPTAB_FN (CHECK_WAR_PTRS, ECF_CONST | ECF_NOTHROW,
> diff --git a/gcc/optabs.def b/gcc/optabs.def
> index 6c064ff4993..f31b69c5d85 100644
> --- a/gcc/optabs.def
> +++ b/gcc/optabs.def
> @@ -488,3 +488,4 @@ OPTAB_DC (vec_series_optab, "vec_series$a", VEC_SERIES)
>  OPTAB_D (vec_shl_insert_optab, "vec_shl_insert_$a")
>  OPTAB_D (len_load_optab, "len_load_$a")
>  OPTAB_D (len_store_optab, "len_store_$a")
> +OPTAB_D (select_vl_optab, "select_vl$a")
> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> index 3f735945e67..1c8100c1a1c 100644
> --- a/gcc/tree-vect-loop-manip.cc
> +++ b/gcc/tree-vect-loop-manip.cc
> @@ -534,7 +534,7 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>  	   _10 = (unsigned long) count_12(D);
>  	   ...
>  	   # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
> -	   _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
> +	   _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
>  	   ...
>  	   vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
>  	   ...
> @@ -549,15 +549,28 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>        tree step = rgc->controls.length () == 1 ? rgc->controls[0]
>  					       : make_ssa_name (iv_type);
>        /* Create decrement IV.  */
> -      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> -		 &incr_gsi, insert_after, &index_before_incr,
> -		 &index_after_incr);
> -      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
> -							    index_before_incr,
> -							    nitems_step));
> +      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
> +	{
> +	  create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
> +		     insert_after, &index_before_incr, &index_after_incr);
> +	  tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
> +				   index_before_incr, nitems_step);
> +	  gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
> +	}
> +      else
> +	{
> +	  create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> +		     &incr_gsi, insert_after, &index_before_incr,
> +		     &index_after_incr);
> +	  gimple_seq_add_stmt (header_seq,
> +			       gimple_build_assign (step, MIN_EXPR,
> +						    index_before_incr,
> +						    nitems_step));
> +	}
>        *iv_step = step;
>        *compare_step = nitems_step;
> -      return index_before_incr;
> +      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
> +						       : index_before_incr;
>      }
>  
>    /* Create increment IV.  */
> @@ -888,7 +901,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
>    /* Get a boolean result that tells us whether to iterate.  */
>    edge exit_edge = single_exit (loop);
>    gcond *cond_stmt;
> -  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
> +      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
>      {
>        gcc_assert (compare_step);
>        tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 5b7a0da0034..ace9e759f5b 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -974,6 +974,7 @@ _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
>      can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
>      using_partial_vectors_p (false),
>      using_decrementing_iv_p (false),
> +    using_select_vl_p (false),
>      epil_using_partial_vectors_p (false),
>      partial_load_store_bias (0),
>      peeling_for_gaps (false),
> @@ -2737,6 +2738,77 @@ start_over:
>  			LOOP_VINFO_VECT_FACTOR (loop_vinfo))))
>      LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo) = true;
>  
> +  /* If a loop uses length controls and has a decrementing loop control IV,
> +     we will normally pass that IV through a MIN_EXPR to calcaluate the
> +     basis for the length controls.  E.g. in a loop that processes one
> +     element per scalar iteration, the number of elements would be
> +     MIN_EXPR <N, VF>, where N is the number of scalar iterations left.
> +
> +     This MIN_EXPR approach allows us to use pointer IVs with an invariant
> +     step, since only the final iteration of the vector loop can have
> +     inactive lanes.
> +
> +     However, some targets have a dedicated instruction for calculating the
> +     preferred length, given the total number of elements that still need to
> +     be processed.  This is encapsulated in the SELECT_VL internal function.
> +
> +     If the target supports SELECT_VL, we can use it instead of MIN_EXPR
> +     to determine the basis for the length controls.  However, unlike the
> +     MIN_EXPR calculation, the SELECT_VL calculation can decide to make
> +     lanes inactive in any iteration of the vector loop, not just the last
> +     iteration.  This SELECT_VL approach therefore requires us to use pointer
> +     IVs with variable steps.
> +
> +     Once we've decided how many elements should be processed by one
> +     iteration of the vector loop, we need to populate the rgroup controls.
> +     If a loop has multiple rgroups, we need to make sure that those rgroups
> +     "line up" (that is, they must be consistent about which elements are
> +     active and which aren't).  This is done by vect_adjust_loop_lens_control.
> +
> +     In principle, it would be possible to use vect_adjust_loop_lens_control
> +     on either the result of a MIN_EXPR or the result of a SELECT_VL.
> +     However:
> +
> +     (1) In practice, it only makes sense to use SELECT_VL when a vector
> +	 operation will be controlled directly by the result.  It is not
> +	 worth using SELECT_VL if it would only be the input to other
> +	 calculations.
> +
> +     (2) If we use SELECT_VL for an rgroup that has N controls, each associated
> +	 pointer IV will need N updates by a variable amount (N-1 updates
> +	 within the iteration and 1 update to move to the next iteration).
> +
> +     Because of this, we prefer to use the MIN_EXPR approach whenever there
> +     is more than one length control.
> +
> +     In addition, SELECT_VL always operates to a granularity of 1 unit.
> +     If we wanted to use it to control an SLP operation on N consecutive
> +     elements, we would need to make the SELECT_VL inputs measure scalar
> +     iterations (rather than elements) and then multiply the SELECT_VL
> +     result by N.  But using SELECT_VL this way is inefficient because
> +     of (1) above.
> +
> +     2. We don't apply SELECT_VL on single-rgroup when both (1) and (2) are
> +	satisfied:
> +
> +     (1). LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo) is true.
> +     (2). LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant () is true.
> +
> +     Since SELECT_VL (variable step) will make SCEV analysis failed and then
> +     we will fail to gain benefits of following unroll optimizations. We prefer
> +     using the MIN_EXPR approach in this situation.  */
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +    {
> +      tree iv_type = LOOP_VINFO_RGROUP_IV_TYPE (loop_vinfo);
> +      if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
> +					  OPTIMIZE_FOR_SPEED)
> +	  && LOOP_VINFO_LENS (loop_vinfo).length () == 1
> +	  && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
> +	  && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> +	      || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
> +	LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
> +    }
> +
>    /* If we're vectorizing an epilogue loop, the vectorized loop either needs
>       to be able to handle fewer than VF scalars, or needs to have a lower VF
>       than the main loop.  */
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index c7e4e71d3c5..34e4ad3c922 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -3148,6 +3148,52 @@ vect_get_data_ptr_increment (vec_info *vinfo,
>    return iv_step;
>  }
>  
> +/* Prepare the pointer IVs which needs to be updated by a variable amount.
> +   Such variable amount is the outcome of .SELECT_VL. In this case, we can
> +   allow each iteration process the flexible number of elements as long as
> +   the number <= vf elments.
> +
> +   Return data reference according to SELECT_VL.
> +   If new statements are needed, insert them before GSI.  */
> +
> +static tree
> +vect_get_loop_variant_data_ptr_increment (
> +  vec_info *vinfo, tree aggr_type, gimple_stmt_iterator *gsi,
> +  vec_loop_lens *loop_lens, dr_vec_info *dr_info,
> +  vect_memory_access_type memory_access_type)
> +{
> +  loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo);
> +  tree step = vect_dr_behavior (vinfo, dr_info)->step;
> +
> +  /* TODO: We don't support gather/scatter or load_lanes/store_lanes for pointer
> +     IVs are updated by variable amount but we will support them in the future.
> +   */
> +  gcc_assert (memory_access_type != VMAT_GATHER_SCATTER
> +	      && memory_access_type != VMAT_LOAD_STORE_LANES);
> +
> +  /* When we support SELECT_VL pattern, we dynamic adjust
> +     the memory address by .SELECT_VL result.
> +
> +     The result of .SELECT_VL is the number of elements to
> +     be processed of each iteration. So the memory address
> +     adjustment operation should be:
> +
> +     addr = addr + .SELECT_VL (ARG..) * step;
> +  */
> +  tree loop_len
> +    = vect_get_loop_len (loop_vinfo, gsi, loop_lens, 1, aggr_type, 0, 0);
> +  tree len_type = TREE_TYPE (loop_len);
> +  /* Since the outcome of .SELECT_VL is element size, we should adjust
> +     it into bytesize so that it can be used in address pointer variable
> +     amount IVs adjustment.  */
> +  tree tmp = fold_build2 (MULT_EXPR, len_type, loop_len,
> +			  wide_int_to_tree (len_type, wi::to_widest (step)));
> +  tree bump = make_temp_ssa_name (len_type, NULL, "ivtmp");
> +  gassign *assign = gimple_build_assign (bump, tmp);
> +  gsi_insert_before (gsi, assign, GSI_SAME_STMT);
> +  return bump;
> +}
> +
>  /* Check and perform vectorization of BUILT_IN_BSWAP{16,32,64,128}.  */
>  
>  static bool
> @@ -8655,11 +8701,21 @@ vectorizable_store (vec_info *vinfo,
>  					 slp_node, &gs_info, &dataref_ptr,
>  					 &vec_offsets);
>  	  else
> -	    dataref_ptr
> -	      = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -					  simd_lane_access_p ? loop : NULL,
> -					  offset, &dummy, gsi, &ptr_incr,
> -					  simd_lane_access_p, bump);
> +	    {
> +	      if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +		  && memory_access_type != VMAT_INVARIANT)
> +		bump = vect_get_loop_variant_data_ptr_increment (
> +		  vinfo, aggr_type, gsi, loop_lens, dr_info,
> +		  memory_access_type);
> +
> +	      if (j != 0)
> +		gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));

this is in a if (j == 0) branch, please put the assert into the
else {} block of it instead.  You still run the 'bump' computation
before the loop, so if you intend to never handle j != 0 you could
put it there, too.  But at least avoid calling
vect_get_data_ptr_increment in that code when you re-do it here.

Btw, 

> +	      dataref_ptr
> +		= vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +					    simd_lane_access_p ? loop : NULL,
> +					    offset, &dummy, gsi, &ptr_incr,
> +					    simd_lane_access_p, bump);
> +	    }
>  	}
>        else
>  	{
> @@ -10046,11 +10102,21 @@ vectorizable_load (vec_info *vinfo,
>  					   &vec_offsets);
>  	    }
>  	  else
> -	    dataref_ptr
> -	      = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -					  at_loop,
> -					  offset, &dummy, gsi, &ptr_incr,
> -					  simd_lane_access_p, bump);
> +	    {
> +	      if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +		  && memory_access_type != VMAT_INVARIANT)
> +		bump = vect_get_loop_variant_data_ptr_increment (
> +		  vinfo, aggr_type, gsi, loop_lens, dr_info,
> +		  memory_access_type);
> +
> +	      if (j != 0)
> +		gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));

same here

> +	      dataref_ptr
> +		= vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +					    at_loop, offset, &dummy, gsi,
> +					    &ptr_incr, simd_lane_access_p,
> +					    bump);

btw, I wonder how vect_create_data_ref_ptr and
vect_get_data_ptr_increment handle LEN when not using .SELECT_VL?
Are they always using the constant VF here?
Can't this be done in vect_get_data_ptr_increment by instead
of using VF for LOOP_VINFO_USING_SELECT_VL_P use
vect_get_loop_len () and so only change one place?

> +	    }
>  	  if (mask)
>  	    vec_mask = vec_masks[0];
>  	}
> diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
> index 1e44f8542d7..af25d20bd7e 100644
> --- a/gcc/tree-vectorizer.h
> +++ b/gcc/tree-vectorizer.h
> @@ -825,6 +825,11 @@ public:
>  	(b) can iterate more than once.  */
>    bool using_decrementing_iv_p;
>  
> +  /* True if we've decided to use output of select_vl to adjust IV of
> +     both loop control and data reference pointer. This is only true
> +     for single-rgroup control.  */
> +  bool using_select_vl_p;
> +
>    /* True if we've decided to use partially-populated vectors for the
>       epilogue of loop.  */
>    bool epil_using_partial_vectors_p;
> @@ -898,6 +903,7 @@ public:
>  #define LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P(L) (L)->can_use_partial_vectors_p
>  #define LOOP_VINFO_USING_PARTIAL_VECTORS_P(L) (L)->using_partial_vectors_p
>  #define LOOP_VINFO_USING_DECREMENTING_IV_P(L) (L)->using_decrementing_iv_p
> +#define LOOP_VINFO_USING_SELECT_VL_P(L) (L)->using_select_vl_p
>  #define LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P(L)                             \
>    (L)->epil_using_partial_vectors_p
>  #define LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS(L) (L)->partial_load_store_bias
>
  
juzhe.zhong@rivai.ai June 9, 2023, 8:23 a.m. UTC | #3
Hi, Richi.

Thanks for comments.

>>this is in a if (j == 0) branch, please put the assert into the
>>else {} block of it instead.  You still run the 'bump' computation
>>before the loop, so if you intend to never handle j != 0 you could
>>put it there, too.  But at least avoid calling
>>vect_get_data_ptr_increment in that code when you re-do it here.
Ok

>> btw, I wonder how vect_create_data_ref_ptr and
>>vect_get_data_ptr_increment handle LEN when not using .SELECT_VL?
>>Are they always using the constant VF here?
Yes, when we are not using SELECT_VL, we always use VF.

>>Can't this be done in vect_get_data_ptr_increment by instead
>>of using VF for LOOP_VINFO_USING_SELECT_VL_P use
>>vect_get_loop_len () and so only change one place?
oK, I will try that in V6 patch.

Thanks.


juzhe.zhong@rivai.ai
 
From: Richard Biener
Date: 2023-06-09 16:13
To: Ju-Zhe Zhong
CC: gcc-patches; richard.sandiford
Subject: Re: [PATCH V5] VECT: Add SELECT_VL support
On Thu, 8 Jun 2023, juzhe.zhong@rivai.ai wrote:
 
> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
> 
> Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> This patch address comments from Richard && Richi and rebase to trunk.
> 
> This patch is adding SELECT_VL middle-end support
> allow target have target dependent optimization in case of
> length calculation.
> 
> This patch is inspired by RVV ISA and LLVM:
> https://reviews.llvm.org/D99750
> 
> The SELECT_VL is same behavior as LLVM "get_vector_length" with
> these following properties:
> 
> 1. Only apply on single-rgroup.
> 2. non SLP.
> 3. adjust loop control IV.
> 4. adjust data reference IV.
> 5. allow non-vf elements processing in non-final iteration
> 
> Code:
>    # void vvaddint32(size_t n, const int*x, const int*y, int*z)
>     # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }
> 
> Take RVV codegen for example:
> 
> Before this patch:
> vvaddint32:
>         ble     a0,zero,.L6
>         csrr    a4,vlenb
>         srli    a6,a4,2
> .L4:
>         mv      a5,a0
>         bleu    a0,a6,.L3
>         mv      a5,a6
> .L3:
>         vsetvli zero,a5,e32,m1,ta,ma
>         vle32.v v2,0(a1)
>         vle32.v v1,0(a2)
>         vsetvli a7,zero,e32,m1,ta,ma
>         sub     a0,a0,a5
>         vadd.vv v1,v1,v2
>         vsetvli zero,a5,e32,m1,ta,ma
>         vse32.v v1,0(a3)
>         add     a2,a2,a4
>         add     a3,a3,a4
>         add     a1,a1,a4
>         bne     a0,zero,.L4
> .L6:
>         ret
> 
> After this patch:
> 
> vvaddint32:
>     vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
>     vle32.v v0, (a1)         # Get first vector
>       sub a0, a0, t0         # Decrement number done
>       slli t0, t0, 2         # Multiply number done by 4 bytes
>       add a1, a1, t0         # Bump pointer
>     vle32.v v1, (a2)         # Get second vector
>       add a2, a2, t0         # Bump pointer
>     vadd.vv v2, v0, v1       # Sum vectors
>     vse32.v v2, (a3)         # Store result
>       add a3, a3, t0         # Bump pointer
>       bnez a0, vvaddint32    # Loop back
>       ret                    # Finished
> 
> gcc/ChangeLog:
> 
>         * doc/md.texi: Add SELECT_VL support.
>         * internal-fn.def (SELECT_VL): Ditto.
>         * optabs.def (OPTAB_D): Ditto.
>         * tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
>         * tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
>         * tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
>         (vectorizable_store): Ditto.
>         (vectorizable_load): Ditto.
>         * tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.
>
> Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> ---
>  gcc/doc/md.texi             | 22 ++++++++++
>  gcc/internal-fn.def         |  1 +
>  gcc/optabs.def              |  1 +
>  gcc/tree-vect-loop-manip.cc | 32 ++++++++++----
>  gcc/tree-vect-loop.cc       | 72 +++++++++++++++++++++++++++++++
>  gcc/tree-vect-stmts.cc      | 86 ++++++++++++++++++++++++++++++++-----
>  gcc/tree-vectorizer.h       |  6 +++
>  7 files changed, 201 insertions(+), 19 deletions(-)
> 
> diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
> index 6a435eb4461..95f7fe1f802 100644
> --- a/gcc/doc/md.texi
> +++ b/gcc/doc/md.texi
> @@ -4974,6 +4974,28 @@ for (i = 1; i < operand3; i++)
>    operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
>  @end smallexample
>  
> +@cindex @code{select_vl@var{m}} instruction pattern
> +@item @code{select_vl@var{m}}
> +Set operand 0 to the number of scalar iterations that should be handled
> +by one iteration of a vector loop.  Operand 1 is the total number of
> +scalar iterations that the loop needs to process and operand 2 is a
> +maximum bound on the result (also known as the maximum ``vectorization
> +factor'').
> +
> +The maximum value of operand 0 is given by:
> +@smallexample
> +operand0 = MIN (operand1, operand2)
> +@end smallexample
> +However, targets might choose a lower value than this, based on
> +target-specific criteria.  Each iteration of the vector loop might
> +therefore process a different number of scalar iterations, which in turn
> +means that induction variables will have a variable step.  Because of
> +this, it is generally not useful to define this instruction if it will
> +always calculate the maximum value.
> +
> +This optab is only useful on targets that implement @samp{len_load_@var{m}}
> +and/or @samp{len_store_@var{m}}.
> +
>  @cindex @code{check_raw_ptrs@var{m}} instruction pattern
>  @item @samp{check_raw_ptrs@var{m}}
>  Check whether, given two pointers @var{a} and @var{b} and a length @var{len},
> diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
> index 3ac9d82aace..5d638de6d06 100644
> --- a/gcc/internal-fn.def
> +++ b/gcc/internal-fn.def
> @@ -177,6 +177,7 @@ DEF_INTERNAL_OPTAB_FN (VEC_SET, 0, vec_set, vec_set)
>  DEF_INTERNAL_OPTAB_FN (LEN_STORE, 0, len_store, len_store)
>  
>  DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
> +DEF_INTERNAL_OPTAB_FN (SELECT_VL, ECF_CONST | ECF_NOTHROW, select_vl, binary)
>  DEF_INTERNAL_OPTAB_FN (CHECK_RAW_PTRS, ECF_CONST | ECF_NOTHROW,
>         check_raw_ptrs, check_ptrs)
>  DEF_INTERNAL_OPTAB_FN (CHECK_WAR_PTRS, ECF_CONST | ECF_NOTHROW,
> diff --git a/gcc/optabs.def b/gcc/optabs.def
> index 6c064ff4993..f31b69c5d85 100644
> --- a/gcc/optabs.def
> +++ b/gcc/optabs.def
> @@ -488,3 +488,4 @@ OPTAB_DC (vec_series_optab, "vec_series$a", VEC_SERIES)
>  OPTAB_D (vec_shl_insert_optab, "vec_shl_insert_$a")
>  OPTAB_D (len_load_optab, "len_load_$a")
>  OPTAB_D (len_store_optab, "len_store_$a")
> +OPTAB_D (select_vl_optab, "select_vl$a")
> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> index 3f735945e67..1c8100c1a1c 100644
> --- a/gcc/tree-vect-loop-manip.cc
> +++ b/gcc/tree-vect-loop-manip.cc
> @@ -534,7 +534,7 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>     _10 = (unsigned long) count_12(D);
>     ...
>     # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
> -    _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
> +    _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
>     ...
>     vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
>     ...
> @@ -549,15 +549,28 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>        tree step = rgc->controls.length () == 1 ? rgc->controls[0]
>         : make_ssa_name (iv_type);
>        /* Create decrement IV.  */
> -      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> - &incr_gsi, insert_after, &index_before_incr,
> - &index_after_incr);
> -      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
> -     index_before_incr,
> -     nitems_step));
> +      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
> + {
> +   create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
> +      insert_after, &index_before_incr, &index_after_incr);
> +   tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
> +    index_before_incr, nitems_step);
> +   gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
> + }
> +      else
> + {
> +   create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> +      &incr_gsi, insert_after, &index_before_incr,
> +      &index_after_incr);
> +   gimple_seq_add_stmt (header_seq,
> +        gimple_build_assign (step, MIN_EXPR,
> +     index_before_incr,
> +     nitems_step));
> + }
>        *iv_step = step;
>        *compare_step = nitems_step;
> -      return index_before_incr;
> +      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
> +        : index_before_incr;
>      }
>  
>    /* Create increment IV.  */
> @@ -888,7 +901,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
>    /* Get a boolean result that tells us whether to iterate.  */
>    edge exit_edge = single_exit (loop);
>    gcond *cond_stmt;
> -  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
> +      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
>      {
>        gcc_assert (compare_step);
>        tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 5b7a0da0034..ace9e759f5b 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -974,6 +974,7 @@ _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
>      can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
>      using_partial_vectors_p (false),
>      using_decrementing_iv_p (false),
> +    using_select_vl_p (false),
>      epil_using_partial_vectors_p (false),
>      partial_load_store_bias (0),
>      peeling_for_gaps (false),
> @@ -2737,6 +2738,77 @@ start_over:
>  LOOP_VINFO_VECT_FACTOR (loop_vinfo))))
>      LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo) = true;
>  
> +  /* If a loop uses length controls and has a decrementing loop control IV,
> +     we will normally pass that IV through a MIN_EXPR to calcaluate the
> +     basis for the length controls.  E.g. in a loop that processes one
> +     element per scalar iteration, the number of elements would be
> +     MIN_EXPR <N, VF>, where N is the number of scalar iterations left.
> +
> +     This MIN_EXPR approach allows us to use pointer IVs with an invariant
> +     step, since only the final iteration of the vector loop can have
> +     inactive lanes.
> +
> +     However, some targets have a dedicated instruction for calculating the
> +     preferred length, given the total number of elements that still need to
> +     be processed.  This is encapsulated in the SELECT_VL internal function.
> +
> +     If the target supports SELECT_VL, we can use it instead of MIN_EXPR
> +     to determine the basis for the length controls.  However, unlike the
> +     MIN_EXPR calculation, the SELECT_VL calculation can decide to make
> +     lanes inactive in any iteration of the vector loop, not just the last
> +     iteration.  This SELECT_VL approach therefore requires us to use pointer
> +     IVs with variable steps.
> +
> +     Once we've decided how many elements should be processed by one
> +     iteration of the vector loop, we need to populate the rgroup controls.
> +     If a loop has multiple rgroups, we need to make sure that those rgroups
> +     "line up" (that is, they must be consistent about which elements are
> +     active and which aren't).  This is done by vect_adjust_loop_lens_control.
> +
> +     In principle, it would be possible to use vect_adjust_loop_lens_control
> +     on either the result of a MIN_EXPR or the result of a SELECT_VL.
> +     However:
> +
> +     (1) In practice, it only makes sense to use SELECT_VL when a vector
> + operation will be controlled directly by the result.  It is not
> + worth using SELECT_VL if it would only be the input to other
> + calculations.
> +
> +     (2) If we use SELECT_VL for an rgroup that has N controls, each associated
> + pointer IV will need N updates by a variable amount (N-1 updates
> + within the iteration and 1 update to move to the next iteration).
> +
> +     Because of this, we prefer to use the MIN_EXPR approach whenever there
> +     is more than one length control.
> +
> +     In addition, SELECT_VL always operates to a granularity of 1 unit.
> +     If we wanted to use it to control an SLP operation on N consecutive
> +     elements, we would need to make the SELECT_VL inputs measure scalar
> +     iterations (rather than elements) and then multiply the SELECT_VL
> +     result by N.  But using SELECT_VL this way is inefficient because
> +     of (1) above.
> +
> +     2. We don't apply SELECT_VL on single-rgroup when both (1) and (2) are
> + satisfied:
> +
> +     (1). LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo) is true.
> +     (2). LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant () is true.
> +
> +     Since SELECT_VL (variable step) will make SCEV analysis failed and then
> +     we will fail to gain benefits of following unroll optimizations. We prefer
> +     using the MIN_EXPR approach in this situation.  */
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +    {
> +      tree iv_type = LOOP_VINFO_RGROUP_IV_TYPE (loop_vinfo);
> +      if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
> +   OPTIMIZE_FOR_SPEED)
> +   && LOOP_VINFO_LENS (loop_vinfo).length () == 1
> +   && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
> +   && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> +       || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
> + LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
> +    }
> +
>    /* If we're vectorizing an epilogue loop, the vectorized loop either needs
>       to be able to handle fewer than VF scalars, or needs to have a lower VF
>       than the main loop.  */
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index c7e4e71d3c5..34e4ad3c922 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -3148,6 +3148,52 @@ vect_get_data_ptr_increment (vec_info *vinfo,
>    return iv_step;
>  }
>  
> +/* Prepare the pointer IVs which needs to be updated by a variable amount.
> +   Such variable amount is the outcome of .SELECT_VL. In this case, we can
> +   allow each iteration process the flexible number of elements as long as
> +   the number <= vf elments.
> +
> +   Return data reference according to SELECT_VL.
> +   If new statements are needed, insert them before GSI.  */
> +
> +static tree
> +vect_get_loop_variant_data_ptr_increment (
> +  vec_info *vinfo, tree aggr_type, gimple_stmt_iterator *gsi,
> +  vec_loop_lens *loop_lens, dr_vec_info *dr_info,
> +  vect_memory_access_type memory_access_type)
> +{
> +  loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo);
> +  tree step = vect_dr_behavior (vinfo, dr_info)->step;
> +
> +  /* TODO: We don't support gather/scatter or load_lanes/store_lanes for pointer
> +     IVs are updated by variable amount but we will support them in the future.
> +   */
> +  gcc_assert (memory_access_type != VMAT_GATHER_SCATTER
> +       && memory_access_type != VMAT_LOAD_STORE_LANES);
> +
> +  /* When we support SELECT_VL pattern, we dynamic adjust
> +     the memory address by .SELECT_VL result.
> +
> +     The result of .SELECT_VL is the number of elements to
> +     be processed of each iteration. So the memory address
> +     adjustment operation should be:
> +
> +     addr = addr + .SELECT_VL (ARG..) * step;
> +  */
> +  tree loop_len
> +    = vect_get_loop_len (loop_vinfo, gsi, loop_lens, 1, aggr_type, 0, 0);
> +  tree len_type = TREE_TYPE (loop_len);
> +  /* Since the outcome of .SELECT_VL is element size, we should adjust
> +     it into bytesize so that it can be used in address pointer variable
> +     amount IVs adjustment.  */
> +  tree tmp = fold_build2 (MULT_EXPR, len_type, loop_len,
> +   wide_int_to_tree (len_type, wi::to_widest (step)));
> +  tree bump = make_temp_ssa_name (len_type, NULL, "ivtmp");
> +  gassign *assign = gimple_build_assign (bump, tmp);
> +  gsi_insert_before (gsi, assign, GSI_SAME_STMT);
> +  return bump;
> +}
> +
>  /* Check and perform vectorization of BUILT_IN_BSWAP{16,32,64,128}.  */
>  
>  static bool
> @@ -8655,11 +8701,21 @@ vectorizable_store (vec_info *vinfo,
>  slp_node, &gs_info, &dataref_ptr,
>  &vec_offsets);
>    else
> -     dataref_ptr
> -       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -   simd_lane_access_p ? loop : NULL,
> -   offset, &dummy, gsi, &ptr_incr,
> -   simd_lane_access_p, bump);
> +     {
> +       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +   && memory_access_type != VMAT_INVARIANT)
> + bump = vect_get_loop_variant_data_ptr_increment (
> +   vinfo, aggr_type, gsi, loop_lens, dr_info,
> +   memory_access_type);
> +
> +       if (j != 0)
> + gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
 
this is in a if (j == 0) branch, please put the assert into the
else {} block of it instead.  You still run the 'bump' computation
before the loop, so if you intend to never handle j != 0 you could
put it there, too.  But at least avoid calling
vect_get_data_ptr_increment in that code when you re-do it here.
 
Btw, 
 
> +       dataref_ptr
> + = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +     simd_lane_access_p ? loop : NULL,
> +     offset, &dummy, gsi, &ptr_incr,
> +     simd_lane_access_p, bump);
> +     }
>  }
>        else
>  {
> @@ -10046,11 +10102,21 @@ vectorizable_load (vec_info *vinfo,
>     &vec_offsets);
>      }
>    else
> -     dataref_ptr
> -       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -   at_loop,
> -   offset, &dummy, gsi, &ptr_incr,
> -   simd_lane_access_p, bump);
> +     {
> +       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +   && memory_access_type != VMAT_INVARIANT)
> + bump = vect_get_loop_variant_data_ptr_increment (
> +   vinfo, aggr_type, gsi, loop_lens, dr_info,
> +   memory_access_type);
> +
> +       if (j != 0)
> + gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
 
same here
 
> +       dataref_ptr
> + = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +     at_loop, offset, &dummy, gsi,
> +     &ptr_incr, simd_lane_access_p,
> +     bump);
 
btw, I wonder how vect_create_data_ref_ptr and
vect_get_data_ptr_increment handle LEN when not using .SELECT_VL?
Are they always using the constant VF here?
Can't this be done in vect_get_data_ptr_increment by instead
of using VF for LOOP_VINFO_USING_SELECT_VL_P use
vect_get_loop_len () and so only change one place?
 
> +     }
>    if (mask)
>      vec_mask = vec_masks[0];
>  }
> diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
> index 1e44f8542d7..af25d20bd7e 100644
> --- a/gcc/tree-vectorizer.h
> +++ b/gcc/tree-vectorizer.h
> @@ -825,6 +825,11 @@ public:
>  (b) can iterate more than once.  */
>    bool using_decrementing_iv_p;
>  
> +  /* True if we've decided to use output of select_vl to adjust IV of
> +     both loop control and data reference pointer. This is only true
> +     for single-rgroup control.  */
> +  bool using_select_vl_p;
> +
>    /* True if we've decided to use partially-populated vectors for the
>       epilogue of loop.  */
>    bool epil_using_partial_vectors_p;
> @@ -898,6 +903,7 @@ public:
>  #define LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P(L) (L)->can_use_partial_vectors_p
>  #define LOOP_VINFO_USING_PARTIAL_VECTORS_P(L) (L)->using_partial_vectors_p
>  #define LOOP_VINFO_USING_DECREMENTING_IV_P(L) (L)->using_decrementing_iv_p
> +#define LOOP_VINFO_USING_SELECT_VL_P(L) (L)->using_select_vl_p
>  #define LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P(L)                             \
>    (L)->epil_using_partial_vectors_p
>  #define LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS(L) (L)->partial_load_store_bias
> 
 
-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
  
juzhe.zhong@rivai.ai June 9, 2023, 8:40 a.m. UTC | #4
Hi, Richi. I have fixed by following your suggestions
Could you take a look at it?

V6 patch:
https://gcc.gnu.org/pipermail/gcc-patches/2023-June/621122.html 

Thanks.


juzhe.zhong@rivai.ai
 
From: Richard Biener
Date: 2023-06-09 16:13
To: Ju-Zhe Zhong
CC: gcc-patches; richard.sandiford
Subject: Re: [PATCH V5] VECT: Add SELECT_VL support
On Thu, 8 Jun 2023, juzhe.zhong@rivai.ai wrote:
 
> From: Ju-Zhe Zhong <juzhe.zhong@rivai.ai>
> 
> Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> This patch address comments from Richard && Richi and rebase to trunk.
> 
> This patch is adding SELECT_VL middle-end support
> allow target have target dependent optimization in case of
> length calculation.
> 
> This patch is inspired by RVV ISA and LLVM:
> https://reviews.llvm.org/D99750
> 
> The SELECT_VL is same behavior as LLVM "get_vector_length" with
> these following properties:
> 
> 1. Only apply on single-rgroup.
> 2. non SLP.
> 3. adjust loop control IV.
> 4. adjust data reference IV.
> 5. allow non-vf elements processing in non-final iteration
> 
> Code:
>    # void vvaddint32(size_t n, const int*x, const int*y, int*z)
>     # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }
> 
> Take RVV codegen for example:
> 
> Before this patch:
> vvaddint32:
>         ble     a0,zero,.L6
>         csrr    a4,vlenb
>         srli    a6,a4,2
> .L4:
>         mv      a5,a0
>         bleu    a0,a6,.L3
>         mv      a5,a6
> .L3:
>         vsetvli zero,a5,e32,m1,ta,ma
>         vle32.v v2,0(a1)
>         vle32.v v1,0(a2)
>         vsetvli a7,zero,e32,m1,ta,ma
>         sub     a0,a0,a5
>         vadd.vv v1,v1,v2
>         vsetvli zero,a5,e32,m1,ta,ma
>         vse32.v v1,0(a3)
>         add     a2,a2,a4
>         add     a3,a3,a4
>         add     a1,a1,a4
>         bne     a0,zero,.L4
> .L6:
>         ret
> 
> After this patch:
> 
> vvaddint32:
>     vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
>     vle32.v v0, (a1)         # Get first vector
>       sub a0, a0, t0         # Decrement number done
>       slli t0, t0, 2         # Multiply number done by 4 bytes
>       add a1, a1, t0         # Bump pointer
>     vle32.v v1, (a2)         # Get second vector
>       add a2, a2, t0         # Bump pointer
>     vadd.vv v2, v0, v1       # Sum vectors
>     vse32.v v2, (a3)         # Store result
>       add a3, a3, t0         # Bump pointer
>       bnez a0, vvaddint32    # Loop back
>       ret                    # Finished
> 
> gcc/ChangeLog:
> 
>         * doc/md.texi: Add SELECT_VL support.
>         * internal-fn.def (SELECT_VL): Ditto.
>         * optabs.def (OPTAB_D): Ditto.
>         * tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
>         * tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
>         * tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
>         (vectorizable_store): Ditto.
>         (vectorizable_load): Ditto.
>         * tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.
>
> Co-authored-by: Richard Sandiford <richard.sandiford@arm.com>
> Co-authored-by: Richard Biener <rguenther@suse.de>
> 
> ---
>  gcc/doc/md.texi             | 22 ++++++++++
>  gcc/internal-fn.def         |  1 +
>  gcc/optabs.def              |  1 +
>  gcc/tree-vect-loop-manip.cc | 32 ++++++++++----
>  gcc/tree-vect-loop.cc       | 72 +++++++++++++++++++++++++++++++
>  gcc/tree-vect-stmts.cc      | 86 ++++++++++++++++++++++++++++++++-----
>  gcc/tree-vectorizer.h       |  6 +++
>  7 files changed, 201 insertions(+), 19 deletions(-)
> 
> diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
> index 6a435eb4461..95f7fe1f802 100644
> --- a/gcc/doc/md.texi
> +++ b/gcc/doc/md.texi
> @@ -4974,6 +4974,28 @@ for (i = 1; i < operand3; i++)
>    operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
>  @end smallexample
>  
> +@cindex @code{select_vl@var{m}} instruction pattern
> +@item @code{select_vl@var{m}}
> +Set operand 0 to the number of scalar iterations that should be handled
> +by one iteration of a vector loop.  Operand 1 is the total number of
> +scalar iterations that the loop needs to process and operand 2 is a
> +maximum bound on the result (also known as the maximum ``vectorization
> +factor'').
> +
> +The maximum value of operand 0 is given by:
> +@smallexample
> +operand0 = MIN (operand1, operand2)
> +@end smallexample
> +However, targets might choose a lower value than this, based on
> +target-specific criteria.  Each iteration of the vector loop might
> +therefore process a different number of scalar iterations, which in turn
> +means that induction variables will have a variable step.  Because of
> +this, it is generally not useful to define this instruction if it will
> +always calculate the maximum value.
> +
> +This optab is only useful on targets that implement @samp{len_load_@var{m}}
> +and/or @samp{len_store_@var{m}}.
> +
>  @cindex @code{check_raw_ptrs@var{m}} instruction pattern
>  @item @samp{check_raw_ptrs@var{m}}
>  Check whether, given two pointers @var{a} and @var{b} and a length @var{len},
> diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
> index 3ac9d82aace..5d638de6d06 100644
> --- a/gcc/internal-fn.def
> +++ b/gcc/internal-fn.def
> @@ -177,6 +177,7 @@ DEF_INTERNAL_OPTAB_FN (VEC_SET, 0, vec_set, vec_set)
>  DEF_INTERNAL_OPTAB_FN (LEN_STORE, 0, len_store, len_store)
>  
>  DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
> +DEF_INTERNAL_OPTAB_FN (SELECT_VL, ECF_CONST | ECF_NOTHROW, select_vl, binary)
>  DEF_INTERNAL_OPTAB_FN (CHECK_RAW_PTRS, ECF_CONST | ECF_NOTHROW,
>         check_raw_ptrs, check_ptrs)
>  DEF_INTERNAL_OPTAB_FN (CHECK_WAR_PTRS, ECF_CONST | ECF_NOTHROW,
> diff --git a/gcc/optabs.def b/gcc/optabs.def
> index 6c064ff4993..f31b69c5d85 100644
> --- a/gcc/optabs.def
> +++ b/gcc/optabs.def
> @@ -488,3 +488,4 @@ OPTAB_DC (vec_series_optab, "vec_series$a", VEC_SERIES)
>  OPTAB_D (vec_shl_insert_optab, "vec_shl_insert_$a")
>  OPTAB_D (len_load_optab, "len_load_$a")
>  OPTAB_D (len_store_optab, "len_store_$a")
> +OPTAB_D (select_vl_optab, "select_vl$a")
> diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
> index 3f735945e67..1c8100c1a1c 100644
> --- a/gcc/tree-vect-loop-manip.cc
> +++ b/gcc/tree-vect-loop-manip.cc
> @@ -534,7 +534,7 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>     _10 = (unsigned long) count_12(D);
>     ...
>     # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
> -    _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
> +    _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
>     ...
>     vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
>     ...
> @@ -549,15 +549,28 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
>        tree step = rgc->controls.length () == 1 ? rgc->controls[0]
>         : make_ssa_name (iv_type);
>        /* Create decrement IV.  */
> -      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> - &incr_gsi, insert_after, &index_before_incr,
> - &index_after_incr);
> -      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
> -     index_before_incr,
> -     nitems_step));
> +      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
> + {
> +   create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
> +      insert_after, &index_before_incr, &index_after_incr);
> +   tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
> +    index_before_incr, nitems_step);
> +   gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
> + }
> +      else
> + {
> +   create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
> +      &incr_gsi, insert_after, &index_before_incr,
> +      &index_after_incr);
> +   gimple_seq_add_stmt (header_seq,
> +        gimple_build_assign (step, MIN_EXPR,
> +     index_before_incr,
> +     nitems_step));
> + }
>        *iv_step = step;
>        *compare_step = nitems_step;
> -      return index_before_incr;
> +      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
> +        : index_before_incr;
>      }
>  
>    /* Create increment IV.  */
> @@ -888,7 +901,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
>    /* Get a boolean result that tells us whether to iterate.  */
>    edge exit_edge = single_exit (loop);
>    gcond *cond_stmt;
> -  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
> +      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
>      {
>        gcc_assert (compare_step);
>        tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;
> diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
> index 5b7a0da0034..ace9e759f5b 100644
> --- a/gcc/tree-vect-loop.cc
> +++ b/gcc/tree-vect-loop.cc
> @@ -974,6 +974,7 @@ _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
>      can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
>      using_partial_vectors_p (false),
>      using_decrementing_iv_p (false),
> +    using_select_vl_p (false),
>      epil_using_partial_vectors_p (false),
>      partial_load_store_bias (0),
>      peeling_for_gaps (false),
> @@ -2737,6 +2738,77 @@ start_over:
>  LOOP_VINFO_VECT_FACTOR (loop_vinfo))))
>      LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo) = true;
>  
> +  /* If a loop uses length controls and has a decrementing loop control IV,
> +     we will normally pass that IV through a MIN_EXPR to calcaluate the
> +     basis for the length controls.  E.g. in a loop that processes one
> +     element per scalar iteration, the number of elements would be
> +     MIN_EXPR <N, VF>, where N is the number of scalar iterations left.
> +
> +     This MIN_EXPR approach allows us to use pointer IVs with an invariant
> +     step, since only the final iteration of the vector loop can have
> +     inactive lanes.
> +
> +     However, some targets have a dedicated instruction for calculating the
> +     preferred length, given the total number of elements that still need to
> +     be processed.  This is encapsulated in the SELECT_VL internal function.
> +
> +     If the target supports SELECT_VL, we can use it instead of MIN_EXPR
> +     to determine the basis for the length controls.  However, unlike the
> +     MIN_EXPR calculation, the SELECT_VL calculation can decide to make
> +     lanes inactive in any iteration of the vector loop, not just the last
> +     iteration.  This SELECT_VL approach therefore requires us to use pointer
> +     IVs with variable steps.
> +
> +     Once we've decided how many elements should be processed by one
> +     iteration of the vector loop, we need to populate the rgroup controls.
> +     If a loop has multiple rgroups, we need to make sure that those rgroups
> +     "line up" (that is, they must be consistent about which elements are
> +     active and which aren't).  This is done by vect_adjust_loop_lens_control.
> +
> +     In principle, it would be possible to use vect_adjust_loop_lens_control
> +     on either the result of a MIN_EXPR or the result of a SELECT_VL.
> +     However:
> +
> +     (1) In practice, it only makes sense to use SELECT_VL when a vector
> + operation will be controlled directly by the result.  It is not
> + worth using SELECT_VL if it would only be the input to other
> + calculations.
> +
> +     (2) If we use SELECT_VL for an rgroup that has N controls, each associated
> + pointer IV will need N updates by a variable amount (N-1 updates
> + within the iteration and 1 update to move to the next iteration).
> +
> +     Because of this, we prefer to use the MIN_EXPR approach whenever there
> +     is more than one length control.
> +
> +     In addition, SELECT_VL always operates to a granularity of 1 unit.
> +     If we wanted to use it to control an SLP operation on N consecutive
> +     elements, we would need to make the SELECT_VL inputs measure scalar
> +     iterations (rather than elements) and then multiply the SELECT_VL
> +     result by N.  But using SELECT_VL this way is inefficient because
> +     of (1) above.
> +
> +     2. We don't apply SELECT_VL on single-rgroup when both (1) and (2) are
> + satisfied:
> +
> +     (1). LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo) is true.
> +     (2). LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant () is true.
> +
> +     Since SELECT_VL (variable step) will make SCEV analysis failed and then
> +     we will fail to gain benefits of following unroll optimizations. We prefer
> +     using the MIN_EXPR approach in this situation.  */
> +  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
> +    {
> +      tree iv_type = LOOP_VINFO_RGROUP_IV_TYPE (loop_vinfo);
> +      if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
> +   OPTIMIZE_FOR_SPEED)
> +   && LOOP_VINFO_LENS (loop_vinfo).length () == 1
> +   && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
> +   && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
> +       || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
> + LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
> +    }
> +
>    /* If we're vectorizing an epilogue loop, the vectorized loop either needs
>       to be able to handle fewer than VF scalars, or needs to have a lower VF
>       than the main loop.  */
> diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
> index c7e4e71d3c5..34e4ad3c922 100644
> --- a/gcc/tree-vect-stmts.cc
> +++ b/gcc/tree-vect-stmts.cc
> @@ -3148,6 +3148,52 @@ vect_get_data_ptr_increment (vec_info *vinfo,
>    return iv_step;
>  }
>  
> +/* Prepare the pointer IVs which needs to be updated by a variable amount.
> +   Such variable amount is the outcome of .SELECT_VL. In this case, we can
> +   allow each iteration process the flexible number of elements as long as
> +   the number <= vf elments.
> +
> +   Return data reference according to SELECT_VL.
> +   If new statements are needed, insert them before GSI.  */
> +
> +static tree
> +vect_get_loop_variant_data_ptr_increment (
> +  vec_info *vinfo, tree aggr_type, gimple_stmt_iterator *gsi,
> +  vec_loop_lens *loop_lens, dr_vec_info *dr_info,
> +  vect_memory_access_type memory_access_type)
> +{
> +  loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo);
> +  tree step = vect_dr_behavior (vinfo, dr_info)->step;
> +
> +  /* TODO: We don't support gather/scatter or load_lanes/store_lanes for pointer
> +     IVs are updated by variable amount but we will support them in the future.
> +   */
> +  gcc_assert (memory_access_type != VMAT_GATHER_SCATTER
> +       && memory_access_type != VMAT_LOAD_STORE_LANES);
> +
> +  /* When we support SELECT_VL pattern, we dynamic adjust
> +     the memory address by .SELECT_VL result.
> +
> +     The result of .SELECT_VL is the number of elements to
> +     be processed of each iteration. So the memory address
> +     adjustment operation should be:
> +
> +     addr = addr + .SELECT_VL (ARG..) * step;
> +  */
> +  tree loop_len
> +    = vect_get_loop_len (loop_vinfo, gsi, loop_lens, 1, aggr_type, 0, 0);
> +  tree len_type = TREE_TYPE (loop_len);
> +  /* Since the outcome of .SELECT_VL is element size, we should adjust
> +     it into bytesize so that it can be used in address pointer variable
> +     amount IVs adjustment.  */
> +  tree tmp = fold_build2 (MULT_EXPR, len_type, loop_len,
> +   wide_int_to_tree (len_type, wi::to_widest (step)));
> +  tree bump = make_temp_ssa_name (len_type, NULL, "ivtmp");
> +  gassign *assign = gimple_build_assign (bump, tmp);
> +  gsi_insert_before (gsi, assign, GSI_SAME_STMT);
> +  return bump;
> +}
> +
>  /* Check and perform vectorization of BUILT_IN_BSWAP{16,32,64,128}.  */
>  
>  static bool
> @@ -8655,11 +8701,21 @@ vectorizable_store (vec_info *vinfo,
>  slp_node, &gs_info, &dataref_ptr,
>  &vec_offsets);
>    else
> -     dataref_ptr
> -       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -   simd_lane_access_p ? loop : NULL,
> -   offset, &dummy, gsi, &ptr_incr,
> -   simd_lane_access_p, bump);
> +     {
> +       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +   && memory_access_type != VMAT_INVARIANT)
> + bump = vect_get_loop_variant_data_ptr_increment (
> +   vinfo, aggr_type, gsi, loop_lens, dr_info,
> +   memory_access_type);
> +
> +       if (j != 0)
> + gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
 
this is in a if (j == 0) branch, please put the assert into the
else {} block of it instead.  You still run the 'bump' computation
before the loop, so if you intend to never handle j != 0 you could
put it there, too.  But at least avoid calling
vect_get_data_ptr_increment in that code when you re-do it here.
 
Btw, 
 
> +       dataref_ptr
> + = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +     simd_lane_access_p ? loop : NULL,
> +     offset, &dummy, gsi, &ptr_incr,
> +     simd_lane_access_p, bump);
> +     }
>  }
>        else
>  {
> @@ -10046,11 +10102,21 @@ vectorizable_load (vec_info *vinfo,
>     &vec_offsets);
>      }
>    else
> -     dataref_ptr
> -       = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> -   at_loop,
> -   offset, &dummy, gsi, &ptr_incr,
> -   simd_lane_access_p, bump);
> +     {
> +       if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
> +   && memory_access_type != VMAT_INVARIANT)
> + bump = vect_get_loop_variant_data_ptr_increment (
> +   vinfo, aggr_type, gsi, loop_lens, dr_info,
> +   memory_access_type);
> +
> +       if (j != 0)
> + gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
 
same here
 
> +       dataref_ptr
> + = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
> +     at_loop, offset, &dummy, gsi,
> +     &ptr_incr, simd_lane_access_p,
> +     bump);
 
btw, I wonder how vect_create_data_ref_ptr and
vect_get_data_ptr_increment handle LEN when not using .SELECT_VL?
Are they always using the constant VF here?
Can't this be done in vect_get_data_ptr_increment by instead
of using VF for LOOP_VINFO_USING_SELECT_VL_P use
vect_get_loop_len () and so only change one place?
 
> +     }
>    if (mask)
>      vec_mask = vec_masks[0];
>  }
> diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
> index 1e44f8542d7..af25d20bd7e 100644
> --- a/gcc/tree-vectorizer.h
> +++ b/gcc/tree-vectorizer.h
> @@ -825,6 +825,11 @@ public:
>  (b) can iterate more than once.  */
>    bool using_decrementing_iv_p;
>  
> +  /* True if we've decided to use output of select_vl to adjust IV of
> +     both loop control and data reference pointer. This is only true
> +     for single-rgroup control.  */
> +  bool using_select_vl_p;
> +
>    /* True if we've decided to use partially-populated vectors for the
>       epilogue of loop.  */
>    bool epil_using_partial_vectors_p;
> @@ -898,6 +903,7 @@ public:
>  #define LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P(L) (L)->can_use_partial_vectors_p
>  #define LOOP_VINFO_USING_PARTIAL_VECTORS_P(L) (L)->using_partial_vectors_p
>  #define LOOP_VINFO_USING_DECREMENTING_IV_P(L) (L)->using_decrementing_iv_p
> +#define LOOP_VINFO_USING_SELECT_VL_P(L) (L)->using_select_vl_p
>  #define LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P(L)                             \
>    (L)->epil_using_partial_vectors_p
>  #define LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS(L) (L)->partial_load_store_bias
> 
 
-- 
Richard Biener <rguenther@suse.de>
SUSE Software Solutions Germany GmbH, Frankenstrasse 146, 90461 Nuernberg,
Germany; GF: Ivo Totev, Andrew Myers, Andrew McDonald, Boudien Moerman;
HRB 36809 (AG Nuernberg)
  

Patch

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index 6a435eb4461..95f7fe1f802 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -4974,6 +4974,28 @@  for (i = 1; i < operand3; i++)
   operand0[i] = operand0[i - 1] && (operand1 + i < operand2);
 @end smallexample
 
+@cindex @code{select_vl@var{m}} instruction pattern
+@item @code{select_vl@var{m}}
+Set operand 0 to the number of scalar iterations that should be handled
+by one iteration of a vector loop.  Operand 1 is the total number of
+scalar iterations that the loop needs to process and operand 2 is a
+maximum bound on the result (also known as the maximum ``vectorization
+factor'').
+
+The maximum value of operand 0 is given by:
+@smallexample
+operand0 = MIN (operand1, operand2)
+@end smallexample
+However, targets might choose a lower value than this, based on
+target-specific criteria.  Each iteration of the vector loop might
+therefore process a different number of scalar iterations, which in turn
+means that induction variables will have a variable step.  Because of
+this, it is generally not useful to define this instruction if it will
+always calculate the maximum value.
+
+This optab is only useful on targets that implement @samp{len_load_@var{m}}
+and/or @samp{len_store_@var{m}}.
+
 @cindex @code{check_raw_ptrs@var{m}} instruction pattern
 @item @samp{check_raw_ptrs@var{m}}
 Check whether, given two pointers @var{a} and @var{b} and a length @var{len},
diff --git a/gcc/internal-fn.def b/gcc/internal-fn.def
index 3ac9d82aace..5d638de6d06 100644
--- a/gcc/internal-fn.def
+++ b/gcc/internal-fn.def
@@ -177,6 +177,7 @@  DEF_INTERNAL_OPTAB_FN (VEC_SET, 0, vec_set, vec_set)
 DEF_INTERNAL_OPTAB_FN (LEN_STORE, 0, len_store, len_store)
 
 DEF_INTERNAL_OPTAB_FN (WHILE_ULT, ECF_CONST | ECF_NOTHROW, while_ult, while)
+DEF_INTERNAL_OPTAB_FN (SELECT_VL, ECF_CONST | ECF_NOTHROW, select_vl, binary)
 DEF_INTERNAL_OPTAB_FN (CHECK_RAW_PTRS, ECF_CONST | ECF_NOTHROW,
 		       check_raw_ptrs, check_ptrs)
 DEF_INTERNAL_OPTAB_FN (CHECK_WAR_PTRS, ECF_CONST | ECF_NOTHROW,
diff --git a/gcc/optabs.def b/gcc/optabs.def
index 6c064ff4993..f31b69c5d85 100644
--- a/gcc/optabs.def
+++ b/gcc/optabs.def
@@ -488,3 +488,4 @@  OPTAB_DC (vec_series_optab, "vec_series$a", VEC_SERIES)
 OPTAB_D (vec_shl_insert_optab, "vec_shl_insert_$a")
 OPTAB_D (len_load_optab, "len_load_$a")
 OPTAB_D (len_store_optab, "len_store_$a")
+OPTAB_D (select_vl_optab, "select_vl$a")
diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 3f735945e67..1c8100c1a1c 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -534,7 +534,7 @@  vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
 	   _10 = (unsigned long) count_12(D);
 	   ...
 	   # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
-	   _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
+	   _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
 	   ...
 	   vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
 	   ...
@@ -549,15 +549,28 @@  vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
       tree step = rgc->controls.length () == 1 ? rgc->controls[0]
 					       : make_ssa_name (iv_type);
       /* Create decrement IV.  */
-      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
-		 &incr_gsi, insert_after, &index_before_incr,
-		 &index_after_incr);
-      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
-							    index_before_incr,
-							    nitems_step));
+      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
+	{
+	  create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
+		     insert_after, &index_before_incr, &index_after_incr);
+	  tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
+				   index_before_incr, nitems_step);
+	  gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
+	}
+      else
+	{
+	  create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
+		     &incr_gsi, insert_after, &index_before_incr,
+		     &index_after_incr);
+	  gimple_seq_add_stmt (header_seq,
+			       gimple_build_assign (step, MIN_EXPR,
+						    index_before_incr,
+						    nitems_step));
+	}
       *iv_step = step;
       *compare_step = nitems_step;
-      return index_before_incr;
+      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
+						       : index_before_incr;
     }
 
   /* Create increment IV.  */
@@ -888,7 +901,8 @@  vect_set_loop_condition_partial_vectors (class loop *loop,
   /* Get a boolean result that tells us whether to iterate.  */
   edge exit_edge = single_exit (loop);
   gcond *cond_stmt;
-  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
+  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
+      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
     {
       gcc_assert (compare_step);
       tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 5b7a0da0034..ace9e759f5b 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -974,6 +974,7 @@  _loop_vec_info::_loop_vec_info (class loop *loop_in, vec_info_shared *shared)
     can_use_partial_vectors_p (param_vect_partial_vector_usage != 0),
     using_partial_vectors_p (false),
     using_decrementing_iv_p (false),
+    using_select_vl_p (false),
     epil_using_partial_vectors_p (false),
     partial_load_store_bias (0),
     peeling_for_gaps (false),
@@ -2737,6 +2738,77 @@  start_over:
 			LOOP_VINFO_VECT_FACTOR (loop_vinfo))))
     LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo) = true;
 
+  /* If a loop uses length controls and has a decrementing loop control IV,
+     we will normally pass that IV through a MIN_EXPR to calcaluate the
+     basis for the length controls.  E.g. in a loop that processes one
+     element per scalar iteration, the number of elements would be
+     MIN_EXPR <N, VF>, where N is the number of scalar iterations left.
+
+     This MIN_EXPR approach allows us to use pointer IVs with an invariant
+     step, since only the final iteration of the vector loop can have
+     inactive lanes.
+
+     However, some targets have a dedicated instruction for calculating the
+     preferred length, given the total number of elements that still need to
+     be processed.  This is encapsulated in the SELECT_VL internal function.
+
+     If the target supports SELECT_VL, we can use it instead of MIN_EXPR
+     to determine the basis for the length controls.  However, unlike the
+     MIN_EXPR calculation, the SELECT_VL calculation can decide to make
+     lanes inactive in any iteration of the vector loop, not just the last
+     iteration.  This SELECT_VL approach therefore requires us to use pointer
+     IVs with variable steps.
+
+     Once we've decided how many elements should be processed by one
+     iteration of the vector loop, we need to populate the rgroup controls.
+     If a loop has multiple rgroups, we need to make sure that those rgroups
+     "line up" (that is, they must be consistent about which elements are
+     active and which aren't).  This is done by vect_adjust_loop_lens_control.
+
+     In principle, it would be possible to use vect_adjust_loop_lens_control
+     on either the result of a MIN_EXPR or the result of a SELECT_VL.
+     However:
+
+     (1) In practice, it only makes sense to use SELECT_VL when a vector
+	 operation will be controlled directly by the result.  It is not
+	 worth using SELECT_VL if it would only be the input to other
+	 calculations.
+
+     (2) If we use SELECT_VL for an rgroup that has N controls, each associated
+	 pointer IV will need N updates by a variable amount (N-1 updates
+	 within the iteration and 1 update to move to the next iteration).
+
+     Because of this, we prefer to use the MIN_EXPR approach whenever there
+     is more than one length control.
+
+     In addition, SELECT_VL always operates to a granularity of 1 unit.
+     If we wanted to use it to control an SLP operation on N consecutive
+     elements, we would need to make the SELECT_VL inputs measure scalar
+     iterations (rather than elements) and then multiply the SELECT_VL
+     result by N.  But using SELECT_VL this way is inefficient because
+     of (1) above.
+
+     2. We don't apply SELECT_VL on single-rgroup when both (1) and (2) are
+	satisfied:
+
+     (1). LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo) is true.
+     (2). LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant () is true.
+
+     Since SELECT_VL (variable step) will make SCEV analysis failed and then
+     we will fail to gain benefits of following unroll optimizations. We prefer
+     using the MIN_EXPR approach in this situation.  */
+  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
+    {
+      tree iv_type = LOOP_VINFO_RGROUP_IV_TYPE (loop_vinfo);
+      if (direct_internal_fn_supported_p (IFN_SELECT_VL, iv_type,
+					  OPTIMIZE_FOR_SPEED)
+	  && LOOP_VINFO_LENS (loop_vinfo).length () == 1
+	  && LOOP_VINFO_LENS (loop_vinfo)[0].factor == 1 && !slp
+	  && (!LOOP_VINFO_NITERS_KNOWN_P (loop_vinfo)
+	      || !LOOP_VINFO_VECT_FACTOR (loop_vinfo).is_constant ()))
+	LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) = true;
+    }
+
   /* If we're vectorizing an epilogue loop, the vectorized loop either needs
      to be able to handle fewer than VF scalars, or needs to have a lower VF
      than the main loop.  */
diff --git a/gcc/tree-vect-stmts.cc b/gcc/tree-vect-stmts.cc
index c7e4e71d3c5..34e4ad3c922 100644
--- a/gcc/tree-vect-stmts.cc
+++ b/gcc/tree-vect-stmts.cc
@@ -3148,6 +3148,52 @@  vect_get_data_ptr_increment (vec_info *vinfo,
   return iv_step;
 }
 
+/* Prepare the pointer IVs which needs to be updated by a variable amount.
+   Such variable amount is the outcome of .SELECT_VL. In this case, we can
+   allow each iteration process the flexible number of elements as long as
+   the number <= vf elments.
+
+   Return data reference according to SELECT_VL.
+   If new statements are needed, insert them before GSI.  */
+
+static tree
+vect_get_loop_variant_data_ptr_increment (
+  vec_info *vinfo, tree aggr_type, gimple_stmt_iterator *gsi,
+  vec_loop_lens *loop_lens, dr_vec_info *dr_info,
+  vect_memory_access_type memory_access_type)
+{
+  loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo);
+  tree step = vect_dr_behavior (vinfo, dr_info)->step;
+
+  /* TODO: We don't support gather/scatter or load_lanes/store_lanes for pointer
+     IVs are updated by variable amount but we will support them in the future.
+   */
+  gcc_assert (memory_access_type != VMAT_GATHER_SCATTER
+	      && memory_access_type != VMAT_LOAD_STORE_LANES);
+
+  /* When we support SELECT_VL pattern, we dynamic adjust
+     the memory address by .SELECT_VL result.
+
+     The result of .SELECT_VL is the number of elements to
+     be processed of each iteration. So the memory address
+     adjustment operation should be:
+
+     addr = addr + .SELECT_VL (ARG..) * step;
+  */
+  tree loop_len
+    = vect_get_loop_len (loop_vinfo, gsi, loop_lens, 1, aggr_type, 0, 0);
+  tree len_type = TREE_TYPE (loop_len);
+  /* Since the outcome of .SELECT_VL is element size, we should adjust
+     it into bytesize so that it can be used in address pointer variable
+     amount IVs adjustment.  */
+  tree tmp = fold_build2 (MULT_EXPR, len_type, loop_len,
+			  wide_int_to_tree (len_type, wi::to_widest (step)));
+  tree bump = make_temp_ssa_name (len_type, NULL, "ivtmp");
+  gassign *assign = gimple_build_assign (bump, tmp);
+  gsi_insert_before (gsi, assign, GSI_SAME_STMT);
+  return bump;
+}
+
 /* Check and perform vectorization of BUILT_IN_BSWAP{16,32,64,128}.  */
 
 static bool
@@ -8655,11 +8701,21 @@  vectorizable_store (vec_info *vinfo,
 					 slp_node, &gs_info, &dataref_ptr,
 					 &vec_offsets);
 	  else
-	    dataref_ptr
-	      = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
-					  simd_lane_access_p ? loop : NULL,
-					  offset, &dummy, gsi, &ptr_incr,
-					  simd_lane_access_p, bump);
+	    {
+	      if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
+		  && memory_access_type != VMAT_INVARIANT)
+		bump = vect_get_loop_variant_data_ptr_increment (
+		  vinfo, aggr_type, gsi, loop_lens, dr_info,
+		  memory_access_type);
+
+	      if (j != 0)
+		gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
+	      dataref_ptr
+		= vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
+					    simd_lane_access_p ? loop : NULL,
+					    offset, &dummy, gsi, &ptr_incr,
+					    simd_lane_access_p, bump);
+	    }
 	}
       else
 	{
@@ -10046,11 +10102,21 @@  vectorizable_load (vec_info *vinfo,
 					   &vec_offsets);
 	    }
 	  else
-	    dataref_ptr
-	      = vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
-					  at_loop,
-					  offset, &dummy, gsi, &ptr_incr,
-					  simd_lane_access_p, bump);
+	    {
+	      if (loop_lens && LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo)
+		  && memory_access_type != VMAT_INVARIANT)
+		bump = vect_get_loop_variant_data_ptr_increment (
+		  vinfo, aggr_type, gsi, loop_lens, dr_info,
+		  memory_access_type);
+
+	      if (j != 0)
+		gcc_assert (!LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo));
+	      dataref_ptr
+		= vect_create_data_ref_ptr (vinfo, first_stmt_info, aggr_type,
+					    at_loop, offset, &dummy, gsi,
+					    &ptr_incr, simd_lane_access_p,
+					    bump);
+	    }
 	  if (mask)
 	    vec_mask = vec_masks[0];
 	}
diff --git a/gcc/tree-vectorizer.h b/gcc/tree-vectorizer.h
index 1e44f8542d7..af25d20bd7e 100644
--- a/gcc/tree-vectorizer.h
+++ b/gcc/tree-vectorizer.h
@@ -825,6 +825,11 @@  public:
 	(b) can iterate more than once.  */
   bool using_decrementing_iv_p;
 
+  /* True if we've decided to use output of select_vl to adjust IV of
+     both loop control and data reference pointer. This is only true
+     for single-rgroup control.  */
+  bool using_select_vl_p;
+
   /* True if we've decided to use partially-populated vectors for the
      epilogue of loop.  */
   bool epil_using_partial_vectors_p;
@@ -898,6 +903,7 @@  public:
 #define LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P(L) (L)->can_use_partial_vectors_p
 #define LOOP_VINFO_USING_PARTIAL_VECTORS_P(L) (L)->using_partial_vectors_p
 #define LOOP_VINFO_USING_DECREMENTING_IV_P(L) (L)->using_decrementing_iv_p
+#define LOOP_VINFO_USING_SELECT_VL_P(L) (L)->using_select_vl_p
 #define LOOP_VINFO_EPIL_USING_PARTIAL_VECTORS_P(L)                             \
   (L)->epil_using_partial_vectors_p
 #define LOOP_VINFO_PARTIAL_LOAD_STORE_BIAS(L) (L)->partial_load_store_bias