RISC-V: optimize '(a >= 0) ? b : 0' to srai + andn, if compiling for Zbb
Checks
Commit Message
If-conversion is turning '(a >= 0) ? b : 0' into a branchless sequence
not a5,a0
srai a5,a5,63
and a0,a1,a5
missing the opportunity to combine the NOT and AND into an ANDN.
This adds a define_split to help the combiner reassociate the NOT with
the AND.
gcc/ChangeLog:
* config/riscv/bitmanip.md: New define_split.
gcc/testsuite/ChangeLog:
* gcc.target/riscv/zbb-srai-andn.c: New test.
---
gcc/config/riscv/bitmanip.md | 13 +++++++++++++
gcc/testsuite/gcc.target/riscv/zbb-srai-andn.c | 15 +++++++++++++++
2 files changed, 28 insertions(+)
create mode 100644 gcc/testsuite/gcc.target/riscv/zbb-srai-andn.c
Comments
On 11/8/22 12:54, Philipp Tomsich wrote:
> If-conversion is turning '(a >= 0) ? b : 0' into a branchless sequence
> not a5,a0
> srai a5,a5,63
> and a0,a1,a5
> missing the opportunity to combine the NOT and AND into an ANDN.
>
> This adds a define_split to help the combiner reassociate the NOT with
> the AND.
>
>
> gcc/ChangeLog:
>
> * config/riscv/bitmanip.md: New define_split.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/riscv/zbb-srai-andn.c: New test.
OK.
FWIW, combine can be pretty sneaky in manipulating the result of a scc
style insn. I've seen a port with pages and pages of special patterns
to match what simplify_if_then_else would do.
Jeff
Applied to master. Thanks!
--Philipp.
On Sun, 13 Nov 2022 at 01:24, Jeff Law <jeffreyalaw@gmail.com> wrote:
>
>
> On 11/8/22 12:54, Philipp Tomsich wrote:
> > If-conversion is turning '(a >= 0) ? b : 0' into a branchless sequence
> > not a5,a0
> > srai a5,a5,63
> > and a0,a1,a5
> > missing the opportunity to combine the NOT and AND into an ANDN.
> >
> > This adds a define_split to help the combiner reassociate the NOT with
> > the AND.
> >
> >
> > gcc/ChangeLog:
> >
> > * config/riscv/bitmanip.md: New define_split.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * gcc.target/riscv/zbb-srai-andn.c: New test.
>
> OK.
>
>
> FWIW, combine can be pretty sneaky in manipulating the result of a scc
> style insn. I've seen a port with pages and pages of special patterns
> to match what simplify_if_then_else would do.
>
>
> Jeff
>
@@ -128,6 +128,19 @@
[(set_attr "type" "bitmanip")
(set_attr "mode" "<X:MODE>")])
+;; '(a >= 0) ? b : 0' is emitted branchless (from if-conversion). Without a
+;; bit of extra help for combine (i.e., the below split), we end up emitting
+;; not/srai/and instead of combining the not into an andn.
+(define_split
+ [(set (match_operand:DI 0 "register_operand")
+ (and:DI (neg:DI (ge:DI (match_operand:DI 1 "register_operand")
+ (const_int 0)))
+ (match_operand:DI 2 "register_operand")))
+ (clobber (match_operand:DI 3 "register_operand"))]
+ "TARGET_ZBB"
+ [(set (match_dup 3) (ashiftrt:DI (match_dup 1) (const_int 63)))
+ (set (match_dup 0) (and:DI (not:DI (match_dup 3)) (match_dup 2)))])
+
(define_insn "*xor_not<mode>"
[(set (match_operand:X 0 "register_operand" "=r")
(not:X (xor:X (match_operand:X 1 "register_operand" "r")
new file mode 100644
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "-march=rv64gc_zbb -mabi=lp64" } */
+/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-Os" "-Oz" } } */
+
+long long foo0(long long a, long long b)
+{
+ if (a >= 0)
+ return b;
+
+ return 0;
+}
+
+/* { dg-final { scan-assembler-times "srai\t" 1 } } */
+/* { dg-final { scan-assembler-times "andn\t" 1 } } */
+