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[Committed] RISC-V: zicond: Fix opt2 pattern

Message ID 20230905152015.9738-1-vineetg@rivosinc.com
State Unresolved
Headers
Series [Committed] RISC-V: zicond: Fix opt2 pattern |

Checks

Context Check Description
snail/gcc-patch-check warning Git am fail log

Commit Message

Vineet Gupta Sept. 5, 2023, 3:20 p.m. UTC 
  Fixes: 1d5bc3285e8a ("[committed][RISC-V] Fix 20010221-1.c with zicond")

This was tripping up gcc.c-torture/execute/pr60003.c at -O1 since in
failing case, pattern semantics were not matching with asm czero.nez

We start with the following src code snippet:

      if (a == 0)
	return 0;
      else
	return x;
    }

which is equivalent to:  "x = (a != 0) ? x : a" where x is NOT 0.
                                                ^^^^^^^^^^^^^^^^

and matches define_insn "*czero.nez.<GPR:mode><X:mode>.opt2"

| (insn 41 20 38 3 (set (reg/v:DI 136 [ x ])
|        (if_then_else:DI (ne (reg/v:DI 134 [ a ])
|                (const_int 0 [0]))
|            (reg/v:DI 136 [ x ])
|            (reg/v:DI 134 [ a ]))) {*czero.nez.didi.opt2}

The corresponding asm pattern generates
    czero.nez x, x, a   ; %0, %2, %1

which implies
    "x = (a != 0) ? 0 : a"

clearly not what the pattern wants to do.

Essentially "(a != 0) ? x : a" cannot be expressed with CZERO.nez if X
is not guaranteed to be 0.

However this can be fixed with a small tweak

"x = (a != 0) ? x : a"

   is same as

"x = (a == 0) ? a : x"

and since middle operand is 0 when a == 0, it is equivalent to

"x = (a == 0) ? 0 : x"

which can be expressed with CZERO.eqz

before fix		after fix
-----------------	-----------------
li        a5,1	        li        a5,1
ld        a4,8(sp)	ld        a4,8(sp)
czero.nez a0,a4,a5 	czero.eqz a0,a4,a5

The issue only happens at -O1 as at higher optimization levels, the
whole conditional move gets optimized away.

This fixes 4 testsuite failues in a zicond build:

FAIL: gcc.c-torture/execute/pr60003.c   -O1  execution test
FAIL: gcc.dg/setjmp-3.c execution test
FAIL: gcc.dg/torture/stackalign/setjmp-3.c   -O1  execution test
FAIL: gcc.dg/torture/stackalign/setjmp-3.c   -O1 -fpic execution test

gcc/ChangeLog:
	* config/riscv/zicond.md: Fix op2 pattern.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>
---
 gcc/config/riscv/zicond.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/gcc/config/riscv/zicond.md b/gcc/config/riscv/zicond.md
index 4619220ef8ac..1721e1011ea8 100644
--- a/gcc/config/riscv/zicond.md
+++ b/gcc/config/riscv/zicond.md
@@ -60,7 +60,7 @@ 
                           (match_operand:GPR 2 "register_operand" "r")
                           (match_operand:GPR 3 "register_operand" "1")))]
   "TARGET_ZICOND && rtx_equal_p (operands[1], operands[3])"
-  "czero.nez\t%0,%2,%1"
+  "czero.eqz\t%0,%2,%1"
 )
 
 ;; Combine creates this form in some cases (particularly the coremark