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i386: Introduce peephole2 to use flags from CMPXCHG more [PR96189]

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CMPXCHG instruction sets ZF flag if the values in the destination operand
and EAX register are equal; otherwise the ZF flag is cleared and value
from destination operand is loaded to EAX. Following assembly:

        movl    %esi, %eax
        lock cmpxchgl   %edx, (%rdi)
        cmpl    %esi, %eax
        sete    %al

can be optimized by removing the unneeded comparison, since set ZF flag
signals that no update to EAX happened.

2020-15-07  Uroš Bizjak  <ubizjak@gmail.com>

gcc/ChangeLog:
	PR target/95355
	* config/i386/sync.md
	(peephole2 to remove unneded compare after CMPXCHG): New pattern.

gcc/testsuite/ChangeLog:
	PR target/95355
	* gcc.target/i386/pr96189.c: New test.
This commit is contained in:
Uros Bizjak 2020-07-15 21:27:00 +02:00
parent 51542d9254
commit 6c2848ad02
2 changed files with 47 additions and 0 deletions
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35
gcc/config/i386/sync.md
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@ -594,6 +594,41 @@
"TARGET_CMPXCHG"
"lock{%;} %K4cmpxchg{<imodesuffix>}\t{%3, %1|%1, %3}")
(define_peephole2
[(set (match_operand:SWI 0 "register_operand")
(match_operand:SWI 1 "general_operand"))
(parallel [(set (match_dup 0)
(unspec_volatile:SWI
[(match_operand:SWI 2 "memory_operand")
(match_dup 0)
(match_operand:SWI 3 "register_operand")
(match_operand:SI 4 "const_int_operand")]
UNSPECV_CMPXCHG))
(set (match_dup 2)
(unspec_volatile:SWI [(const_int 0)] UNSPECV_CMPXCHG))
(set (reg:CCZ FLAGS_REG)
(unspec_volatile:CCZ [(const_int 0)] UNSPECV_CMPXCHG))])
(set (reg:CCZ FLAGS_REG)
(compare:CCZ (match_operand:SWI 5 "register_operand")
(match_operand:SWI 6 "general_operand")))]
"(rtx_equal_p (operands[0], operands[5])
&& rtx_equal_p (operands[1], operands[6]))
|| (rtx_equal_p (operands[0], operands[6])
&& rtx_equal_p (operands[1], operands[5]))"
[(set (match_dup 0)
(match_dup 1))
(parallel [(set (match_dup 0)
(unspec_volatile:SWI
[(match_dup 2)
(match_dup 0)
(match_dup 3)
(match_dup 4)]
UNSPECV_CMPXCHG))
(set (match_dup 2)
(unspec_volatile:SWI [(const_int 0)] UNSPECV_CMPXCHG))
(set (reg:CCZ FLAGS_REG)
(unspec_volatile:CCZ [(const_int 0)] UNSPECV_CMPXCHG))])])
;; For operand 2 nonmemory_operand predicate is used instead of
;; register_operand to allow combiner to better optimize atomic
;; additions of constants.

12
gcc/testsuite/gcc.target/i386/pr96189.c Normal file
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@ -0,0 +1,12 @@
/* PR target/96176 */
/* { dg-do compile { target lp64 } } */
/* { dg-options "-O2" } */
/* { dg-final { scan-assembler-not "\tcmpb\t" } } */
_Bool
foo (unsigned char *x, unsigned char y, unsigned char z)
{
unsigned char y_old = y;
__atomic_compare_exchange_n (x, &y, z, 0, __ATOMIC_RELAXED, __ATOMIC_RELAXED);
return y == y_old;
}