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PR tree-optimization/91384: peephole2 to eliminate testl after negl.

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This patch is my proposed solution to PR tree-optimization/91384 which is
a missed-optimization/code quality regression on x86_64.  The problematic
idiom is "if (r = -a)" which is equivalent to both "r = -a; if (r != 0)"
and alternatively "r = -a; if (a != 0)".  In this particular case, on
x86_64, we prefer to use the condition codes from the negation, rather
than require an explicit testl instruction.

Unfortunately, combine can't help, as it doesn't attempt to merge pairs
of instructions that share the same operand(s), only pairs/triples of
instructions where the result of each instruction feeds the next.  But
I doubt there's sufficient benefit to attempt this kind of "combination"
(that wouldn't already be caught by the tree-ssa passes).

Fortunately, it's relatively easy to fix this up (addressing the
regression) during peephole2 to eliminate the unnecessary testl in:

        movl    %edi, %ebx
        negl    %ebx
        testl   %edi, %edi
        je      .L2

2022-02-28  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR tree-optimization/91384
	* config/i386/i386.md (peephole2): Eliminate final testl insn
	from the sequence *movsi_internal, *negsi_1, *cmpsi_ccno_1 by
	transforming using *negsi_2 for the negation.

gcc/testsuite/ChangeLog
	PR tree-optimization/91384
	* gcc.target/i386/pr91384.c: New test case.
This commit is contained in:
Roger Sayle 2022-02-28 22:30:27 +00:00
parent 7e5c6edeb1
commit 28068d1115
2 changed files with 33 additions and 0 deletions
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13
gcc/config/i386/i386.md
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@ -11011,6 +11011,19 @@
[(set_attr "type" "negnot")
(set_attr "mode" "<MODE>")])
;; Optimize *negsi_1 followed by *cmpsi_ccno_1 (PR target/91384)
(define_peephole2
[(set (match_operand:SWI 0 "general_reg_operand")
(match_operand:SWI 1 "general_reg_operand"))
(parallel [(set (match_dup 0) (neg:SWI (match_dup 0)))
(clobber (reg:CC FLAGS_REG))])
(set (reg:CCZ FLAGS_REG) (compare:CCZ (match_dup 1) (const_int 0)))]
""
[(set (match_dup 0) (match_dup 1))
(parallel [(set (reg:CCZ FLAGS_REG)
(compare:CCZ (neg:SWI (match_dup 0)) (const_int 0)))
(set (match_dup 0) (neg:SWI (match_dup 0)))])])
;; Special expand pattern to handle integer mode abs
(define_expand "abs<mode>2"

20
gcc/testsuite/gcc.target/i386/pr91384.c Normal file
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@ -0,0 +1,20 @@
/* { dg-do compile } */
/* { dg-options "-O2" } */
void foo (void);
void bar (void);
int
test (int a)
{
int r;
if (r = -a)
foo ();
else
bar ();
return r;
}
/* { dg-final { scan-assembler-not "testl" } } */