[Bug target/28691] New: missed optimization, redundant scalar SSE comparisons

tbptbp at gmail dot com gcc-bugzilla@gcc.gnu.org
Fri Aug 11 05:33:00 GMT 2006


Symptoms: gcc fails to notice flags already available and generates redundant
comparisons.

After tracking down a real world performance issue i couldn't rid of without
ressorting to inline asm, i've bugged Uros; he produced that reduced testcase.

int test(float a, float b)
{
 int lt = a < b;
 int eq = a == b;

 return lt + eq;
}

with gcc-4.2-20060805/cygwin, -O3 -march=k8 -mfpmath=sse -fomit-frame-pointer
-ffast-math:
00401050 <test(float, float)>:
  401050:       movss  0x4(%esp),%xmm1
  401056:       xor    %eax,%eax
  401058:       movss  0x8(%esp),%xmm0
  40105e:       comiss %xmm0,%xmm1
  401061:       sete   %al
  401064:       xor    %edx,%edx
  401066:       comiss %xmm0,%xmm1
  401069:       setb   %dl
  40106c:       add    %edx,%eax
  40106e:       ret

Also happens on x86-64.
This is not only related to PR 17390, but also integer compares :)


-- 
           Summary: missed optimization, redundant scalar SSE comparisons
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tbptbp at gmail dot com
  GCC host triplet: x86*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28691



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