[Bug target/18589] New: could optimize FP multiplies better

["debian-gcc at lists dot debian dot org" <gcc-bugzilla at gcc dot gnu dot org>]

[forwarded from http://bugs.debian.org/268115]

  Matthias

The bug submitter writes:


compiling this function:
double baz(double foo, double bar)
{
   return foo*foo*foo*foo*bar*bar*bar*bar;
}

 on amd64 with -O6 -ffast-math, gcc emits this code:

foo.o:     file format elf64-x86-64

Disassembly of section .text:

... (some similar functions that I was messing around with) ...
0000000000000050 <ddbar>:
  50:	f2 0f 59 c0          	mulsd  %xmm0,%xmm0
  54:	f2 0f 59 c0          	mulsd  %xmm0,%xmm0
  58:	f2 0f 59 c1          	mulsd  %xmm1,%xmm0
  5c:	f2 0f 59 c1          	mulsd  %xmm1,%xmm0
  60:	f2 0f 59 c1          	mulsd  %xmm1,%xmm0
  64:	f2 0f 59 c1          	mulsd  %xmm1,%xmm0
  68:	c3                   	retq   


 So, it notices that it can do foo*foo*foo*foo with two mulsd instructions,
but it misses the same optimization for bar*bar*bar*bar.  It would save one
FP multiply overall to do:
mulsd %xmm0, %xmm0
mulsd %xmm1, %xmm1
mulsd %xmm0, %xmm0
mulsd %xmm1, %xmm1
mulsd %xmm1, %xmm0
retq
 Also, the two non-dependent muls could run in parallel.
 
 Without -ffast-math, of course, gcc can't take advantage of the laws of
arithmetic like that and has to do all the multiplies the straightforward
way.

-- 
           Summary: could optimize FP multiplies better
           Product: gcc
           Version: 3.4.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: target
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: debian-gcc at lists dot debian dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: amd64-linux
  GCC host triplet: amd64-linux
GCC target triplet: amd64-linux


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18589