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List:       mingw-users
Subject:    [Mingw-users] Optimizer produces suboptimal code for e.g. x = x ^
From:       Jasper Neumann <jasper.neumann () web ! de>
Date:       2011-02-25 19:27:51
Message-ID: 4D6802B7.406 () web ! de
[Download RAW message or body]

Hello folks!

When I compile the following OPT.CPP with gcc 4.5.2...
===
int test(int x)
{
   x = x ^ (x >> 1);

   int x1=x;
   x = x >> 2;
   x = x ^ x1;

   return x;
}
===

...a call to
gpp -O3 -S OPT.CPP
produces this OPT.s:
===
         .file   "OPT.CPP"
         .text
         .p2align 2,,3
.globl __Z4testi
         .def    __Z4testi;      .scl    2;      .type   32;     .endef
__Z4testi:
LFB0:
         pushl   %ebp
LCFI0:
         movl    %esp, %ebp
LCFI1:
         movl    8(%ebp), %eax
         movl    %eax, %edx
         sarl    %edx
         xorl    %eax, %edx
         movl    %edx, %eax
         sarl    $2, %eax
         xorl    %edx, %eax
         leave
LCFI2:
         ret
LFE0:
===

The problem I see is that in
         movl    %eax, %edx
         sarl    %edx
         xorl    %eax, %edx

         movl    %edx, %eax
         sarl    $2, %eax
         xorl    %edx, %eax
gcc produces code which presumably costs 6 cycles
(edx end then eax is modified 3 times in a row)
whereas the equivalent statements
         movl    %eax, %edx
         sarl    %eax
         xorl    %eax, %edx

         movl    %edx, %eax
         sarl    $2, %edx
         xorl    %edx, %eax
cost only 4 cycles since the mov and the shift can go in parallel.
I would have expected this at least for explicit form in
   int x1=x;
   x = x >> 2;
   x = x ^ x1;
I found no way to get gcc to output my version.

Have I missed something?


By the way: If I produce an output in Intel syntax
the statement "sar eax" should be "sar eax,1".
Otherwise some assemblers will complain.

Jasper

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