[Bug target/56309] -O3 optimizer generates conditional moves instead of compare and branch resulting in almost 2x slower code

[arturomdn at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56309



--- Comment #9 from arturomdn at gmail dot com 2013-02-14 16:00:49 UTC ---

I found in the Intel optimization guide an example of this idiom of comparing

once and issuing two cmov back-to-back... so the problem isn't the two cmov,

but possibly introducing the 2nd compare that splits them.



not_equal:

   movzx eax, BYTE PTR[esi+edx]

   movzx edx, BYTE PTR[edi+edx]

   cmp eax, edx

   cmova eax, ONE

   cmovb eax, NEG_ONE

   jmp ret_tag



Taken from example 10-16 of

http://www.intel.com/content/dam/doc/manual/64-ia-32-architectures-optimization-manual.pdf