[llvm-commits] [llvm] r120932 - in /llvm/trunk: lib/Target/X86/X86ISelLowering.cpp test/CodeGen/X86/select.ll

[Chris Lattner]

On Dec 5, 2010, at 3:32 PM, Evan Cheng wrote:

>> 
>> but this also helps if the other value is an imm or load, because those can be folded into orq but not into cmov.
> 
> That may be true. I'd still call this optimization suspect. A couple of cycles of latencies means very little for modern x86. Instruction throughput is probably just as important and the extra instruction here may actually be an pessimisation.
> 
> Both gcc and icc just generates something like this. What are we missing? It doesn't seem like they are concerned with overflow?
>        salq    $2, %rdi
>        jmp     __Znam

Right, they don't handle the integer overflow case.  In this particular example, what we really want is a way to say that the select is heavily biased, so that we get a jump instead of cmov (or cmov equivalent).  We really want:

  mulq <reg>
  jo Lfoo
  jmp __Znam
Lfoo:
  mov rdi, -1
  jmp __Znam
  
-Chris