[llvm-commits] [llvm] r122364 - in /llvm/trunk: lib/Target/X86/X86ISelLowering.cpp test/CodeGen/X86/conditional-indecrement.ll

[Benjamin Kramer]

On 22.12.2010, at 03:13, Chris Lattner wrote:

> 
> On Dec 21, 2010, at 1:41 PM, Benjamin Kramer wrote:
> 
>> Author: d0k
>> Date: Tue Dec 21 15:41:44 2010
>> New Revision: 122364
>> 
>> URL: http://llvm.org/viewvc/llvm-project?rev=122364&view=rev
>> Log:
>> Add some x86 specific dagcombines for conditional increments.
>> 
>> (add Y, (sete  X, 0)) -> cmp X, 1; adc  0, Y
>> (add Y, (setne X, 0)) -> cmp X, 1; sbb -1, Y
>> (sub (sete  X, 0), Y) -> cmp X, 1; sbb  0, Y
>> (sub (setne X, 0), Y) -> cmp X, 1; adc -1, Y
> 
> Very nice.  Do we handle the cases when X and Y are the same value?  "X == 0 ? X+1 : X" simplifies to "X == 0 ? 1 : X".

$ cat t.c
unsigned foo(unsigned x) {
  if (x == 0) x++;
  return x;
}

$ clang -O3 t.c
_foo:
	cmpl	$1, %edi
	movl	%edi, %eax
	adcl	$0, %eax
	ret

$ gcc -O3 t.c
_foo:
	testl	%edi, %edi
	movl	$1, %eax
	cmove	%eax, %edi
	movl	%edi, %eax
	ret