[LLVMdev] Which floating-point comparison?

[Jeffrey Yasskin]

On Sun, Mar 28, 2010 at 7:45 AM, Russell Wallace
<russell.wallace at gmail.com> wrote:
> I notice llvm provides both ordered and unordered variants of
> floating-point comparison. Which of these is the right one to use by
> default? I suppose the two criteria would be, in order of importance:
>
> 1. Which is more efficient (more directly maps to typical hardware)?

You can figure this out by looking at the output of llc:

$ cat test.ll
define i1 @less(double %x, double %y) nounwind readnone {
entry:
  %0 = fcmp ult double %x, %y                     ; <i1> [#uses=1]
  ret i1 %0
}
$ Debug/bin/llc  <test.ll
	.section	__TEXT,__text,regular,pure_instructions
	.globl	_less
	.align	4, 0x90
_less:                                  ## @less
## BB#0:                                ## %entry
	movsd	4(%esp), %xmm0
	ucomisd	12(%esp), %xmm0
	sbbb	%al, %al
	andb	$1, %al
	ret

> 2. Which is more familiar (more like the way C and Fortran do it)?

You can use http://llvm.org/demo/ to figure that out.