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)?
2. Which is more familiar (more like the way C and Fortran do it)?
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 Try out LLVM and Clang in your browser! to figure that out.
That works, thanks! It turns out that x86/SSE at least handles both
equally well, but the ordered version is what C uses.