llvm dev - Oct 2006 - [LLVMdev] floating point exceptions in compare instructions

Looking at the llvm code I have the impression that ordered fp
comparisons should use exception throwing instructions. Is it true?

Thanks,
Rafael

On Fri, 13 Oct 2006, [UTF-8] Rafael Esp?ndola wrote:
> Looking at the llvm code I have the impression that ordered fp
> comparisons should use exception throwing instructions. Is it true?
Nope, you want non-trapping instructions.  If you use trapping 
instructions, C99 functions like isgreater will not work correctly with 
NAN's.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

> Nope, you want non-trapping instructions.  If you use trapping
> instructions, C99 functions like isgreater will not work correctly with
> NAN's.
How do I know when to use a trapping instruction? For example consider
this C function:
------------------------
int f(float a, float b) {
        return a <= b;
}
------------------------

gcc uses fcmpes instead of fcmps...
> -Chris
Thanks,
Rafael