[llvm-dev] different output with fast-math flag

[sangeeta chowdhary via llvm-dev]

This is of course not homework. I am trying to understand how fast math
optimizations work in llvm. When I compared IR for both the programs, the
only thing I have noticed is that fdiv and fmul are replaced with fdiv fast
and fmul fast. Not sure what happens in fdiv fast and fmul fast.
I feel that its because d/max is really small number and fast-math does not
care about small numbers and consider them to zero but this is so
incorrect.

On Tue, Aug 21, 2018 at 12:45 PM Stephen Canon <scanon at apple.com> wrote:

> On Aug 21, 2018, at 11:17 AM, sangeeta chowdhary via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
> Why the output is different for this below program when compiled using
> clang with fast-math optimization
>
> #include<stdio.h>
>
> int main() {
>   double d = 1.0;
>   double max = 1.79769e+308;
>
>   d /= max;
>   printf("d:%e:\n", d);
>   d *= max;
>   printf("d:%e:\n", d);
>   return 0;
> }
>
> prints 0 with fast math but 1 without fast math.
>
>
> Please do not ask llvm-dev to do your homework. If this is genuinely not a
> school assignment, reply to me off-list and I’ll help you understand what’s
> happening here.
>
> – Steve
>
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