[LLVMdev] [RFC] Extend LLVM IR to express "fast-math" at a per-instruction level

[Dan Gohman]

On Tue, Oct 30, 2012 at 8:28 PM, Michael Ilseman <milseman at apple.com> wrote:

>

> This is similar to how gcc defines *-fno-signed-zeros:*
> "Allow optimizations for floating point arithmetic that ignore the
> signedness of zero. IEEE arithmetic specifies the behavior of distinct
> +0.0 and -0.0 values, which then prohibits simplification of expressions
> such as x+0.0 or 0.0*x (even with *-ffinite-math-only*). This option
> implies that the sign of a zero result isn't significant."
>
> I'll revise my description to also mention that the sign of a zero result
> isn't significant.
>

Ok, I see what you're saying here now.


>
> Also, even when you do have the second sentence, it seems to contradict
> the first sentence.
>
>
> Why does it contradict the first sentence? I meant it as a clarification
> or reinforcement of the first, not a contradiction.
>

Suppose I'm writing a backend for a target which has an instruction that
traps on any kind of NaN. Assuming I care about NaNs, I can't use such an
instruction for regular floating-point operations. However, would it be ok
to use it when the N flag is set?

If the "optimizer" may truly ignore the possibility of NaNs under the N
flag, this would seem to be ok. However, a trap is outside the boundaries
of "undefined result". So, which half is right?

Dan
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