[llvm-dev] [cfe-dev] Should isnan be optimized out in fast-math mode?

[Aaron Ballman via llvm-dev]

On Mon, Sep 20, 2021 at 1:04 PM Mehdi AMINI via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> On Mon, Sep 20, 2021 at 1:23 AM Serge Pavlov <sepavloff at gmail.com> wrote:
>>
>> On Fri, Sep 17, 2021 at 11:17 PM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>>
>>> On Thu, Sep 16, 2021 at 11:19 PM Serge Pavlov <sepavloff at gmail.com> wrote:
>> Also if we agree that NaNs can appear in the code compiled with -ffinite-math-only, there must be a way to check if a number is a NaN.
>
>
> I'd find it unfortunate though that in a mode which specifies that the result of floating point arithmetic can't have NaN we can't constant fold isnan(x) where x is a potentially complex expression (think that x could be dead-code if not for the isnan).

I think part of my concern is with the "result of floating point
arithmetic can't have NaN" statement. I consider this case:

if (isnan(foo / bar)) {}

to be fundamentally different from this case:

if (isnan(foo)) {}

because the first example can never result in the if branch being
taken with -ffinite-math-only while the second example can. Assuming
that all values are the result of floating-point arithmetic is a
faulty assumption.

~Aaron