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

[David Edelsohn via cfe-dev]

On Tue, Sep 14, 2021 at 11:15 AM Arthur O'Dwyer via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> On Tue, Sep 14, 2021 at 9:22 AM Serge Pavlov via cfe-dev <cfe-dev at lists.llvm.org> wrote:
>>
>> On Tue, Sep 14, 2021 at 8:21 PM Krzysztof Parzyszek <kparzysz at quicinc.com> wrote:
>>>
>>> If `has_nan` returns "true", it means that the explanation "there are no NaNs" does not work anymore and something more complex is needed to explain the effect of the option. In this case it is difficult to say that this approach is "intuitively clear".
>>>
>>>
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>>> If your program has “x = *p”, it means that at this point p is never a null pointer.  Does this imply that the type of p can no longer represent a null pointer?
>>
>>
>> Good example! If you use integer division `r = a / b`, you promise that `b` is not zero. It however does not mean  that preceding check `b == 0` may be optimized to `false`.
>
>
> In C and C++, it actually does mean that, although of the compilers I just tested on Godbolt, only MSVC seems to take advantage of that permission.
> https://godbolt.org/z/11ss5T7e8
>
> The question of whether it is acceptable to treat as equivalent the statements "p is known to be dereferenced in all successors of B" and "p is known to be non-null in B," was discussed extensively about 20 years ago, and then again 12 years ago when it bit someone in the Linux kernel:
> https://www.gnu.org/software/gcc/news/null.html
> https://lwn.net/Articles/342330/
> https://lwn.net/Articles/342420/
> https://qinsb.blogspot.com/2018/03/ub-will-delete-your-null-checks.html
>
>>> On Mon, Sep 13, 2021 at 10:28 PM Arthur O'Dwyer <arthur.j.odwyer at gmail.com> wrote:
>>>
>>> Btw, I don't think this thread has paid enough attention to Richard Smith's suggestion:
>>>
>>> I can only subscribe to James Y Knight's opinion. Indeed, it can be a good criterion of which operations should work in finite-math-only mode and which can not work. The only thing which I worry about is the possibility of checking the operation result for infinity (and nan for symmetry). But the suggested criterion is formulated in terms of arguments, not results, so it must allow such checks.
>
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> What is the opinion to which you subscribe?
>
> Anyway, Richard's "quiet is signaling and signals are unspecified values" is really the only way out of the difficulty, as far as compiler people are concerned. You two (Serge and Krzysztof) can keep talking past each other at the application level, but the compiler people are going to have to do something in the code eventually, and that something is going to have to be expressed in terms similar to what Richard and I have been saying, because these are the terms that the compiler understands.

Hopefully the clarified semantics can be coordinated between and
implemented in a consistent manner in both LLVM and GCC.

Thanks, David