[llvm-dev] Invalid transformation in LibCallSimplifier::replacePowWithSqrt?

[Hubert Tong via llvm-dev]

On Mon, Sep 14, 2020 at 3:00 PM Sanjay Patel <spatel at rotateright.com> wrote:

> Sorry - I misread your example and the problem. I see now where
> LibCallSimplifier creates the select...but we are immediately erasing that
> select with the code from the godbolt example.
> Does the real motivating case have no uses of the pow() result value?
>
Using the result value (assign to global volatile) doesn't cause the sqrt
call to be avoided either.


>
> On Mon, Sep 14, 2020 at 1:03 PM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
>> Yes, I mean just bail out on the transform in
>> LibCallSimplifier::replacePowWithSqrt() -> getSqrtCall(). If we can't prove
>> the call behaves the same with errno, then give up.
>> I'm not sure where the select / branching happens, but I don't see that
>> happening in the initial transform (called from -instcombine)
>>
>> On Mon, Sep 14, 2020 at 12:58 PM Hubert Tong <
>> hubert.reinterpretcast at gmail.com> wrote:
>>
>>> On Mon, Sep 14, 2020 at 12:45 PM Sanjay Patel <spatel at rotateright.com>
>>> wrote:
>>>
>>>> Yes, that looks like a bug. The transform is ok in general for negative
>>>> numbers, but -Inf is a special-case for pow(), right?
>>>> If so, we probably need an extra check of the input with
>>>> "isKnownNeverInfinity()".
>>>>
>>> There is an extra check there already, but it uses "select" instead of
>>> branching. One question is if branching is okay to use instead. Or perhaps
>>> you mean the transform should not be done unless "isKnownNeverInfinity()"
>>> returns true.
>>>
>>>
>>>> If there are other errno divergences for edge case values, we may need
>>>> to check other conditions.
>>>>
>>>> On Sat, Sep 12, 2020 at 9:37 PM Hubert Tong via llvm-dev <
>>>> llvm-dev at lists.llvm.org> wrote:
>>>>
>>>>> The transformation in LibCallSimplifier::replacePowWithSqrt with
>>>>> respect to -Inf uses a select instruction, which based on the observed
>>>>> behaviour, incorporates the side effects of the unchosen branch. This means
>>>>> that (for pow) a call to sqrt(-Inf) is materialized. Such a call is
>>>>> specified as having a domain error (C17 subclause 7.12.7.5) since the
>>>>> operand is less than zero. Contrast this with pow(-Inf, 0.5), which
>>>>> is specified by C17 subclause F.10.4.4 as having a result of +Inf
>>>>> (indicating an exact result for the operation and, since IEEE Std 754-2008
>>>>> subclause 9.1.1 states that domain errors are to be indicated by a NaN
>>>>> result, a lack of a domain error).
>>>>>
>>>>> It is noted that the above statements were made notwithstanding the
>>>>> ERRORS section of pow() in POSIX.1-2017 XSH Chapter 3, which specifies a
>>>>> domain error except perhaps by deference to the C standard due to a
>>>>> conflict between the POSIX and the C wording.
>>>>>
>>>>> The transformation in question causes EDOM for  pow(-Inf, 0.5) even
>>>>> on platforms where the system library implementation of pow does not
>>>>> cause this situation to arise.
>>>>>
>>>>> A sample program that (on some platforms, such as Linux on x86-64)
>>>>> completes successfully with optimizations off, and aborts with LLVM's
>>>>> optimization follows; -fmath-errno does not help, and it is not
>>>>> expected to, because it is designed to retain setting errno to non-zero
>>>>> (not to prevent spuriously setting errno).
>>>>>
>>>>> #include <errno.h>
>>>>> volatile double inf = -__builtin_inf();
>>>>>
>>>>> double pow(double, double);
>>>>> void abort(void);
>>>>> int main(void) {
>>>>>   errno = 0;
>>>>>   pow(inf, 0.5);
>>>>>   if (errno != 0) abort();
>>>>> }
>>>>>
>>>>> Compiler Explorer link: https://godbolt.org/z/5Wr66M
>>>>>
>>>>> Is the transformation actually valid in some way? If not, I see no
>>>>> instances where the LibCallSimplier generates conditional branches.
>>>>> Retaining the transformation in some way without generating conditional
>>>>> branches would probably involve bailing out if infinities are still in play.
>>>>>
>>>>> _______________________________________________
>>>>> LLVM Developers mailing list
>>>>> llvm-dev at lists.llvm.org
>>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>>
>>>>
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