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

Serge Pavlov via cfe-dev cfe-dev at lists.llvm.org
Mon Sep 20 01:22:56 PDT 2021

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:
>> On Fri, Sep 17, 2021 at 10:53 AM Mehdi AMINI <joker.eph at gmail.com> wrote:
>>> On Thu, Sep 16, 2021 at 8:23 PM Serge Pavlov via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>> On Fri, Sep 17, 2021 at 3:11 AM Chris Tetreault <ctetreau at quicinc.com>
>>>> wrote:
>>>>> The difference there is that doing pointer arithmetic on null pointers
>>>>> doesn't *usually* work, unless you turn on -ffast-pointers.
>>>>> It seems to me that  most confusion related to -ffast-math is likely
>>>>> caused by people who are transitioning to using it. I have some codebase,
>>>>> and I turn on fast math, and then a few months down the road I notice a
>>>>> strangeness that I did not catch during the initial transition period. If
>>>>> you're writing new code with fast-math, you don't do things like try to use
>>>>> NaN as a sentinel value in a TU with fast math turned on. This is the sort
>>>>> of thing you catch when you try to transition an existing codebase. Forgive
>>>>> me for the uncharitable interpretation, but it's much easier to ask the
>>>>> compiler to change to accommodate your use case than it is to refactor your
>>>>> code.
>>>> It is a common way to explain problems with -ffinite-math-only by user
>>>> ignorance. However user misunderstandings and complaints may indicate a
>>>> flaw in compiler implementation, which I believe we have in this case.
>>>> Using NaN as sentinels is a natural way when you cannot spend extra
>>>> memory for keeping flags for each item, spend extra cycles to read that
>>>> flag and do not want to pollute cache. It does not depend on reading
>>>> documentation or writing the code from scratch. It is simply the best
>>>> solution for storing data. If performance of the data processing is
>>>> critical, -ffast-math is a good solution. This is a fairly legitimate use
>>>> case. The fact that the compiler does not allow it is a compiler drawback.
>>>>> To me, I think Mehdi had the best solution: The algorithm that is the
>>>>> bottleneck, and experiences the huge speedup using fast-math should be
>>>>> separated into its own source file. This source file, and only this source
>>>>> file should be compiled with fast-math. The outer driver loop should not be
>>>>> compiled with fast math. This solution is clean, (probably) easy, and
>>>>> doesn't require a change in the compiler.
>>>> It is a workaround, it works in some cases but does not in others. ML
>>>> kernel often is a single translation unit, there may be no such thing as
>>>> linker for that processor. At the same time it is computation intensive and
>>>> using fast-math in it may be very profitable.
>>> Switching mode in a single TU seems valuable, but could this be handled
>>> with pragmas or function attributes instead?
>> GCC allows it by using `#pragma GCC optimize()`, but clang does not
>> support it. No suitable function attribute exists for that.
> Right, I know that clang does not support it, but it could :)
> So since we're looking at what provides the best user-experience: isn't
> that it? Shouldn't we look into providing this level of granularity?
> (whether function-level or finer grain)

It could mitigate the problem if it were implemented. A user who needs to
handle NaNs in -ffinite-math-only compilation and writes the code from
scratch could use this facility to get things working. I also think such
pragma, implemented with enough degree of flexibility, could be useful
irrespective of this topic.

However, in general it does not solve the problem. The most important issue
which remains unaddressed is inconsistency of the implementation.

The handling of `isnan` in -ffinite-math-only by clang is not consistent
- It differs from what other compilers do. Namely MSVC and Intel compiler
do not throw away `isnan` in this mode: https://godbolt.org/z/qTaz47qhP.
- It depends on optimization options. With -O2 the check is removed but
with -O0 remains: https://godbolt.org/z/cjYePv7s7. Other options also can
affect the behavior, for example with `-ffp-model=strict` the check is
generated irrespective of the optimization mode (see the same link).
- It is inconsistent with libc implementations. If `isnan` is provided by
libc, it is a real check, but the compiler may drop it.
It would not be an issue if `isnan` removal were just an optimization. It
however changes semantics in the presence of NaNs, so such removal can
break user code.

In the typical use case a user puts a call to `isnan` to ensure no
operations on NaNs occur. The call can also be present in some header that
implements some functionality for the general case. It may work because
`isnan` is provided by libc. Later on when configuration changes or libc is
updated the code may be broken, because implementation of `isnan` changes,
as it happened after https://reviews.llvm.org/D69806.

If clang kept calls to `isnan`, it would be consistent with ICC and MSVC
and with all libc implementations. The behavior would be different from
gcc, but clang would be on the winning side, because the number of programs
that work with clang would be larger.

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.

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