[cfe-dev] [llvm-dev] Do we need intrinsics for floating-point classification functions?

[Serge Pavlov via cfe-dev]

On Fri, Sep 3, 2021 at 12:58 AM Sanjay Patel <spatel at rotateright.com> wrote:

> Thank you for restarting this proposal. It's a bit more work, but let's
> make sure we have a good design, so we don't miss any details.
>
> What optimizations/analysis does this intrinsic enable **in IR** in
> strict-FP mode?
> If it is only constant folding, we don't need to add an intrinsic. We fold
> lots of math library calls with no corresponding LLVM intrinsic under
> llvm::ConstantFoldCall().
> We also do transforms based on operands/uses of library calls in
> SimplifyLibCalls.
>

Optimization is not the motivation for this intrinsic. If FP exceptions are
ignored, unordered comparison can be used to implement `isnan`. If not, we
need to make the test without affecting FP exceptions. Without this
intrinsic there is no way to represent such a test.


> So I'm really asking: why do we translate isnan() in clang into something
> else in the first place? The backend can lower that call into whatever
> suits the target without involving LLVM IR.
>

It must be a function, known to the backend without
declarations/definitions, that is an intrinsic.


>
> On Thu, Sep 2, 2021 at 8:33 AM Serge Pavlov via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi all,
>>
>> Some time ago a new intrinsic `llvm.isnan` was introduced, which was
>> intended to represent IEEE-754 operation `isNaN` as well as a family of C
>> library functions `isnan*`. Then a concern was raised (see
>> https://reviews.llvm.org/D104854) that this functionality should be
>> removed. Discussion in the subsequent RFC (
>> https://lists.llvm.org/pipermail/llvm-dev/2021-August/152257.html) came
>> to consensus that such intrinsic is necessary. Nevertheless the patches
>> related to the new intrinsic were reverted. I have to restart the
>> discussion in hope to convince the community that this intrinsic and other
>> classification functions are necessary.
>>
>> There are two main reasons why this intrinsic is necessary:
>> 1. It allows correct implementation of `isnan` if strict floating point
>> semantics is in effect,
>> 2. It allows preserving the check in -ffast-math compilation.
>>
>> To facilitate the discussion let's concentrate on the first problem.
>>
>> Previously the frontend intrinsic `__builtin_isnan` was converted into
>> `cmp uno` during IR generation in clang codegen. This solution is not
>> suitable if FP exceptions are not ignored, because compare instructions
>> raise exceptions if its argument is signaling NaN. Both IEEE-754 (5.7.2) an
>> C standard  (http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2596.pdf,
>> F.3p6) demand that this function does not raise floating point exceptions.
>> There was no target-independent IR construct that could represent `isnan`.
>>
>> This drawback was significant enough and some attempts to alleviate it
>> were undertaken. In https://reviews.llvm.org/D95948 `isnan` was
>> implemented using integer operations in strictfp functions. It however is
>> not suitable for targets where a more efficient way exists, like dedicated
>> instruction. Another solution was implemented in
>> https://reviews.llvm.org/D96568, where a hook
>> `clang::TargetCodeGenInfo::testFPKind` was introduced, which injects target
>> specific code into IR. Such a solution makes IR more target-dependent and
>> prevents some IR-level optimizations.
>>
>> To have a solution suitable for all cases, a new intrinsic function
>> `llvm.isnan` was introduced (https://reviews.llvm.org/D104854). It
>> protects the check from undesirable optimizations and preserves it till
>> selector, where it can be lowered in optimal for a particular target way.
>>
>> Other classification functions also need their own intrinsics. In
>> strictfp mode even a check for zero (`iszero`) cannot be made by comparing
>> a value against zero, - if the value is signaling NaN, FP exceptions would
>> be raised. James Y Knight in the previous discussion (
>> https://lists.llvm.org/pipermail/llvm-dev/2021-August/152282.html)
>> listed such "non-computational" functions, which should not signal if
>> provided with an sNAN argument.
>>
>> It looks like new intrinsic is the only consistent and in target-agnostic
>> way to implement these checks in all environments including the case when
>> FP exceptions are not ignored.
>>
>> Any feedback is welcome.
>>
>> Thanks,
>> --Serge
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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