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

[Serge Pavlov via cfe-dev]

Hi all,

If nobody argues, in a couple of days I will put back the `llvm.isnan`
implementation excluding the changes for fast-mode.

Thanks,
--Serge


On Fri, Sep 3, 2021 at 1:46 PM Serge Pavlov <sepavloff at gmail.com> wrote:

> Thank you for summarizing.
>
> I would prefer Option 2, a separate tool for separate tasks. It seems to
> me that a specialized function is easier to implement than a universal one.
> As implementation of llvm.isnan demonstrated, there may be various issues
> in implementing such functions for different targets. It is easier to
> provide optimized versions of small functions which are used more
> frequently than `fpclassify`. Besides, `fpclassify` may be used itself and
> a user may choose arbitrary constants, which complicates code generation.
> It seems that it is more acceptable to tolerate some inefficiency in
> `fpclassify` than in basic classification intrinsics. And yes, we should
> consider `signbit` with them.
>
> Another consideration in favor of dedicated intrinsic rather than ordinary
> function. Compiler may do some optimizations when it knows the semantics of
> the function. In https://reviews.llvm.org/D104854 this intrinsic was
> optimized out if its argument was  provided by an operation with 'nnan'
> flag. We can also think about an optimization that determines basic blocks
> guarded by `llvm.isnan` and assign flag 'nnan' in them. It could make code
> faster in many practical cases, and make use of notorious -ffast-math less
> attractive.
>
> Thanks,
> --Serge
>
>
> On Fri, Sep 3, 2021 at 12:22 PM James Y Knight <jyknight at google.com>
> wrote:
>
>> As you say, we don't strictly need a new intrinsic -- we can emit the
>> code to do the correct integer-based bit-checking in the frontend -- as was
>> done before. Yet, that is not ideal. The rationale to have an intrinsic
>> instead of the frontend generating integer-bit-manipulation seem good, IMO.
>>
>> So the question that arises: *which* new intrinsic(s) do we want to add?
>> I list 3 options below. I think Option 1 should be discarded from
>> consideration, and I'm not sure which of 2 or 3 is best. I'm leaning
>> towards 3 -- it seems like it may be simpler -- although I'm not certain.
>>
>> *Option 1*: we could theoretically address the problem with an
>> llvm.experimental.constrained.superquiet_fcmp, which would be the same as
>> fcmp, except it would not raise an fp exception even for an sNAN. This
>> would be a straightforward substitution for fcmp in Clang's existing
>> codegen, and it should work.
>>
>> However, I don't think this would be a good idea, because it's a poor
>> match for hardware. The "superquiet_fcmp" operation doesn't map to CPU
>> functionality in any ISA i'm aware of. Generally, the implementation would
>> have to be to filter out sNANs prior to using the hardware fp-compare
>> instruction. And how you you detect an sNAN without raising an fp
>> exceptoin? Fall down to integer ops. But by doing that, you've done work
>> almost equivalent to what you needed for the actual classification function
>> that you were trying to implement in the first place. Not real useful.
>>
>> *Option 2*: We could add a whole family of classification intrinsics. I
>> think that would be:
>>
>> llvm.isnan
>> llvm.issignaling
>> llvm.isinf
>> llvm.isfinite
>> llvm.isnormal
>> llvm.issubnormal
>> llvm.iszero
>> (Note, some of these are missing corresponding __builtin_is* in Clang at
>> the moment -- we have no __builtin_issignaling, __builtin_issubnormal, or
>> __builtin_iszero. Probably ought to.)
>>
>> We don't necessarily need an intrinsic for fpclassify if we have the
>> above intrinsics, since it can be built with llvm.isnan, llvm.isinf,
>> llvm.iszero, and llvm.isnormal.
>>
>> *Option 3*: Add only an fpclassify intrinsic.
>>
>> That is, something like:
>>    i32 llvm.fpclassify.i32.f32(i32 %if_snan, i32 %if_qnan, i32
>> %if_infinite, i32 %if_normal, i32 %if_subnormal, i32 %if_zero, float %value)
>> which classifies the given value, returning the value of the argument
>> corresponding to its categorization. We can say the %if_* args are required
>> to be constant integers, if we like, for simplicity of implementation.
>>
>> Thus, Clang would codegen __builtin_isnan/etc like:
>>   %isnan = call i1 llvm.fpclassify.i1.f32(i1 1, i1 1, i1 0, i1 0, i1 0,
>> i1 0, float %value)
>> And for fpclassify, we might generate something like:
>>   %ret = call i32 llvm.fpclassify.i32.f32(i32 0, i32 0, i32 1, i32 4, i32
>> 3, i32 2, float %value)
>>
>> On most architectures, we'd expand this intrinsic into appropriate
>> integer operations (skipping the parts of classification which are
>> irrelevant for the given constant arguments), since there's no
>> corresponding hardware instructions available for float classification.
>> Or, for non-strictfp functions, we could continue to expand into an
>> fcmp-based set of tests...although looking at the asm we currently
>> generate, the integer versions may well be faster, other than isnan.
>>
>> On SystemZ/s390x, this intrinsic would translate almost directly into the
>> "test data class" instruction -- so long as the %if_* arguments are all
>> 0/1. That's kinda nice. ("Test data class" takes a fp value and a bitmask,
>> and returns true if a bit is set in the position corresponding to the
>> classification of the fp value.)
>>
>>
>> *Separately*, we have the signbit operation. I think that's the only
>> other operation that needs to be addressed related to this RFC. Currently,
>> Clang always generates integer bit-ops for __builtin_signbit in the
>> frontend. This is arguably OK as is. Yet, completing the set of IR fp
>> classification intrinsics seems like it'd be a good idea. So, we could also
>> (along with any of the above options) add:
>>   i1 llvm.signbit.f32(float %value)
>>
>>
>> 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.
>>>
>>
>>
>>
>
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