[llvm-dev] [cfe-dev] Intrinsic llvm::isnan

[Roman Lebedev via llvm-dev]

Thank you for posting the RFC!

I do not believe we should conflate StrictFP support, and
`-ffast-math` handling, these are two separate/separatable concerns.

As for the latter, right now i'm not convinced that we should
second-guess/override explicit user request.
This is inconsistent, and does not match how at least the GCC deals with it.
I think changing the status-quo (before said patch) should be a separate RFC,
and that change should be undone until after that RFC is accepted.

As for the latter, the main point of confusion is,
why is `@llvm.isnan` still used in non-StrictFP code?
The argument that we need `@llvm.isnan` because we *might* transition
in and out of StrictFP section does not seem to hold for me, because
https://llvm.org/docs/LangRef.html#constrainedfp says:

> If any FP operation in a function is constrained then they all must be constrained. This is required for correct LLVM IR.

So presumably when codegen'ing a function, we already know that we
will use StrictFP ops, and that should be the knob to use `@llvm.isnan`,
i think.


Roman




On Mon, Aug 23, 2021 at 1:57 PM Serge Pavlov via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> Some time ago a new intrinsic `llvm.isnan` was introduced, which is intended to represent IEEE-754 operation `isNaN` as well as a family of C library functions `isnan*`. Recently during post-commit review concern was raised (see  https://reviews.llvm.org/D104854) that this functionality must have had RFC to make sure there is consensus on semantics.
>
> Previously the frontend intrinsic `__builtin_isnan` was converted into `cmp uno` during IR generation in clang codegen. There are two main reasons why this solution is not satisfactory.
>
> 1.  Strict floating-point semantics.
>
> If FP exceptions are not ignored, `cmp uno` must be replaced with its constrained counterpart, namely `llvm.experimental.constrained.fcmp` or `llvm.experimental.constrained.fcmps`. None of them is compatible with the semantics of `isnan`. 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. Both the constrained compare intrinsics raise an exception if either operand is a SNAN (https://llvm.org/docs/LangRef.html#id1131). So there was no target-independent IR construct that could express `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.
>
> 2. Compilation with -ffast-math
>
> The option '-ffast-math' is often used for performance critical code, as it can produce faster code. In this case the user must ensure that NaNs are not used as operand values. `isnan` is just proposed for such checks, but it was unusable when `isnan` was represented by compare instruction, because the latter may be optimized out. One of use cases is data in memory, which is processed by a function compiled with `-ffast-math`. Some items in the data are NaNs to denote absence of values.
>
> This point requires some remarks about using NaNs when a function is compiled with `-ffast-math`. GCC manual does not specify how this option works, it only states about `-ffinite-math-only` (https://gcc.gnu.org/onlinedocs/gcc-11.2.0/gcc/Optimize-Options.html#Optimize-Options):
>
> `Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.`
>
> `isnan` does not do any arithmetic, only check, so this statement apparently does not apply to it. There is a GCC bug report https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949, where investigation conforms that std::isnan() and std::fpclassify() should works with NaNs as specified even in -ffast-math mode.
>
> Extending NaN restrictions in -ffast-math mode to functions like `isnan` does not make code faster, but is a source of broken user expectations. If a user writes `isnan` they usually expect an actual check. Silently removing the check is a stronger action than assuming that float value contains only real numbers.
>
> Intrinsic `llvm.isnan` solves these problems. It
> - represents the check throughout the IR pipeline and saves it from undesired optimizations,
> - is lowered in selector, which can choose the most suitable implementation for particular target,
> - helps keeping IR target-independent,
> - facilitates program analysis as the operation is presented explicitly and is not hidden behind general nodes.
>
> Note that `llvm.isnan` is optimized out if its argument is an operation with `nnan` flag, this behavior agrees with the definition of this flag in LLVM documentation.
>
> Any feedback is welcome.
>
> Thanks,
> --Serge
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