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

[Serge Pavlov via llvm-dev]

On Mon, Aug 23, 2021 at 6:12 PM Roman Lebedev <lebedev.ri at gmail.com> wrote:

> 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.
>

You are right, they are separate, but they originate from the
implementation of the same function and can be solved with the same
solution.


>
> 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.
>
>
Actually we have two explicit user requests, a call of 'isnan' and an
option '-ffast-math'. IMHO they do not contradict each other as 'isnan' is
not an arithmetic operation. There is a discussion in
https://reviews.llvm.org/D18513#387418, which also expresses the opinion
that limitations imposed by '-ffast-math' should be applied only to 'math'
functions but not to 'tests'. As for GCC behavior, they agree that this
behavior is a bag: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84949.
Intel and Microsoft compilers do not replace 'isnan' with assumed value.


> As for the latter, the main point of confusion is,
> why is `@llvm.isnan` still used in non-StrictFP code?
>

We have to introduce an intrinsic to represent `isnan` in strictfp
environment. It is natural to use it for the default environment as
well. Besides, a target may have a more efficient way to represent `isnan`
than unordered comparison.

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.
>

There was no such intention. The primary motivation was strict fp
exceptions.


> 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
> > _______________________________________________
> > cfe-dev mailing list
> > cfe-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
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