[cfe-dev] Intrinsic llvm::isnan

[Sanjay Patel via cfe-dev]

I'm confused about the definition of:
https://llvm.org/docs/LangRef.html#llvm-experimental-constrained-fcmp-and-llvm-experimental-constrained-fcmps-intrinsics

These intrinsics require an "exception behavior" argument. That argument
can take the value “fpexcept.ignore” which is defined as:
"optimization passes may assume that the exception status flags will not be
read and that floating-point exceptions will be masked"

i1 @llvm.experimental.constrained.fcmp.f64(double %x, double %x, metadata
!"uno", metadata !"fpexcept.ignore")

How is this call in LLVM different than the semantics of "isnan(x)" that is
required by IEEE-754 or the C standard?

On Mon, Aug 23, 2021 at 9:00 AM Serge Pavlov via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
> 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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