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

[Sanjay Patel via cfe-dev]

Ok, does this edit to the LangRef make sense for the definition of "ignore":
"optimization passes may assume that the exception status flags will not be
read and that floating-point exceptions **will** be masked" -->
"optimization passes may assume that the exception status flags will not be
read and that floating-point exceptions **may** be masked"

It's still not clear to me if there's a benefit from having an intrinsic
vs. one more exception mode ("none" or "off").
That answer might depend on how many more of these intrinsics we'll need?
<cmath> has these:
isfinite()
isinf()
isnormal()
signbit()

others?

On Mon, Aug 23, 2021 at 2:25 PM Kaylor, Andrew <andrew.kaylor at intel.com>
wrote:

> > Why would a target be allowed to lower the constrained fcmp intrinsic
> with "ignore" to an operation that might raise an exception?
>
>
>
> Because the “fpexcept.ignore” argument means that exceptions are being
> ignored. That is, it doesn’t matter whether exceptions are raised or not.
> It’s a promise that exceptions are not unmasked or being explicitly checked
> in this part of the code.
>
>
>
> The “fpexcept.ignore” argument isn’t used for the fully strict mode. It is
> used for two cases:
>
>
>
> 1) To get back to the “default” state (in combination with
> “fpround.tonearest”) when a non-constrained operation is inlined into a
> function that has constrained operations.
>
> 2) To enable dynamic rounding (-frounding-math) without requiring strict
> exception semantics.
>
>
>
>
>
> Regarding NaN comparison support in the presence of fast-math flags, I
> think this is an excellent reason to have the intrinsic. I needed this for
> a (non-C/C++) downstream compiler before the llvm.isnan intrinsic was
> introduced and found that the only ways to achieve it were either to
> introduce an intrinsic for the comparison or to remove the ‘nnan’ flag
> everywhere.
>
>
>
> -Andy
>
>
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Sanjay
> Patel via llvm-dev
> *Sent:* Monday, August 23, 2021 10:56 AM
> *To:* Wang, Pengfei <Pengfei.Wang at intel.com>
> *Cc:* llvm-dev at lists.llvm.org; Clang Dev <cfe-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [cfe-dev] Intrinsic llvm::isnan
>
>
>
> Why would a target be allowed to lower the constrained fcmp intrinsic with
> "ignore" to an operation that might raise an exception?
>
> If that scenario does not exist, then make the definition of fcmp "ignore"
> stronger by saying that it *requires* a lowering that does not raise an
> exception.
>
> If refining the definition of "ignore" in this case is too hard to
> reconcile with other FP ops, how about adding another exception mode
> ("none") to specify that exceptions are guaranteed not to be raised?
>
>
>
> On Mon, Aug 23, 2021 at 11:04 AM Wang, Pengfei <pengfei.wang at intel.com>
> wrote:
>
> AFAIU, the fpexcept metadata is just a hint for optimizations, which have
> to respect the sequence of FP instructions. “ignore” is not a command to
> ask backend to mask the exception. It just tells optimizations the
> exceptions are not concerned by user, so that the FP instructions can be
> scheduled.
>
> I think an non exception intrinsic is needed, because it is a commend to
> request backend not to emit instructions that may raise exception.
>
>
>
> Thanks
>
> Pengfei
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of *Sanjay
> Patel via llvm-dev
> *Sent:* Monday, August 23, 2021 10:13 PM
> *To:* Serge Pavlov <sepavloff at gmail.com>
> *Cc:* LLVM Developers <llvm-dev at lists.llvm.org>; Clang Dev <
> cfe-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] [cfe-dev] Intrinsic llvm::isnan
>
>
>
> If the FP state is not made invisible/irrelevant by "fpexcept.ignore",
> then why is that argument on this intrinsic call at all?
>
> Ie, why is that argument not used by the backend to decide to lower to a
> CMP instruction or special isnan instruction or library call?
>
>
>
> An example would be helpful - in what case would these two lower
> differently given that SNAN always raises a visible exception?
>
> call i1 @llvm.experimental.constrained.fcmp.f64(double %x, double %x,
> metadata !"uno", metadata !"fpexcept.ignore") ; "floating-point exceptions
> will be masked"
>
> call i1 @llvm.experimental.constrained.fcmp.f64(double %x, double %x,
> metadata !"uno", metadata !"fpexcept.strict")  ; "this mode can also be
> used with code that unmasks FP exceptions"
>
>
>
>
>
> On Mon, Aug 23, 2021 at 9:50 AM Serge Pavlov <sepavloff at gmail.com> wrote:
>
> When codegen lowers this call, it does not know if this is a regular
> compare and it may create CMP instruction, as for default environment, or
> this is 'isnan' for which it should generate different code, which does not
> influence FP state.
>
>
>
> On most architectures compare instructions change FP state, in default
> environment it is just ignored, but actually hardware registers can be
> modified, For 'isnan' instructions must actually leave FP state intact.
>
>
> Thanks,
> --Serge
>
>
>
>
>
> On Mon, Aug 23, 2021 at 8:43 PM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
> You're saying that the function definition text overrides the argument
> definition text. Why are we choosing that interpretation rather than the
> inverse (and documenting it one way or the other)?
>
>
>
> On Mon, Aug 23, 2021 at 9:38 AM Serge Pavlov <sepavloff at gmail.com> wrote:
>
> > How is this call in LLVM different than the semantics of "isnan(x)" that
> is required by IEEE-754 or the C standard?
>
>
>
> If either of the arguments of `llvm.experimental.constrained.fcmp` is
> signaling NaN, this function should raise an 'Invalid' exception. 'isnan'
> never raises exceptions.
>
>
> Thanks,
> --Serge
>
>
>
>
>
> On Mon, Aug 23, 2021 at 8:10 PM Sanjay Patel <spatel at rotateright.com>
> wrote:
>
> 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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