[llvm-dev] [FPEnv] undef and constrained intrinsics?

Fri Jul 23 03:24:15 PDT 2021

Thank you for the reference. I saw an even older discussion on this topic
in the IRC channel. It looks like the problem of understanding `undef` has
been persisting since long ago. Probably it is because `undef` is "one of
the set" value, but the set itself is not specified. For floating point
values it generally includes all possible values, but for example if
`-fffast-math` is in action, NaNs are not in this set.

Another source of problems is replacing `undef` with concrete value. It
turns "one of the set" into one value and this contraction cannot be
equally good for all cases. For example:

  %A = select undef, %X, %Y
  %B = select undef, %X, 42
  %C = icmp eq %A, %B

Contraction of `select` instructions to the first operands, as recommended
in LLVM Language Reference Manual would make the compiler deduce that %C is
true, which is not correct in general case.

The concept of poison seems more clear and consistent. I wonder if we could
make transformations like:

%r = fadd undef, %x
-->
poison

and similar for constrained intrinsics. Using `poison` is consistent with
using `undef` for values on which the result does not depend. When poison
needs representation in machine code, it could be lowered to NaN, which
behaves similarly in runtime. The same solution is already made for
shufflevector. Does anything prevents from such transformation?

Thanks,
--Serge

On Thu, Jul 22, 2021 at 7:29 PM Sanjay Patel <spatel at rotateright.com> wrote:

> Unfortunately, it's not as easy as "any undef in --> undef out". That's a
> big reason for moving away from undef in IR.
>
> If you read this page bottom-up (there must be a better link somewhere?)
> and then read the follow-ups in the thread, you'll see how we arrived at
> the current rules for the standard FP ops:
> https://lists.llvm.org/pipermail/llvm-dev/2018-March/121481.html
>
> On Thu, Jul 22, 2021 at 6:34 AM Serge Pavlov <sepavloff at gmail.com> wrote:
>
>> The concept of undefined value has always been obscure and caused many
>> questions. I'd like to share my opinion, however I am not sure if I
>> understand this concept correctly.
>>
>> LLVM documentation (https://llvm.org/docs/LangRef.html#undefined-values)
>> describes undefined values:
>> "Undefined values are useful because they indicate to the compiler that
>> the program is well defined no matter what value is used". So these are
>> values on which the result of program execution does not depend. This is
>> why an undefined value may be replaced by an arbitrary value of proper type
>> and range. The choice of the replacement value is dictated mainly by
>> convenience. If however the produced result depends on this choice, it
>> means the value of `undef` affects results, so the initial supposition is
>> broken and we have undefined behavior.
>>
>> I agree with Sanjay that constrained intrinsics should behave in the same
>> way as regular FP operations with respect to `undef`. Control modes (like
>> rounding mode) influence result value, but we know that particular value of
>> `undef` is not important. FP exceptions are a bit more complex. If the
>> value of `undef` may be arbitrary, it is not possible to guarantee that FP
>> exceptions would be the same for all possible values. So we can assume that
>> `undef` operands do not affect FP exceptions. Either such operation is
>> eliminated, because its value is not used, or the operation itself does not
>> use the `undef` argument.
>>
>> If any of standard IR FP operations has undef argument, the result may be
>> either `undef` or any FP value. It is convenient to use NaN in such cases.
>> It does not make the program more correct but it can help to detect
>> undefined behavior in some FP environments. However `undef` result seems
>> better choice than NaN, because in this case the user of `undef` value may
>> choose a convenient representation for `undef`.
>>
>> I do not see any reason to distinguish between the cases "all operands
>> are undefs" and "only one operand is undef". In both cases we get a value
>> that is not used in the correct program.
>>
>> So I would propose transformations:
>>
>> %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>> float undef, metadata !"round.dynamic", metadata !"fpexcept.strict")
>>   -->
>>   %r = undef
>>
>> And
>>
>> %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>> float %x, metadata !"round.dynamic", metadata !"fpexcept.strict")
>>   -->
>>   %r = undef
>>
>> What do you think about it?
>>
>> Thanks,
>> --Serge
>>
>>
>> On Wed, Jul 21, 2021 at 8:15 PM Sanjay Patel <spatel at rotateright.com>
>> wrote:
>>
>>> Can we use the regular FP instructions (fadd, fmul, etc.) as a model?
>>>
>>> If both operands to any of the binops are undef, then the result is
>>> undef. So for the corresponding constrained intrinsic, if both operands are
>>> undef, the result is undef and the exception state is also undef:
>>>
>>>   %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>>> float undef, metadata !"round.dynamic", metadata !"fpexcept.strict")
>>>   -->
>>>   %r = undef
>>>
>>>   %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>>> float undef, metadata !"round.dynamic", metadata !"fpexcept.maytrap")
>>>   -->
>>>   %r = undef
>>>
>>>
>>> If one operand is undef and the other is regular value, assume that the
>>> undef value takes on some encoding of SNaN:
>>>
>>>   %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>>> float %x, metadata !"round.dynamic", metadata !"fpexcept.strict")
>>>   -->
>>>   %r = call float @llvm.experimental.constrained.fadd.f32(float SNaN,
>>> float %x, metadata !"round.dynamic", metadata !"fpexcept.strict") ; raise
>>> invalid op exception
>>>   (%r could be folded to QNaN here, but we can't get rid of the call, so
>>> don't bother?)
>>>
>>>   %r = call float @llvm.experimental.constrained.fadd.f32(float undef,
>>> float %x, metadata !"round.dynamic", metadata !"fpexcept.maytrap")
>>>   -->
>>>   %r = QNaN ; exception state does not have to be preserved
>>>
>>> Does that match the proposed behavior in
>>> https://reviews.llvm.org/D102673 (cc @sepavloff)?
>>>
>>> We could go further (potentially reduce to poison) if we have
>>> fast-math-flags on the calls -- just as we partially do with the regular
>>> instructions -- but it probably doesn't matter much to real code.
>>>
>>>
>>> On Fri, Jul 9, 2021 at 12:06 PM Kevin Neal via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> How should the constrained FP intrinsics behave when called with an
>>>> operand that is “undef” and the FP environment is _*not*_ the default
>>>> environment? I’m specifically working in the middle end passes if it
>>>> matters. Let me start with the assumption that the rounding mode is not
>>>> relevant. That still leaves the exception handling as a factor:
>>>>
>>>> With “fpexcept.maytrap” we are allowed to drop instructions that could
>>>> or would cause a trap at run-time. Does this imply we can fold the entire
>>>> instruction to a new undef?
>>>>
>>>> With “fpexcept.strict” we are _*not*_ allowed to lose or reorder
>>>> traps. So how does that affect undef? What happens in the backend? Perhaps
>>>> the middle end should leave the instruction with the undef and let the
>>>> backend do something reasonable?
>>>>
>>>> The “maytrap” case is the one I’m most interested in. An earlier
>>>> version of D103169 would fold away undef constrained intrinsics in the
>>>> maytrap case. This was removed so it could be handled without affecting the
>>>> rest of the patch I believe.
>>>>
>>>> Opinions?
>>>> --
>>>> Kevin P. Neal
>>>> SAS/C and SAS/C++ Compiler
>>>> Compute Services
>>>> SAS Institute, Inc.
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
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