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

Juneyoung Lee via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 23 08:35:24 PDT 2021 

Hi Serge,

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

Supporting this transformation is slightly complex because a value can be
partially undef:

%r = fadd undef, %x ; let's assume that this is poison
%r = fadd (or undef, 1), %x ; what about this?
%r = fadd (or undef, 0x7F..FF), %x ; this has a single undef bit only;
maybe it isn't undef enough to yield poison.

So, relying on 'fadd poison %x -> poison' and making poison appear as
frequently as possible might be a cleaner option (in my opinion).


About moving away from undef:
Let me share how things are going, since people might be interested in it.
There are three sources of undef currently, and for each of them some kind
of progress has been made (thanks to reviewers and people):


1. Undef is being used as a don't-care value.
A creation of a vector value is done via a sequence of insertelement,
e.g., insertelement(insertelement undef, x, 1), y, 2.
As shown in this expression, undef is used as a don't-care value.
There are a few patches landed to make instructions to use poison instead.
For example, IRBuilder::CreateShuffleVector is now using poison for its
second vector operand.
However, there are so many places where UndefValue is being used, so they
aren't fully updated. :(
Updating transformations to use poison for
insertelement/insertvalue/phi/etc's don't-care value will facilitate
further optimizations.
One successful case I observed was InstCombine's unit test removing
unreachable instructions after switching to poison.

2. Undef value is used in the semantics of shufflevector's undef mask.
Shufflevector is currently defined to yield undef if the mask is undef.
This is because optimizations want to regard shufflevector of a specific
form to be equivalent to 'insertelement undef, ...'.
If don't-care values are fully updated to be poison (item 1), the semantics
of shufflevector can be finally updated to return poison.
There is another blocker though; X86-64's mm*_undefined* intrinsics are
supposed to return an uninitialized vector which is not *undefined*; unlike
undef, each read should return a consistent value.
Using shufflevector with undef mask to encode mm*_undefined* was already
wrong, and making shufflevector return poison will make it worse.

3. Undef value is used to represent the value of the uninitialized memory.
Poison can be used instead, but (in the case of C/C++) two cases should be
treated carefully:
(1) Translating bit fields into IR: since there is no bitwise load and
store in IR, poison bits can contaminate the whole loaded value.
(2) A variable whose address is escaped: I heard from a few people studying
C standard that an uninitialized variable whose address is escaped contains
an unspecified value, which is more defined than both undef and poison IIUC.
Precisely encoding this case will come at a cost.


Fully addressing these requires correctly understanding a number of
transformations/analyses in LLVM; it is pretty scary to fix them as well :/

Best,
Juneyoung

On Fri, Jul 23, 2021 at 7:24 PM Serge Pavlov via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> 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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-- 

Juneyoung Lee
Software Foundation Lab, Seoul National University
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