[llvm-dev] [RFC] Optional parameter tuples

[Reid Kleckner via llvm-dev]

+1 for operand bundles, I think they are what you are looking for.

On Wed, Feb 12, 2020, 6:33 AM Johannes Doerfert via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> On 02/12, Simon Moll via llvm-dev wrote:
> > Hi,
> >
> > this is an RFC for optional, named parameter tuples for intrinsics. The
> proposed syntax is:
> >
> > %z = call @llvm.some.intrinsic(%a, %b) optional_tuple(%x, %y, %z)
> >
> > where from the perspective of the call site %x, %y and %z are simply
> additional parameters.
> >
> > Optional parameter tuples would be very useful for constrained fp
> intrinsics and vector predication. Some examples:
> >
> > ; Default fpenv fadd (isomorphic to the fadd instruction)
> > %z = call double @llvm.fadd(%a, %b)
> >
> >
> > ; Constrained fp add
> > %x = call double @llvm.fadd(%a, %b) fpenv(metadata !fpround.tonearest,
> metadata !fpexcept.strict)
> >
> >
> > ; Constrained fp add with vector predication (
> https://reviews.llvm.org/D57504)
> > %x = call <256 x double> @llvm.fadd(%a, %b) mask(<256 x i1> %mask),
> evl(i32 %evl), fpenv(metadata !fpround.tozero, metadata !fpexcept.strict)
> >
> > For constrained fp and VP, we only require a very restricted form of
> this. That is:
> >
> > - Tuples are only allowed on intrinsic function calls.
> >
> > - There is a builtin list of allowed tuple types (eg tablegen file).
> >
> > - Tuple types are opt in: Intrinsics define a list of permissible tuple
> types.
> >
> > - Tuple parameters have default values when a tuple is absent at a call
> site. Eg, for `fpenv` the default fp environment `fpenv(fpround.tonearest,
> fpexcept.ignore)` is implied in the first example above.
> >
> >
> > Btw, different to operand bundles, optional parameter tuples are much
> more restricted and do not imply side effects of any kind. We might want to
> re-use the OpBundle logic for this though.
>
> I feel this would be the right way forward, especially since your "tuple
> names" are basically operand bundle tags.
>
> You could express
>    %z = call @llvm.some.intrinsic(%a, %b) optional_tuple(%x, %y, %z)
> as
>    %z = call @llvm.some.intrinsic(%a, %b) ["optional_tuple"(%x, %y, %z)]
> or
>    %x = call <256 x double> @llvm.fadd(%a, %b) mask(<256 x i1> %mask),
> evl(i32 %evl), fpenv(metadata !fpround.tozero, metadata !fpexcept.strict)
> as
>    %x = call <256 x double> @llvm.fadd(%a, %b) ["mask"(<256 x i1> %mask),
> "evl"(i32 %evl), "fpenv"(metadata !fpround.tozero, metadata
> !fpexcept.strict)]
>
> Operand bundles tags can come with special semantics so we can teach
> parts to not "assume the worst" for such uses. I say this also because
> we are already heading that way for `llvm.assume`:
>   https://reviews.llvm.org/D74209
>
> Cheers,
>   Johannes
>
> > There are probably more use cases for this and i'd like to hear about
> them.
> >
> > Looking forward to your feedback
> >
> > - Simon
>
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> --
>
> Johannes Doerfert
> Researcher
>
> Argonne National Laboratory
> Lemont, IL 60439, USA
>
> jdoerfert at anl.gov
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