[llvm-dev] [RFC] Optional parameter tuples

[Neil Henning via llvm-dev]

Hey Simon,

It wasn't clear from your RFC: why can't you just have these optional
parameter tuples be normal function arguments to the intrinsics?

Cheers,
-Neil.

On Wed, Feb 12, 2020 at 9:28 AM Simon Moll via llvm-dev <
llvm-dev at lists.llvm.org> 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.
>
> There are probably more use cases for this and i'd like to hear about them.
>
> Looking forward to your feedback
>
> - Simon
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>


-- 
Neil Henning
Senior Software Engineer Compiler
unity.com
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