[llvm-dev] [RFC] Optional parameter tuples

[Simon Moll via llvm-dev]

Hi Neil,

we want to make the representation more concise.

One more thing i just realized is that we could use the tuple types to mask optimizations that do not expect them*.
Eg standard fp optimizations could claim that they handle `llvm.fadd` unless there are any parameter tuples on it.
Likewise, we can then implement a masked constrainedfp, SIMD fadd optimization pass that handles `llvm.fadd` with `fpenv` and/or `mask`.

*: (this comes on the heels of a second to-be-published RFC on generalizing the pattern match logic in LLVM...)

- Simon

On 2/12/20 11:40 AM, Neil Henning wrote:
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<mailto: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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