[PATCH] D30086: Add generic IR vector reductions

[Michael Kuperstein via Phabricator via llvm-commits]

mkuper added a comment.

In https://reviews.llvm.org/D30086#681447, @aemerson wrote:

> In https://reviews.llvm.org/D30086#680635, @mkuper wrote:
>
> > I can see some advantages to forming the intrinsic conditionally, but I'm not sure they're worth the cost of not having a canonical representation.
>
>
> This patch was intended to be a transitional stage, my expectation was that over time targets would decide to move to this representation by opting in and handling them. The reason for the transitional step was that I didn't want to force this form on all targets (even if it lowers to the same code by a lowering step) as it changes the IR during the mid-end stage. At the moment, the only target which requires this intrinsic form without any alternative is SVE. If you think it's ok to change the reduction form for all targets until a late lowering then I'm happy to implement that.


What I'd really like to avoid is having a nominally "target-independent" intrinsic, which can, in practice, only be handled by one or two targets.

There are three ways I can see this going:
a) If we know this is not the right generic representation, this should not go in as a target-independent intrinsic. I haven't seen anyone raise any "deal-breaker" issues so far, though.
b) If we think this is the right generic representation, but we're not sure, we can introduce these intrinsics as llvm.experimental. For an experimental intrinsic, I think it makes more sense to have targets opt-in like this patch does (but we probably still want default lowering for targets that don't opt-in).
c) If we are sure about this, then we should do just it for all targets, and have late lowering by default. I'm not really the right person to determine that, though. I guess that's a question for the backend owners.


Repository:
  rL LLVM

https://reviews.llvm.org/D30086