[llvm-dev] RFC: Generic IR reductions

Simon Pilgrim via llvm-dev llvm-dev at lists.llvm.org
Fri Feb 3 02:15:33 PST 2017

> On 2 Feb 2017, at 01:06, Amara Emerson <amara.emerson at gmail.com> wrote:
>> What stops us from doing so with intrinsics is just the knowledge, so
>> we trade complexity in the back-end to match long patterns for
>> complexity in optimisation passes to know about the new intrinsics.
>> The arguments were:
>> Pro intrinsic:
>> * Simpler and shorter IR
>> * Easier to read, easier for the vectorisers to generate
>> * Easier for the back-end to match
>> * Easier transition towards scalable vectors in the future
> Also want to re-iterate that reductions are useful for testing bits of
> predicate vectors, which can be applicable to other targets than SVE
> for things like early-exit vectorization. Simon mentioned this to me
> off-list. Simon, could you comment here if this proposal would work
> for your needs?

Yes - I’m hoping that we can both vectorise early-out ‘any_of’ predicate tests code and perform early-out breaks from already vectorised cases - nothing I’ve seen suggests this will get in the way. It’s mainly going to be a case of correct recognition in the LV, handling dereference’d arrays etc. and I don’t think these intrinsics will obfuscate these cases/attributes etc. 

Does SVE have an early-out ability or is it an all or nothing mechanism?


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