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[SVE] Add support for scalable vectors with vectorize.scalable.enable loop…

Authored by david-arm on Oct 7 2020, 1:21 AM.

Description

[SVE] Add support for scalable vectors with vectorize.scalable.enable loop attribute

In this patch I have added support for a new loop hint called
vectorize.scalable.enable that says whether we should enable scalable
vectorization or not. If a user wants to instruct the compiler to
vectorize a loop with scalable vectors they can now do this as
follows:

br i1 %exitcond, label %for.end, label %for.body, !llvm.loop !2
...
!2 = !{!2, !3, !4}
!3 = !{!"llvm.loop.vectorize.width", i32 8}
!4 = !{!"llvm.loop.vectorize.scalable.enable", i1 true}

Setting the hint to false simply reverts the behaviour back to the
default, using fixed width vectors.

Differential Revision: https://reviews.llvm.org/D88962