[PATCH] D82660: [RISCV] Optimize multiplication by specific immediates

[Sam Elliott via Phabricator via llvm-commits]

lenary added a comment.

Thanks for the patch!

This optimisation is done by DAGCombine if you instead implement `decomposeMulByConstant` in `RISCVTargetLowering`. Read the comment on the `TargetLoweringBase` class to understand how to use it. This is preferrable, because we don't want to maintain a target-specific copy of this optimisation if we can avoid it.

It would be sensible to base your implementation on the one in the x86 backend: `X86TargetLowering::decomposeMulByConstant`, which deals with some phase ordering issues around legalisation.


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https://reviews.llvm.org/D82660