[PATCH] D113371: [X86] combinePMULH - recognise 'cheap' trunctions via PACKS/PACKUS as well as SEXT/ZEXT

[Simon Pilgrim via Phabricator via llvm-commits]

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combinePMULH currently only truncates vXi32/vXi64 multiplies to PMULHW/PMULUW if the source operands are SEXT/ZEXT instructions for a 'free' truncation.

But we can generalize this to any source operand with sufficient leading sign/zero bits that would allow PACKS/PACKUS to be used as a 'cheap' truncation.

This helps us avoid the wider multiplies, in exchange for truncation on both source operands instead of the result.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D113371

Files:
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/pmulh.ll

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