[PATCH] D32616: [X86] Add more patterns for BZHI isel

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

This patch adds more patterns that a reasonable person might write that can be compiled to BZHI.

This adds support for

  (~0U >> (32 - b)) & a;

and

  a << (32 - b) >> (32 - b);

This was inspired by the code in APInt::clearUnusedBits.

This can pass an index of 32 to the bzhi instruction which a quick test of Haswell hardware shows will not mask any bits. Though the description text in the Intel manual says the "index is saturated to OperandSize-1". The pseudocode in the same manual indicates no bits will be zeroed for this case.

I think this is still missing cases where the subtract portion is an 8-bit operation.


https://reviews.llvm.org/D32616

Files:
  lib/Target/X86/X86InstrInfo.td
  test/CodeGen/X86/bmi.ll

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