[PATCH] D34141: [X86] Recognize constant arrays with special values and replace loads from it with subtract and shift instructions, which then may be replaced by BZHI machine instruction.

[Ayman Musa via Phabricator via llvm-commits]

aymanmus created this revision.

Recognize constant arrays with the following values:

  0x0, 0x1, 0x3, 0x7, 0xF, 0x1F, .... , 2^(size - 1) -1

where //size// is the size of the array.
the result of a load with index //idx// from this array is equivalent to the result of the following:

  (0xFFFFFFFF >> (sub 32, idx))             (assuming the array of type 32-bit integer).

This transformations also prepares the way of a pattern match for bzhi instruction when:

1. Target has BMI2 extension.
2. The result of the load is and operand of an //and// instruction.

See last 2 test cases in the .ll file for better understanding.


https://reviews.llvm.org/D34141

Files:
  include/llvm/Target/TargetLowering.h
  lib/CodeGen/CodeGenPrepare.cpp
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/replace-load-with-sub-shr.ll

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