[PATCH] D17583: [InstCombine] transform bitcasted bitwise logic ops with constants (PR26702)

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: majnemer, hfinkel, joker.eph.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

Given that we're not actually deleting any instructions in the included regression tests, I think we would call this a canonicalization step.

The motivation comes from the example in PR26702:
https://llvm.org/bugs/show_bug.cgi?id=26702

If we hoist the bitwise logic ahead of the bitcast, the previously unoptimizable example of:
  define <4 x i32> @is_negative(<4 x i32> %x) {
    %lobit = ashr <4 x i32> %x, <i32 31, i32 31, i32 31, i32 31>
    %not = xor <4 x i32> %lobit, <i32 -1, i32 -1, i32 -1, i32 -1>
    %bc = bitcast <4 x i32> %not to <2 x i64>
    %notnot = xor <2 x i64> %bc, <i64 -1, i64 -1>
    %bc2 = bitcast <2 x i64> %notnot to <4 x i32>
    ret <4 x i32> %bc2
  }

Simplifies to the expected:
  define <4 x i32> @is_negative(<4 x i32> %x) {
    %lobit = ashr <4 x i32> %x, <i32 31, i32 31, i32 31, i32 31>
    ret <4 x i32> %lobit
  }



http://reviews.llvm.org/D17583

Files:
  lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  test/Transforms/InstCombine/bitcast-bigendian.ll
  test/Transforms/InstCombine/bitcast.ll

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