[PATCH] D73929: [AArch64][GlobalISel] Fold G_XOR into TB(N)Z bit calculation

[Jessica Paquette via Phabricator via llvm-commits]

paquette created this revision.
paquette added a reviewer: aemerson.
Herald added subscribers: Petar.Avramovic, hiraditya, kristof.beyls, rovka.
Herald added a project: LLVM.

This ports the existing case for G_XOR from `getTestBitOperand` in AArch64ISelLowering into GlobalISel.

The idea is to flip between TBZ and TBNZ while walking through G_XORs.

Let's say we have

  tbz (xor x, c), b

Let's say the `b`-th bit in `c` is 1. Then

- If the `b`-th bit in `x` is 1, the `b`-th bit in `(xor x, c)` is 0.
- If the `b`-th bit in `x` is 0, then the `b`-th bit in `(xor x, c)` is 1.

So, then

  tbz (xor x, c), b == tbnz x, b

Let's say the `b`-th bit in `c` is 0. Then

- If the `b`-th bit in `x` is 1, the `b`-th bit in `(xor x, c)` is 1.
- If the `b`-th bit in `x` is 0, then the `b`-th bit in `(xor x, c)` is 0.

So, then

  tbz (xor x, c), b == tbz x, b


https://reviews.llvm.org/D73929

Files:
  llvm/lib/Target/AArch64/AArch64InstructionSelector.cpp
  llvm/test/CodeGen/AArch64/GlobalISel/opt-fold-xor-tbz-tbnz.mir

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