[all-commits] [llvm/llvm-project] 49c356: [AArch64][GlobalISel] Swap compare operands when i...

[Jessica Paquette via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 49c3565b9ba738d0305ab97c2771fa37f2023726
      https://github.com/llvm/llvm-project/commit/49c3565b9ba738d0305ab97c2771fa37f2023726
  Author: Jessica Paquette <jpaquette at apple.com>
  Date:   2021-04-09 (Fri, 09 Apr 2021)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64Combine.td
    M llvm/lib/Target/AArch64/GISel/AArch64GlobalISelUtils.cpp
    M llvm/lib/Target/AArch64/GISel/AArch64GlobalISelUtils.h
    M llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp
    M llvm/lib/Target/AArch64/GISel/AArch64PostLegalizerLowering.cpp
    A llvm/test/CodeGen/AArch64/GlobalISel/postlegalizer-lowering-swap-compare-operands.mir

  Log Message:
  -----------
  [AArch64][GlobalISel] Swap compare operands when it may be profitable

This adds support for swapping comparison operands when it may introduce new
folding opportunities.

This is roughly the same as the code added to AArch64ISelLowering in
162435e7b5e026b9f988c730bb6527683f6aa853.

For an example of a testcase which exercises this, see
llvm/test/CodeGen/AArch64/swap-compare-operands.ll

(Godbolt for that testcase: https://godbolt.org/z/43WEMb)

The idea behind this is that sometimes, we may be able to fold away, say, a
shift or extend in a compare by swapping its operands.

e.g. in the case of this compare:

```
lsl x8, x0, #1
cmp x8, x1
cset w0, lt
```

The following is equivalent:

```
cmp x1, x0, lsl #1
cset w0, gt
```

Most of the code here is just a reimplementation of what already exists in
AArch64ISelLowering.

(See `getCmpOperandFoldingProfit` and `getAArch64Cmp` for the equivalent code.)

Note that most of the AND code in the testcase doesn't actually fold. It seems
like we're missing selection support for that sort of fold right now, since SDAG
happily folds these away (e.g testSwapCmpWithShiftedZeroExtend8_32 in the
original .ll testcase)

Differential Revision: https://reviews.llvm.org/D89422