[all-commits] [llvm/llvm-project] d49f64: [AArch64][GlobalISel] Refactor G_BRCOND selection

[Jessica Paquette via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d49f6491b6d1439b5a65ff6e965b65a66d943b63
      https://github.com/llvm/llvm-project/commit/d49f6491b6d1439b5a65ff6e965b65a66d943b63
  Author: Jessica Paquette <jpaquette at apple.com>
  Date:   2020-12-07 (Mon, 07 Dec 2020)

  Changed paths:
    M llvm/lib/Target/AArch64/GISel/AArch64InstructionSelector.cpp
    M llvm/test/CodeGen/AArch64/GlobalISel/speculative-hardening-brcond.mir

  Log Message:
  -----------
  [AArch64][GlobalISel] Refactor G_BRCOND selection

`selectCompareBranch` was hard to understand.

Also, it was being needlessly pessimistic with the `ProduceNonFlagSettingCondBr`
case. It assumed that everything in `selectCompareBranch` would emit a TB(N)Z
or C(B)NZ. That's not true; the G_FCMP + G_BRCOND case would never emit those
instructions, and the G_ICMP + G_BRCOND case was capable of emitting an integer
compare + Bcc.

- Refactor `selectCompareBranch` into separate functions based off of what is
feeding the G_BRCOND's condition.

- Move G_BRCOND selection code from `select` to `selectCompareBranch`.

- Remove duplicated constraint code from the code originally in `select`;
  `emitTestBit` already handles that, so no need to constrain twice.

- Factor out the G_FCMP + G_BRCOND case into `selectCompareBranchFedByFCmp`.

- Split the G_ICMP + G_BRCOND case into an optimization function,
`tryOptCompareBranchFedByICmp` and a general selection function,
`selectCompareBranchFedByICmp`.

- Reduce the number of things passed to `tryOptAndIntoCompareBranch`.

- Improve documentation.

- Give some variables more descriptive names.

Other than improving the code generation for functions with
speculative_load_hardening by getting the logic correct, this is NFC.

Differential Revision: https://reviews.llvm.org/D92582