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[AArch64][GlobalISel] Refactor G_BRCOND selection

Authored by paquette on Dec 1 2020, 11:58 AM.

Description

[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

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