[all-commits] [llvm/llvm-project] 2c1857: [RISCV] Remove setJumpIsExpensive(). (#74647)

[Craig Topper via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2c185709bca195f3d5e062819bba5dd828330548
      https://github.com/llvm/llvm-project/commit/2c185709bca195f3d5e062819bba5dd828330548
  Author: Craig Topper <craig.topper at sifive.com>
  Date:   2023-12-13 (Wed, 13 Dec 2023)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/double-previous-failure.ll
    M llvm/test/CodeGen/RISCV/select-and.ll
    M llvm/test/CodeGen/RISCV/select-or.ll
    M llvm/test/CodeGen/RISCV/setcc-logic.ll
    M llvm/test/CodeGen/RISCV/zext-with-load-is-free.ll

  Log Message:
  -----------
  [RISCV] Remove setJumpIsExpensive(). (#74647)

Middle end up optimizations can speculate away the short circuit
behavior of C/C++ && and ||. Using i1 and/or or logical select
instructions and a single branch.

SelectionDAGBuilder can turn i1 and/or/select back into multiple
branches, but this is disabled when jump is expensive.

RISC-V can use slt(u)(i) to evaluate a condition into any GPR which
makes us better than other targets that use a flag register. RISC-V also
has single instruction compare and branch. So its not clear from a code
size perspective that using compare+and/or is better.

If the full condition is dependent on multiple loads, using a logic
delays the branch resolution until all the loads are resolved even if
there is a cheap condition that makes the loads unnecessary.

PowerPC and Lanai are the only CPU targets that use setJumpIsExpensive.
NVPTX and AMDGPU also use it but they are GPU targets. PowerPC appears
to have a MachineIR pass that turns AND/OR of CR bits into multiple
branches. I don't know anything about Lanai and their reason for using
setJumpIsExpensive.

I think the decision to use logic vs branches is much more nuanced than
this big hammer. So I propose to make RISC-V match other CPU targets.

Anyone who wants the old behavior can still pass -mllvm
-jump-is-expensive=true.