[all-commits] [llvm/llvm-project] 25eb7b: [DAGCombiner] Fold BRCOND(FREEZE(COND)) to BRCOND(...

[Juneyoung Lee via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 25eb7b08ba77a0b7c9c938490444bb8b5121233c
      https://github.com/llvm/llvm-project/commit/25eb7b08ba77a0b7c9c938490444bb8b5121233c
  Author: Juneyoung Lee <aqjune at gmail.com>
  Date:   2021-01-13 (Wed, 13 Jan 2021)

  Changed paths:
    M llvm/include/llvm/CodeGen/ISDOpcodes.h
    M llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    M llvm/test/CodeGen/X86/select-prof-codegen.ll

  Log Message:
  -----------
  [DAGCombiner] Fold BRCOND(FREEZE(COND)) to BRCOND(COND)

This patch resolves the suboptimal codegen described in http://llvm.org/pr47873 .
When CodeGenPrepare lowers select into a conditional branch, a freeze instruction is inserted.
It is then translated to `BRCOND(FREEZE(SETCC))` in SelDag.
The `FREEZE` in the middle of `SETCC` and `BRCOND` was causing a suboptimal code generation however.
This patch adds `BRCOND(FREEZE(cond))` -> `BRCOND(cond)` fold to DAGCombiner to remove the `FREEZE`.

To make this optimization sound, `BRCOND(UNDEF)` simply should nondeterministically jump to the branch or not, rather than raising UB.
It wasn't clear what happens when the condition was undef according to the comments in ISDOpcodes.h, however.
I updated the comments of `BRCOND` to make it explicit (as well as `BR_CC`, which is also a conditional branch instruction).

Note that it diverges from the semantics of `br` instruction in IR, which is explicitly UB.
Since the UB semantics was necessary to explain optimizations that use branching conditions, and SelDag doesn't seem to have such optimization, I think this divergence is okay.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D92015