[all-commits] [llvm/llvm-project] 36e8e1: [NFC][BlockPlacement]Add an option to renumber blo...

[Mingming Liu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 36e8e19337b9a6f47c7f17c849164df47d45fa65
      https://github.com/llvm/llvm-project/commit/36e8e19337b9a6f47c7f17c849164df47d45fa65
  Author: Mingming Liu <mingmingl at google.com>
  Date:   2022-11-07 (Mon, 07 Nov 2022)

  Changed paths:
    M llvm/lib/CodeGen/MachineBlockPlacement.cpp

  Log Message:
  -----------
  [NFC][BlockPlacement]Add an option to renumber blocks based on function layout order.

Use case:
- When block layout is visualized after MBP pass, the basic blocks are labeled in layout order; meanwhile blocks could be numbered in a different order.
- As a result, it's hard to map between the graph and pass output. With this option on, the basic blocks are renumbered in function layout order.

This option is only useful when a function is to be visualized (i.e., when view options are on) to make it debugging only.

Use https://godbolt.org/z/5WTW36bMr as an example:
- As MBP pass output (shown in godbolt output window), `func2` is in a basic block numbered `2` (`bb.2`), and `func1` is in a basic block numbered `3` (`bb.3`);
    `bb.3` is a block with higher block frequency than `bb.2`, and `bb.3` is placed before `bb.2` in the functin layout.
- Use [1] to get the dot graph (graph uploaded in [2]), the blocks are re-numbered.
   - `func1` is in 'if.end' block, and labeled `1` in visualized dot; `func2` is in 'if.then' blocks, and labeled `3` --> the labeled number and bb number won't map.
   - [[ https://github.com/llvm/llvm-project/blob/b5626ae9751f0d82aa04791a21689b289721738e/llvm/lib/CodeGen/MachineBlockFrequencyInfo.cpp#L127 | DOTGraphTraits<MachineBlockFrequencyInfo *>::getNodeLabel ]] is where labeled numbers are based on function layout number, and [[ https://github.com/llvm/llvm-project/blob/a8d93783f37c042ace67069ae4ca6f8fd849c2d0/llvm/include/llvm/Support/GraphWriter.h#L209
 | called by graph writer ]].
        So call 'MachineFunction::RenumberBlocks' would make labeled number (in dot graph) and block number (in pass output) consistent with each other.

[1] `./bin/clang++ -O3 -S -mllvm -view-block-layout-with-bfi=count -mllvm -view-bfi-func-name=_Z9func_loopv -mllvm -print-after=block-placement -mllvm  -filter-print-funcs=_Z9func_loopv test.c`

[2] {F25201785}

Reviewed By: davidxl

Differential Revision: https://reviews.llvm.org/D137467