[all-commits] [llvm/llvm-project] ac28ef: [SimplifyCFG][TranformUtils]Do not simplify away a...

[Mingming Liu via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: ac28efa6c100cde81304f01b589d44b40b93ffeb
      https://github.com/llvm/llvm-project/commit/ac28efa6c100cde81304f01b589d44b40b93ffeb
  Author: Mingming Liu <mingmingl at google.com>
  Date:   2022-09-28 (Wed, 28 Sep 2022)

  Changed paths:
    M llvm/lib/Transforms/Utils/Local.cpp
    M llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll

  Log Message:
  -----------
  [SimplifyCFG][TranformUtils]Do not simplify away a trivial basic block if both this block and at least one of its predecessors are loop latches.

- Before this patch, loop metadata (if exists) will override the metadata of each predecessor; if the predecessor block already has loop metadata, the orignal loop metadata won't be preserved and could cause missed loop transformations (see 'test2' in llvm/test/Transforms/SimplifyCFG/preserve-llvm-loop-metadata.ll).

To illustrate how inner-loop metadata might be dropped before this patch:

CFG Before

      entry
        |
        v
 ---> while.cond   ------------->  while.end
 |       |
 |       v
 |   while.body
 |       |
 |       v
 |    for.body <---- (md1)
 |       |  |______|
 |       v
 |    while.cond.exit (md2)
 |       |
 |_______|

CFG After

       entry
         |
         v
 ---> while.cond.rewrite  ------------->  while.end
 |       |
 |       v
 |   while.body
 |       |
 |       v
 |    for.body <---- (md2)
 |_______|  |______|

Basically, when 'while.cond.exit' is folded into 'while.cond', 'md2' overrides 'md1' and 'md1' is dropped from the CFG.

Differential Revision: https://reviews.llvm.org/D134152