[all-commits] [llvm/llvm-project] 0ba487: [FixIrreducible] Use CycleInfo instead of a custom...

[Sameer Sahasrabuddhe via All-commits]

  Branch: refs/heads/users/ssahasra/fix-irreducible
  Home:   https://github.com/llvm/llvm-project
  Commit: 0ba4872d47179a4d54a06224008cc160905360dc
      https://github.com/llvm/llvm-project/commit/0ba4872d47179a4d54a06224008cc160905360dc
  Author: Sameer Sahasrabuddhe <sameer.sahasrabuddhe at amd.com>
  Date:   2024-08-13 (Tue, 13 Aug 2024)

  Changed paths:
    M llvm/include/llvm/ADT/GenericCycleInfo.h
    M llvm/lib/Transforms/Utils/FixIrreducible.cpp
    M llvm/test/CodeGen/AMDGPU/llc-pipeline.ll
    M llvm/test/Transforms/FixIrreducible/basic.ll
    M llvm/test/Transforms/FixIrreducible/bug45623.ll
    M llvm/test/Transforms/FixIrreducible/nested.ll
    M llvm/test/Transforms/FixIrreducible/switch.ll
    M llvm/test/Transforms/FixIrreducible/unreachable.ll
    M llvm/test/Transforms/StructurizeCFG/workarounds/needs-fix-reducible.ll
    M llvm/test/Transforms/StructurizeCFG/workarounds/needs-fr-ule.ll

  Log Message:
  -----------
  [FixIrreducible] Use CycleInfo instead of a custom SCC traversal

1. CycleInfo efficiently locates all cycles in a single pass, while the SCC is
   repeated inside every natural loop.

2. CycleInfo provides a hierarchy of irreducible cycles, and the new
   implementation transforms each cycle in this hierarchy separately instead of
   reducing an entire irreducible SCC in a single step. This reduces the number
   of control-flow paths that pass through the header of each newly created
   loop. This is evidenced by the reduced number of predecessors on the "guard"
   blocks in the lit tests, and fewer operands on the corresponding PHI nodes.

3. When an entry of an irreducible cycle is the header of a child natural loop,
   the original implementation destroyed that loop. This is now preserved,
   since the incoming edges on non-header entries are not touched.



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