[all-commits] [llvm/llvm-project] 233659: [LoopFlatten] Enable it by default

[sjoerdmeijer via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 233659c7ae9b83b64a9f739d340736bca39c3d2e
      https://github.com/llvm/llvm-project/commit/233659c7ae9b83b64a9f739d340736bca39c3d2e
  Author: Sjoerd Meijer <sjoerd.meijer at gmail.com>
  Date:   2022-10-17 (Mon, 17 Oct 2022)

  Changed paths:
    M llvm/docs/ReleaseNotes.rst
    M llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
    M llvm/lib/Transforms/Scalar/LoopFlatten.cpp
    M llvm/test/Other/new-pm-defaults.ll
    M llvm/test/Other/new-pm-lto-defaults.ll
    M llvm/test/Other/new-pm-thinlto-defaults.ll
    M llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
    M llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
    M llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
    M llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll

  Log Message:
  -----------
  [LoopFlatten] Enable it by default

LoopFlatten has been in the code base off by default for years, but this
enables it to run by default. Downstream this has been running for
years, so it has been exposed to quite some code. Then around the time
we switched to the NPM, several fixes went in related to updating the
MemorySSA state and we moved it to a loop pass manager, which both
helped preventing rerunning certain analysis passes, and thus helped a
bit with compile-times.

About compile-times, adding a pass isn't free, but this should see only
very minor increases. The pass is relatively simple and there shouldn't
be anything algorithmically expensive because all it does is looking at
inner/outer loops and it checks assumptions on loop increments and
indices. If we see increases, I expect this to mainly come from
invalidation of analysis info, and perhaps subsequent passes to trigger
and do more. Despite its simplicity/restrictions, it triggers in most
code-bases, which makes it worth to enable this by default.

Differential Revision: https://reviews.llvm.org/D109958