[PATCH] D112840: [PassManager] `buildFunctionSimplificationPipeline()`: schedule another `LoopDeletion` pass run before last `LICM`

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri created this revision.
lebedev.ri added reviewers: reames, mkazantsev, fhahn, jdoerfert, nikic.
lebedev.ri added a project: LLVM.
Herald added subscribers: ormris, wenlei, steven_wu, javed.absar, hiraditya.
lebedev.ri requested review of this revision.

Test thanks to Michael Kuklinski from `#llvm`,
originally inspired by Daniel Lemire's https://lemire.me/blog/2021/10/26/in-c-is-empty-faster-than-comparing-the-size-with-zero/

We manage to deduce that the answer does not require looping,
but we do that after the last `LoopDeletion` pass run,
so we end up being stuck with a dead loop.

While perhaps we are (clearly) also missing `LoopDeletion`
in `buildModuleOptimizationPipeline()` part of the pipeline,
we *should* add this one here, to reduce the function size
before inlining. I believe we want to have a new loop pass manager,
so that `LoopSimplifyPass`/`LCSSAPass` are guaranteed to be computed once?

Now, as with all things SCEV, this has a very expected ~+0.5% compile time performance regression:
https://llvm-compile-time-tracker.com/compare.php?from=e5df0a5a6f412965eb1be495d2672b4164c6c3d5&to=4af31d7accb368597bc8b4fc8f125d7c2fdbae6a&stat=instructions

Looking at the transformation stats over vanilla test-suite, i think it's rather expected:

  | statistic name                                   | baseline  | proposed  | Δ      | %      | \|%\|  |
  |--------------------------------------------------|-----------|-----------|--------|--------|--------|
  | scalar-evolution.NumTripCountsNotComputed        | 105592    | 137458    | 31866  | 30.18% | 30.18% |
  | scalar-evolution.NumBruteForceTripCountsComputed | 789       | 955       | 166    | 21.04% | 21.04% |
  | scalar-evolution.NumTripCountsComputed           | 299759    | 348901    | 49142  | 16.39% | 16.39% |
  | loop-delete.NumBackedgesBroken                   | 542       | 557       | 15     | 2.77%  | 2.77%  |
  | regalloc.numExtends                              | 81        | 79        | -2     | -2.47% | 2.47%  |
  | licm.NumSunk                                     | 12167     | 11875     | -292   | -2.40% | 2.40%  |
  | indvars.NumFoldedUser                            | 408       | 400       | -8     | -1.96% | 1.96%  |
  | indvars.NumElimCmp                               | 3831      | 3757      | -74    | -1.93% | 1.93%  |
  | loop-delete.NumDeleted                           | 8055      | 8096      | 41     | 0.51%  | 0.51%  |
  | codegenprepare.NumSelectsExpanded                | 277       | 278       | 1      | 0.36%  | 0.36%  |
  | assume-queries.NumAssumeQueries                  | 120910554 | 121324983 | 414429 | 0.34%  | 0.34%  |
  | loop-unroll.NumRuntimeUnrolled                   | 13841     | 13796     | -45    | -0.33% | 0.33%  |
  | phi-node-elimination.NumCriticalEdgesSplit       | 83054     | 82804     | -250   | -0.30% | 0.30%  |
  | branch-folder.NumBranchOpts                      | 108122    | 107827    | -295   | -0.27% | 0.27%  |
  | correlated-value-propagation.NumCmps             | 1461      | 1465      | 4      | 0.27%  | 0.27%  |
  | branch-folder.NumDeadBlocks                      | 130818    | 130490    | -328   | -0.25% | 0.25%  |
  | branch-folder.NumTailMerge                       | 72501     | 72347     | -154   | -0.21% | 0.21%  |
  | correlated-value-propagation.NumAShrs            | 514       | 515       | 1      | 0.19%  | 0.19%  |
  | loop-unroll.NumUnrolled                          | 40136     | 40069     | -67    | -0.17% | 0.17%  |
  | licm.NumHoisted                                  | 388827    | 388221    | -606   | -0.16% | 0.16%  |
  | machine-cse.NumPREs                              | 3085      | 3080      | -5     | -0.16% | 0.16%  |
  | loop-unroll.NumCompletelyUnrolled                | 9236      | 9222      | -14    | -0.15% | 0.15%  |


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D112840

Files:
  llvm/lib/Passes/PassBuilderPipelines.cpp
  llvm/test/Other/new-pm-defaults.ll
  llvm/test/Other/new-pm-thinlto-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-postlink-samplepgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-pgo-defaults.ll
  llvm/test/Other/new-pm-thinlto-prelink-samplepgo-defaults.ll
  llvm/test/Transforms/PhaseOrdering/deletion-of-loops-that-became-side-effect-free.ll

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