[all-commits] [llvm/llvm-project] 006334: [LoopInterchange] New cost model for loop interchange

[CongzheUalberta via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 006334470d8d1b5d8f630890336fcb45795749d1
      https://github.com/llvm/llvm-project/commit/006334470d8d1b5d8f630890336fcb45795749d1
  Author: Congzhe Cao <congzhe.cao at huawei.com>
  Date:   2022-06-02 (Thu, 02 Jun 2022)

  Changed paths:
    M llvm/lib/Transforms/Scalar/LoopInterchange.cpp
    M llvm/test/Transforms/LICM/lnicm.ll
    M llvm/test/Transforms/LoopInterchange/call-instructions.ll
    M llvm/test/Transforms/LoopInterchange/currentLimitation.ll
    M llvm/test/Transforms/LoopInterchange/debuginfo.ll
    M llvm/test/Transforms/LoopInterchange/inner-indvar-depend-on-outer-indvar.ll
    M llvm/test/Transforms/LoopInterchange/inner-only-reductions.ll
    M llvm/test/Transforms/LoopInterchange/innermost-latch-uses-values-in-middle-header.ll
    M llvm/test/Transforms/LoopInterchange/interchange-flow-dep-outer.ll
    M llvm/test/Transforms/LoopInterchange/interchange-insts-between-indvar.ll
    M llvm/test/Transforms/LoopInterchange/interchange-no-deps.ll
    M llvm/test/Transforms/LoopInterchange/interchangeable-innerloop-multiple-indvars.ll
    M llvm/test/Transforms/LoopInterchange/interchangeable-outerloop-multiple-indvars.ll
    M llvm/test/Transforms/LoopInterchange/interchangeable.ll
    M llvm/test/Transforms/LoopInterchange/interchanged-loop-nest-3.ll
    M llvm/test/Transforms/LoopInterchange/lcssa-preheader.ll
    M llvm/test/Transforms/LoopInterchange/lcssa.ll
    M llvm/test/Transforms/LoopInterchange/loop-interchange-optimization-remarks.ll
    M llvm/test/Transforms/LoopInterchange/not-interchanged-dependencies-1.ll
    M llvm/test/Transforms/LoopInterchange/not-interchanged-loop-nest-3.ll
    M llvm/test/Transforms/LoopInterchange/not-interchanged-tightly-nested.ll
    M llvm/test/Transforms/LoopInterchange/outer-header-jump-to-inner-latch.ll
    M llvm/test/Transforms/LoopInterchange/outer-only-reductions.ll
    M llvm/test/Transforms/LoopInterchange/perserve-lcssa.ll
    M llvm/test/Transforms/LoopInterchange/pr43176-move-to-new-latch.ll
    M llvm/test/Transforms/LoopInterchange/pr43326-ideal-access-pattern.ll
    M llvm/test/Transforms/LoopInterchange/pr43326.ll
    M llvm/test/Transforms/LoopInterchange/pr43473-invalid-lcssa-phis-in-inner-exit.ll
    M llvm/test/Transforms/LoopInterchange/pr43797-lcssa-for-multiple-outer-loop-blocks.ll
    M llvm/test/Transforms/LoopInterchange/pr45743-move-from-inner-preheader.ll
    M llvm/test/Transforms/LoopInterchange/pr48212.ll
    M llvm/test/Transforms/LoopInterchange/profitability.ll
    M llvm/test/Transforms/LoopInterchange/reductions-across-inner-and-outer-loop.ll
    M llvm/test/Transforms/LoopInterchange/update-condbranch-duplicate-successors.ll
    M llvm/test/Transforms/LoopInterchange/vector-gep-operand.ll

  Log Message:
  -----------
  [LoopInterchange] New cost model for loop interchange

This patch proposed to use a new cost model for loop interchange, which
is obtained from loop cache analysis.

Given a loopnest, what loop cache analysis returns is a vector of loops
[loop0, loop1, loop2, ...] where loop0 should be replaced as the outermost
loop, loop1 should be placed one more level inside, and loop2 one more level
inside, etc. What loop cache analysis does is not only more comprehensive than
the current cost model, it is also a "one-shot" query which means that we only
need to query it once during the entire loop interchange pass, which is better
than the current cost model where we query it every time we check whether it is
profitable to interchange two loops. Thus complexity is reduced, especially after
D120386 where we do more interchanges to get the globally optimal loop access pattern.

Updates made to test cases are mostly minor changes and some corrections.
Test coverage for loop interchange is not reduced.

Currently we did not completely remove the legacy cost model, but keep it as
fall-back in case the new cost model did not run successfully. This is because
currently we have some limitations in delinearization, which sometimes makes
loop cache analysis bail out. The longer term goal is to enhance delinearization
and eventually remove the legacy cost model compeletely.

Reviewed By: bmahjour, #loopoptwg

Differential Revision: https://reviews.llvm.org/D124926