[all-commits] [llvm/llvm-project] b94185: [LoopInterchange] New cost model for loop interchange

[CongzheUalberta via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: b941857b40edd7f3f3a9ec2ec85a26db24739774
      https://github.com/llvm/llvm-project/commit/b941857b40edd7f3f3a9ec2ec85a26db24739774
  Author: Congzhe Cao <congzhe.cao at huawei.com>
  Date:   2022-06-28 (Tue, 28 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/phi-ordering.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 is another attempt to land this patch.

The 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. One change that applies to all tests is that we added an option
`-cache-line-size=64` to the RUN lines. This is ensure that loop
cache analysis receives a valid number of cache line size for correct
analysis. 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