[PATCH] D113656: [LICM] Check a heuristic case to hoist load

[JinGu Kang via Phabricator via llvm-commits]

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Let's see an example with MemorySSA.

  loop.header:
  ; 2 = MemoryPhi({entry,liveOnEntry},{loop.body,1})
    %i.0 = phi i32 [ 0, %entry ], [ %inc, %loop.body ]
  ...
  
  loop.body:
  ; MemoryUse(2) MayAlias
    %0 = load i32**, i32*** @a, align 8, !tbaa !8
    %idxprom = zext i32 %i.0 to i64
    %arrayidx = getelementptr inbounds i32*, i32** %0, i64 %idxprom
  ; 1 = MemoryDef(2)
    store i32* null, i32** %arrayidx, align 8, !tbaa !8
  ...

As you can see, there is a dependence with MayAlias between `%0 = load` of MemoryUse(2) and `store` of `1 = MemoryDef(2)`. However, it is WAR dependence at first iteration of the loop and there is no dependence at rest of iteration. In this case, we can hoist the '%0 = load` to preheader.

It fixes https://bugs.llvm.org/show_bug.cgi?id=52143.


https://reviews.llvm.org/D113656

Files:
  llvm/include/llvm/Transforms/Utils/LoopUtils.h
  llvm/lib/Transforms/Scalar/LICM.cpp
  llvm/test/Transforms/LICM/heuristic-hoist-load.ll

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