[PATCH] D44147: [InstCombine] Fold add/sub over phi node

[Hiroshi Inoue via Phabricator via llvm-commits]

inouehrs created this revision.
inouehrs added reviewers: spatel, craig.topper, scanon, hfinkel, efriedma, kbarton, nemanjai.
Herald added subscribers: javed.absar, sanjoy.

This patch adds FoldPHIUserOpIntoPred, which finds a phi node that is

1. used by only one add/sub instruction and
2. all its incoming values are ConstantInt or add/sub instruction used only by this phi node.

For such Phi nodes, we can eliminate add/sub instruction after the phi node by changing immediates of previous add/sub instructions.

Example of redundant add instruction to be optimized:

  BB1:
    %add = add i64 %a, 5 # -> immediate will be changed to 6
    br label %BB3
  BB2:
    %sub = sub i64 %b, 3 # -> immediate will be changed to 2
    br label %BB3
  BB3:
    %phi = phi i64 [ %add, %BB1 ], [ %sub, %BB2 ]
    %rc = add i64 %phi, 1 # -> will be removed

Additionally, if only one incoming value to the phi node does not meet above conditions, we can move the add/sub instruction to avoid partially redundant computation.
This optimization happens about 19k times (4k for full redundant cases and others for partially redundant cases) while building LLVM/Clang.

I am going to add similar optimization for logical operations as well as add/sub as a separate patch later.


https://reviews.llvm.org/D44147

Files:
  lib/Transforms/InstCombine/InstCombineInternal.h
  lib/Transforms/InstCombine/InstCombinePHI.cpp
  test/Analysis/ValueTracking/non-negative-phi-bits.ll
  test/Transforms/IndVarSimplify/rewrite-loop-exit-value.ll
  test/Transforms/InstCombine/stacksaverestore.ll
  test/Transforms/LoopUnroll/runtime-loop-multiple-exits.ll
  test/Transforms/LoopVectorize/X86/masked_load_store.ll
  test/Transforms/SLPVectorizer/AArch64/gather-reduce.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D44147.137177.patch
Type: text/x-patch
Size: 19882 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180306/bf869df5/attachment.bin>