[PATCH] D16340: Cache results of WalkChainUsers() in SelectionDAG

[Z. Zheng via llvm-commits]

zzheng created this revision.
zzheng added reviewers: hfinkel, apazos, atrick, mcrosier, resistor.
zzheng added a subscriber: llvm-commits.
zzheng set the repository for this revision to rL LLVM.

We found the following loop, after unrolled by a large factor, can cause infinite compile-time spent in WalkChainUsers() when -combiner-alias-analysis is enabled. This issue was first report by Chad on: https://llvm.org/bugs/show_bug.cgi?id=21759.

In this patch, we appended the proposed fix in the bug report with a test case. The test case is derived from the loop below with 'opt -loop-unroll -unroll-threshold=700 -unroll-allow-paritial'.

define void @foo(i32* nocapture %a) #0 {
entry:
  br label %for.body

for.body:
  %count = phi i64 [ %dec, %for.body ], [ 3000, %entry ]
  %addr = phi i32* [ %gep, %for.body ], [ %a, %entry ]
  %dec = add nsw i64 %count, -8
  %gep = getelementptr inbounds i32, i32* %addr, i64 8
  %cmp = icmp eq i64 %count, 8
  %cast = bitcast i32* %addr to i8*
  call void @llvm.memset.p0i8.i64(i8* %cast, i8 0, i64 32, i32 4, i1 false)
  br i1 %cmp, label %exit, label %for.body

exit:
  ret void
}

Repository:
  rL LLVM

http://reviews.llvm.org/D16340

Files:
  lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp
  test/CodeGen/Generic/token-compile-time.ll

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