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Bug 44835 - [InstCombine] Infinite loop in min/max load/store combine
Summary: [InstCombine] Infinite loop in min/max load/store combine
Status: RESOLVED FIXED
Alias: None
Product: libraries
Classification: Unclassified
Component: Scalar Optimizations (show other bugs)
Version: 10.0
Hardware: PC Linux
: P enhancement
Assignee: Unassigned LLVM Bugs
URL:
Keywords:
Depends on:
Blocks: release-10.0.0
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Reported: 2020-02-07 13:29 PST by Nikita Popov
Modified: 2020-02-10 02:27 PST (History)
4 users (show)

See Also:
Fixed By Commit(s): 23db9724d0e5490fa5a2a726acf015f84e2c87cf

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Description Nikita Popov 2020-02-07 13:29:33 PST 
define void @test(i32* %p, i32* %p2) {
  %v = load i32, i32* %p, align 4
  %v2 = load i32, i32* %p2, align 4
  %cmp = icmp ult i32 %v2, %v
  %sel = select i1 %cmp, i32* %p2, i32* %p
  %p8 = bitcast i32* %p to i8*
  %sel8 = bitcast i32* %sel to i8*
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 4, i1 false)
  ret void 
}

; Function Attrs: argmemonly nounwind willreturn
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* noalias nocapture writeonly, i8* noalias nocapture readonly, i64, i1 immarg) #0

attributes #0 = { argmemonly nounwind willreturn }

This loops on LLVM 10. It is fixed in master as a side-effect of https://github.com/llvm/llvm-project/commit/80581966771a6d705daddbbf640a91c94652ceb5.

InstCombine log:

INSTCOMBINE ITERATION #1 on test
IC: ADDING: 8 instrs to worklist
IC: Visiting:   %v = load i32, i32* %p, align 4
IC: Visiting:   %v2 = load i32, i32* %p2, align 4
IC: Visiting:   %cmp = icmp ult i32 %v2, %v
IC: Visiting:   %sel = select i1 %cmp, i32* %p2, i32* %p
IC: Visiting:   %p8 = bitcast i32* %p to i8*
IC: Visiting:   %sel8 = bitcast i32* %sel to i8*
IC: Visiting:   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 4, i1 false)
IC: ADD:   %1 = bitcast i8* %sel8 to i32*
IC: ADD:   %2 = bitcast i8* %p8 to i32*
IC: ADD:   %3 = load i32, i32* %1
IC: ADD:   store i32 %3, i32* %2
IC: Mod =   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 4, i1 false)
    New =   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 0, i1 false)
IC: ADD:   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 0, i1 false)
IC: Visiting:   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 0, i1 false)
IC: ERASE   call void @llvm.memcpy.p0i8.p0i8.i64(i8* align 4 %p8, i8* align 4 %sel8, i64 0, i1 false)
IC: ADD:   %p8 = bitcast i32* %p to i8*
IC: ADD:   %sel8 = bitcast i32* %sel to i8*
IC: Visiting:   %sel8 = bitcast i32* %sel to i8*
IC: Visiting:   %p8 = bitcast i32* %p to i8*
IC: Visiting:   store i32 %3, i32* %2, align 4
IC: ADD:   %3 = load i32, i32* %1
IC: ADD:   store i32 %3, i32* %2
IC: ADD:   store i32 %4, i32* %2, align 4
IC: Replacing   %4 = load i32, i32* %1, align 4
    with i32 undef
IC: ERASE   %4 = load i32, i32* %1, align 4
IC: ERASE   store i32 undef, i32* %2, align 4
IC: Visiting:   store i32 %3, i32* %2, align 4
IC: ADD:   %3 = load i32, i32* %1
IC: ADD:   store i32 %3, i32* %2
IC: ADD:   store i32 %4, i32* %2, align 4
IC: Replacing   %4 = load i32, i32* %1, align 4
    with i32 undef
IC: ERASE   %4 = load i32, i32* %1, align 4
IC: ERASE   store i32 undef, i32* %2, align 4

The memcpy() transform leaves behind a redundant bitcast-of-bitcast, which (prior to worklist order fixes) never gets a chance to be cleaned up, because the removeBitcastsFromLoadStoreOnMinMax() transform goes into an infinite loop when it encounters this structure:

define void @test(i32* %p, i32* %p2) {
  %v = load i32, i32* %p, align 4
  %v2 = load i32, i32* %p2, align 4
  %cmp = icmp ult i32 %v2, %v
  %sel = select i1 %cmp, i32* %p2, i32* %p
  %p8 = bitcast i32* %p to i8*
  %sel8 = bitcast i32* %sel to i8*
  %1 = bitcast i8* %sel8 to i32*
  %2 = bitcast i8* %p8 to i32*
  %3 = load i32, i32* %1, align 4
  store i32 %3, i32* %2, align 4
  ret void
}
Comment 1 Nikita Popov 2020-02-08 03:14:29 PST 
Candidate patch: https://reviews.llvm.org/D74278
Comment 2 Nikita Popov 2020-02-08 09:06:35 PST 
Landed in https://reviews.llvm.org/rG23db9724d0e5490fa5a2a726acf015f84e2c87cf, keeping open to track backport.
Comment 3 Hans Wennborg 2020-02-10 02:27:51 PST 
(In reply to Nikita Popov from comment #2)
> Landed in
> https://reviews.llvm.org/rG23db9724d0e5490fa5a2a726acf015f84e2c87cf, keeping
> open to track backport.

Thanks! Cherry-picked as 9db3e5d5156bc2a3ba8ec0d70ab7069a82472fbb