[llvm-bugs] [Bug 44835] New: [InstCombine] Infinite loop in min/max load/store combine

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=44835

            Bug ID: 44835
           Summary: [InstCombine] Infinite loop in min/max load/store
                    combine
           Product: libraries
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: nikita.ppv at gmail.com
                CC: llvm-bugs at lists.llvm.org
            Blocks: 44555

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
}


Referenced Bugs:

https://bugs.llvm.org/show_bug.cgi?id=44555
[Bug 44555] [meta] 10.0.0 Release Blockers
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