[llvm-dev] Possible MemCpyOpt bug?

[Bryant Wong via llvm-dev]

Hi all,

I think I've managed to trick the legacy MemCpyOpt (MCO) into an incorrect
transform, but I would like to confirm the validity of my counterexample
before
working on the fix. Suppose the following IR:

    %T = type { i32, i32 }

    define void @f(%T* %a, %T* %b, %T* %c, %T* %d) {
        %val = load %T, %T* %a, !alias.scope !{!10}

        ; store1
        ; Aliases the load
        store %T { i32 22, i32 22 }, %T* %b, !alias.scope !{!11}

        ; store2
        ; Aliases the store below and the load. No-alias with above store
        store %T { i32 44, i32 44 }, %T* %c, !alias.scope !{!12},
              !noalias !{!11}

        ; store3
        ; MCO wants to combine this store with the load into a memcpy
        store %T %val, %T* %d, !alias.scope !{!13}, !noalias !{!10, !11}
        ret void
    }

!0 = !{!0}
!1 = !{!1}
!2 = !{!2}
!3 = !{!3}

!10 = !{ !10, !0 }
!11 = !{ !11, !1 }
!12 = !{ !12, !2 }
!13 = !{ !13, !3 }

Have I used the metadata nodes correctly? The goal is to make:

- load may-alias store1
- load may-alias store2
- store2 may-alias store3

`-print-may-aliases` seems confirm this:

    opt -disable-output -basicaa -scoped-noalias -aa-eval
-evaluate-aa-metadata -print-may-aliases ohgod.ll
    Function: f: 4 pointers, 0 call sites
      MayAlias:     %T* %a, %T* %b
      MayAlias:     %T* %a, %T* %c
      MayAlias:     %T* %b, %T* %c
      MayAlias:     %T* %a, %T* %d
      MayAlias:     %T* %b, %T* %d
      MayAlias:     %T* %c, %T* %d
      MayAlias:   %val = load %T, %T* %a, !alias.scope !0 <->   store %T {
i32 23, i32 23 }, %T* %b, !alias.scope !3
      MayAlias:   %val = load %T, %T* %a, !alias.scope !0 <->   store %T {
i32 44, i32 44 }, %T* %c, !alias.scope !6, !noalias !3
      MayAlias:   store %T %val, %T* %d, !alias.scope !9, !noalias !12
<->   store %T { i32 44, i32 44 }, %T* %c, !alias.scope !6, !noalias !3

The problem is that MCO happily moves store2 + store3 above store1 (this is
fine) and then the load below store2 (this is not fine) in order to combine
load + store3 into a memcpy:

    opt -S -basicaa -scoped-noalias -aa-eval -evaluate-aa-metadata
-memcpyopt

    %T = type { i32, i32 }

    define void @f(%T* %a, %T* %b, %T* %c, %T* %d) {
      ; store2
      store %T { i32 44, i32 44 }, %T* %c, !alias.scope !0, !noalias !3

      ; combined from load + store3
      call void @llvm.memcpy.p0i8.p0i8.i64(i8* (bitcast %T* %d to i8*),
                                           i8* (bitcast %T* %a to i8*),
                                           i64 8, i32 4, i1 false)

      ; store1
      store %T { i32 23, i32 23 }, %T* %b, !alias.scope !3
      ret void
    }
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