[llvm-dev] Possible MemCpyOpt bug?

[Hal Finkel via llvm-dev]

----- Original Message -----
> From: "Bryant Wong via llvm-dev" <llvm-dev at lists.llvm.org>
> To: llvm-dev at lists.llvm.org
> Sent: Thursday, November 17, 2016 1:00:06 PM
> Subject: [llvm-dev] Possible MemCpyOpt bug?
> 
> 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. 

Looks like you posted a fix for review: https://reviews.llvm.org/D26811

> 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

Looks right.

 -Hal

> 
> `-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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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory