[llvm-dev] llvm MemorySSA def-use chains

[Hayrapetyan, Anahit via llvm-dev]

Hi Sebastian,

Thanks for the answer. I've tried to run memoryssa piping it with alias analysis, as you've suggested, however the output is still the same. As a def for use of a I'm still getting a def of d.

________________________________
From: Sebastian Pop <sebpop.llvm at gmail.com>
Sent: Friday, August 10, 2018 10:58 PM
To: Alexandros.Lamprineas at arm.com
Cc: Hayrapetyan, Anahit; LLVM Dev
Subject: Re: [llvm-dev] llvm MemorySSA def-use chains

Hi,

try adding some alias analysis to the pipeline: see examples in
llvm/test/Analysis/MemorySSA/*.ll
for example assume.ll has
; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>,verify<memoryssa>'

without basic-aa the memory ssa will safely assume that the stores to
%a and %d alias which may be proven disjoint by one of the alias
analyses.
On Thu, Aug 9, 2018 at 8:37 AM Alexandros Lamprineas via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
>
> My understanding is that MemorySSA doesn't know whether memory locations can alias or not.
>
>
> Alexandros
>
> ________________________________
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Hayrapetyan, Anahit via llvm-dev <llvm-dev at lists.llvm.org>
> Sent: Thursday, August 9, 2018 2:00:14 PM
> To: llvm-dev at lists.llvm.org
> Subject: [llvm-dev] llvm MemorySSA def-use chains
>
>
> Hi,
>
>
> I have a question about how llvm MemorySSA works, as seems I misunderstand something.
>
> Consider following code snippet and corresponding IR with MemorySSA annotations (got with opt -print-memoryssa)
>
>
> void foo(int* b) {
>
>    int a = 0;
>
>    int d = 12;
>
>    if (b) {
>
>        a = 42;
>
>        d = 32;
>
>    }
>
>    int c = a;
>
>    int e = d;
>
> }
>
>
> ; Function Attrs: noinline nounwind optnone uwtable
>
> define void @foo(i32* %b) #0 {
>
> entry:
>
>  %b.addr = alloca i32*, align 8
>
>  %a = alloca i32, align 4
>
>  %d = alloca i32, align 4
>
>  %c = alloca i32, align 4
>
>  %e = alloca i32, align 4
>
> ; 1 = MemoryDef(liveOnEntry)
>
>  store i32* %b, i32** %b.addr, align 8
>
> ; 2 = MemoryDef(1)
>
>  store i32 0, i32* %a, align 4
>
> ; 3 = MemoryDef(2)
>
>  store i32 12, i32* %d, align 4
>
> ; MemoryUse(1)
>
>  %0 = load i32*, i32** %b.addr, align 8
>
>  %tobool = icmp ne i32* %0, null
>
>  br i1 %tobool, label %if.then, label %if.end
>
>
> if.then:                                          ; preds = %entry
>
> ; 4 = MemoryDef(3)
>
>  store i32 42, i32* %a, align 4
>
> ; 5 = MemoryDef(4)
>
>  store i32 32, i32* %d, align 4
>
>  br label %if.end
>
>
> if.end:                                           ; preds = %if.then, %entry
>
> ; 9 = MemoryPhi({entry,3},{if.then,5})
>
> ; MemoryUse(9)
>
>  %1 = load i32, i32* %a, align 4
>
> ; 6 = MemoryDef(9)
>
>  store i32 %1, i32* %c, align 4
>
> ; MemoryUse(9)
>
>  %2 = load i32, i32* %d, align 4
>
> ; 7 = MemoryDef(6)
>
>  store i32 %2, i32* %e, align 4
>
>  ret void
>
> }
>
>
> Tracking back def-use chain for `%1 = load i32, i32* %a, align 4` first I get `9 = MemoryPhi({entry,3},{if.then,5})` and then as definitions MemoryDef(2) which corresponds to `store i32 12, i32* %d, align 4` and MemoryDef(4) corresponding to `store i32 32, i32* %d, align 4`.  However neither of these stores define `a`, they both define `d`. What I was expecting to get was MemoryDef(3) and MemoryDef(1), which are real definitions for `a`.
>
> Could you please explain what's the reason MemorySSA gives incorrect definitions, and is there any way to get the correct ones.
>
>
> Thanks.
>
>
>
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