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Hi Sebastian,
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</div>
<div>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
<i>a </i>I'm still getting a def of <i>d.</i><br>
<br>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Sebastian Pop <sebpop.llvm@gmail.com><br>
<b>Sent:</b> Friday, August 10, 2018 10:58 PM<br>
<b>To:</b> Alexandros.Lamprineas@arm.com<br>
<b>Cc:</b> Hayrapetyan, Anahit; LLVM Dev<br>
<b>Subject:</b> Re: [llvm-dev] llvm MemorySSA def-use chains</font>
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<div class="PlainText">Hi,<br>
<br>
try adding some alias analysis to the pipeline: see examples in<br>
llvm/test/Analysis/MemorySSA/*.ll<br>
for example assume.ll has<br>
; RUN: opt -aa-pipeline=basic-aa -passes='print<memoryssa>,verify<memoryssa>'<br>
<br>
without basic-aa the memory ssa will safely assume that the stores to<br>
%a and %d alias which may be proven disjoint by one of the alias<br>
analyses.<br>
On Thu, Aug 9, 2018 at 8:37 AM Alexandros Lamprineas via llvm-dev<br>
<llvm-dev@lists.llvm.org> wrote:<br>
><br>
> Hi,<br>
><br>
><br>
> My understanding is that MemorySSA doesn't know whether memory locations can alias or not.<br>
><br>
><br>
> Alexandros<br>
><br>
> ________________________________<br>
> From: llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Hayrapetyan, Anahit via llvm-dev <llvm-dev@lists.llvm.org><br>
> Sent: Thursday, August 9, 2018 2:00:14 PM<br>
> To: llvm-dev@lists.llvm.org<br>
> Subject: [llvm-dev] llvm MemorySSA def-use chains<br>
><br>
><br>
> Hi,<br>
><br>
><br>
> I have a question about how llvm MemorySSA works, as seems I misunderstand something.<br>
><br>
> Consider following code snippet and corresponding IR with MemorySSA annotations (got with opt -print-memoryssa)<br>
><br>
><br>
> void foo(int* b) {<br>
><br>
> int a = 0;<br>
><br>
> int d = 12;<br>
><br>
> if (b) {<br>
><br>
> a = 42;<br>
><br>
> d = 32;<br>
><br>
> }<br>
><br>
> int c = a;<br>
><br>
> int e = d;<br>
><br>
> }<br>
><br>
><br>
> ; Function Attrs: noinline nounwind optnone uwtable<br>
><br>
> define void @foo(i32* %b) #0 {<br>
><br>
> entry:<br>
><br>
> %b.addr = alloca i32*, align 8<br>
><br>
> %a = alloca i32, align 4<br>
><br>
> %d = alloca i32, align 4<br>
><br>
> %c = alloca i32, align 4<br>
><br>
> %e = alloca i32, align 4<br>
><br>
> ; 1 = MemoryDef(liveOnEntry)<br>
><br>
> store i32* %b, i32** %b.addr, align 8<br>
><br>
> ; 2 = MemoryDef(1)<br>
><br>
> store i32 0, i32* %a, align 4<br>
><br>
> ; 3 = MemoryDef(2)<br>
><br>
> store i32 12, i32* %d, align 4<br>
><br>
> ; MemoryUse(1)<br>
><br>
> %0 = load i32*, i32** %b.addr, align 8<br>
><br>
> %tobool = icmp ne i32* %0, null<br>
><br>
> br i1 %tobool, label %if.then, label %if.end<br>
><br>
><br>
> if.then: ; preds = %entry<br>
><br>
> ; 4 = MemoryDef(3)<br>
><br>
> store i32 42, i32* %a, align 4<br>
><br>
> ; 5 = MemoryDef(4)<br>
><br>
> store i32 32, i32* %d, align 4<br>
><br>
> br label %if.end<br>
><br>
><br>
> if.end: ; preds = %if.then, %entry<br>
><br>
> ; 9 = MemoryPhi({entry,3},{if.then,5})<br>
><br>
> ; MemoryUse(9)<br>
><br>
> %1 = load i32, i32* %a, align 4<br>
><br>
> ; 6 = MemoryDef(9)<br>
><br>
> store i32 %1, i32* %c, align 4<br>
><br>
> ; MemoryUse(9)<br>
><br>
> %2 = load i32, i32* %d, align 4<br>
><br>
> ; 7 = MemoryDef(6)<br>
><br>
> store i32 %2, i32* %e, align 4<br>
><br>
> ret void<br>
><br>
> }<br>
><br>
><br>
> 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`.<br>
><br>
> Could you please explain what's the reason MemorySSA gives incorrect definitions, and is there any way to get the correct ones.<br>
><br>
><br>
> Thanks.<br>
><br>
><br>
><br>
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