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<p style="margin-top:0;margin-bottom:0">Hi,</p>
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<p style="margin-top:0;margin-bottom:0">My understanding is that MemorySSA doesn't know whether memory locations can alias or not.</p>
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<p style="margin-top:0;margin-bottom:0">Alexandros</p>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> on behalf of Hayrapetyan, Anahit via llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Sent:</b> Thursday, August 9, 2018 2:00:14 PM<br>
<b>To:</b> llvm-dev@lists.llvm.org<br>
<b>Subject:</b> [llvm-dev] llvm MemorySSA def-use chains</font>
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<p><span id="x_ms-rterangepaste-start"></span><span id="x_docs-internal-guid-e5e07aeb-7fff-0ea4-f360-b5a57d6b29f9"></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Hi,</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><br>
</p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">I have a question about how llvm MemorySSA works, as seems
I misunderstand something. </span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Consider following code snippet and corresponding IR with MemorySSA
annotations (got with opt -print-memoryssa)</span></p>
<br>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">void foo(int* b)
</span><span style="background-color:transparent; font-family:Arial; font-size:11pt; white-space:pre-wrap">{</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> int a = 0;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> int d = 12;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> if (b) {</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> a = 42;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> d = 32;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> }</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> int c = a;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> int e = d;</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">}</span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">; Function Attrs: noinline nounwind optnone uwtable</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">define void @foo(i32* %b) #0 {</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">entry:</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %b.addr = alloca i32*, align 8</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %a = alloca i32, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %d = alloca i32, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %c = alloca i32, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %e = alloca i32, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 1 = MemoryDef(liveOnEntry)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32* %b, i32** %b.addr, align 8</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 2 = MemoryDef(1)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 0, i32* %a, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 3 = MemoryDef(2)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 12, i32* %d, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; MemoryUse(1)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %0 = load i32*, i32** %b.addr, align 8</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %tobool = icmp ne i32* %0, null</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> br i1 %tobool, label %if.then, label %if.end</span></p>
<br>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">if.then: ; preds =
%entry</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 4 = MemoryDef</i>(3)</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 42, i32* %a, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 5 = MemoryDef(4)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 32, i32* %d, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> br label %if.end</span></p>
<br>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">if.end: ; preds =
%if.then, %entry</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 9 = MemoryPhi({entry,3},{if.then,5})</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">; MemoryUse(9)</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %1 = load i32, i32* %a, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 6 = MemoryDef(9)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 %1, i32* %c, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; MemoryUse(9)</i></span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> %2 = load i32, i32* %d, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"><i>; 7 = MemoryDef(6</i>)</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> store i32 %2, i32* %e, align 4</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap"> ret void</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">}</span></p>
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<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Tracking back def-use chain for
<i>`%1 = load i32, i32* %a, align 4`</i> first I get <i>`9 = MemoryPhi({entry,3},{if.then,5})`</i> 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`.</span></p>
<p dir="ltr" style="line-height:1.38; margin-top:0pt; margin-bottom:0pt"><span style="font-size:11pt; font-family:Arial; background-color:transparent; vertical-align:baseline; white-space:pre-wrap">Could you please explain what's the reason MemorySSA gives
incorrect definitions, and is there any way to get the correct ones. </span></p>
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<span style="font-size:12pt">Thanks.</span><br>
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