<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - store of pointer to object into itself treated as escape by GVN"
href="https://bugs.llvm.org/show_bug.cgi?id=37263">37263</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>store of pointer to object into itself treated as escape by GVN
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Testcase, in C++ (using libstdc++):
#include <string>
void f();
void g() {
std::string s = "foo";
f();
}
We should be able to completely eliminate the local 's' variable here. But we
can't, because we believe that f() might have modified it. The reason is that
(in libstdc++, when the small-string optimization is engaged) s contains a
pointer to itself.
Similar example, reduced to eliminate stdlib dependency:
void f();
void g() {
void *p = &p;
f();
if (p != &p) f();
}
IR:
define dso_local void @_Z1gv() {
entry:
%p = alloca i8*, align 8
%0 = bitcast i8** %p to i8*
store i8* %0, i8** %p, align 8
call void @_Z1fv()
%1 = load i8*, i8** %p, align 8
%2 = bitcast i8** %p to i8*
%cmp = icmp ne i8* %1, %2
br i1 %cmp, label %if.then, label %if.end
if.then: ; preds = %entry
call void @_Z1fv()
br label %if.end
if.end: ; preds = %if.then, %entry
ret void
}
declare dso_local void @_Z1fv()
We should be able to optimize this to just:
call void @_Z1fv()
ret void</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>