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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - missing optimization (alias analysis issue?)"
href="https://llvm.org/bugs/show_bug.cgi?id=30638">30638</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>missing optimization (alias analysis issue?)
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>3.9
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>vanyacpp@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>The following code:
#include <vector>
#include <memory>
struct base
{
virtual ~base()
{}
};
void f(std::vector<std::unique_ptr<base> >& v)
{
v.back().release();
v.pop_back();
}
GCC 6.2 is able to optimize to just two instructions:
sub QWORD PTR [rdi+8], 8
ret
The code generated by clang 3.9.0 is less efficient:
push rbx
mov rax, qword ptr [rdi + 8]
mov qword ptr [rax - 8], 0
mov rax, qword ptr [rdi + 8]
lea rbx, [rax - 8]
mov qword ptr [rdi + 8], rbx
mov rdi, qword ptr [rax - 8]
test rdi, rdi
je .LBB0_2
mov rax, qword ptr [rdi]
call qword ptr [rax + 8]
.LBB0_2:
mov qword ptr [rbx], 0
pop rbx
ret
As [rax - 8] is reloaded after subtraction I tend believe it is an alias
analysis issue.</pre>
</div>
</p>
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