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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimizations for std::optional<int>::emplace()"
href="https://bugs.llvm.org/show_bug.cgi?id=43915">43915</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed optimizations for std::optional<int>::emplace()
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>redbeard0531@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
</td>
</tr></table>
<p>
<div>
<pre><a href="https://godbolt.org/z/6-0fT3">https://godbolt.org/z/6-0fT3</a>
#include <optional>
std::optional<int> x;
void test() {
x.emplace(7);
}
clang -O3 codegen:
test(): # @test()
cmp byte ptr [rip + x+4], 0
je .LBB0_2
mov byte ptr [rip + x+4], 0
.LBB0_2:
mov dword ptr [rip + x], 7
mov byte ptr [rip + x+4], 1
ret
Note the double store to [rip + x+4]. If the initial store was eliminated, that
should also kill the unnecessary comparison+branch because the end result is
the same regardless of the initial state of the object.
gcc -O3 codegen:
test():
mov DWORD PTR x[rip], 7
mov BYTE PTR x[rip+4], 1
ret</pre>
</div>
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