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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missing optimization: *this in constructors should not alias with reference input parameters of the same type"
href="https://bugs.llvm.org/show_bug.cgi?id=37329">37329</a>
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<th>Summary</th>
<td>Missing optimization: *this in constructors should not alias with reference input parameters of the same type
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>C++
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>antoshkka@gmail.com
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<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Following code
struct test {
char data[2] = {1, 2};
test(const test& v);
};
test::test(const test& v)
: data{ v.data[0], v.data[0] }
{}
Produces the following assembly:
test::test(test const&): # @test::test(test const&)
mov al, byte ptr [rsi]
mov byte ptr [rdi], al
mov al, byte ptr [rsi] <=== This line is not required
mov byte ptr [rdi + 1], al
ret
Line 'mov al, byte ptr [rsi]' is for cases when input argument `v` aliases
`this`. Clang already warns on code like `Y v(v)`.
warning: variable 'v' is uninitialized when used within its own initialization
[-Wuninitialized]
Y v(v);
~ ^
C++ Standard covers the above case and allows to assume that argument do not
alias with this.
[class.ctor] paragraph 14:
"During the construction of an object, if the value of the object or any of its
subobjects is accessed through a glvalue that is not obtained, directly or
indirectly, from the constructor’s this pointer, the value of the object or
subobject thus obtained is unspecified."</pre>
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