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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to remove useless reload"
href="https://bugs.llvm.org/show_bug.cgi?id=45737">45737</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to remove useless reload
</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>gabravier@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>struct testStruct
{
struct testStruct* p1;
struct testStruct* p2;
};
void f(struct testStruct *element)
{
element->p1->p2 = element->p2;
element->p2->p1 = element->p1;
}
In this code, `element->p2` only needs to be loaded once. However, LLVM loads
`element->p2` again after `element->p1->p2 = element->p2`. Whether or not
`element->p1 == &element` doesn't matter here, since then `element->p2` would
be assigned `element->p2`, so the value would still be known here.
Code from GCC:
f(testStruct*):
mov rdx, QWORD PTR [rdi]
mov rax, QWORD PTR [rdi+8]
mov QWORD PTR [rdx+8], rax
mov QWORD PTR [rax], rdx
ret
Code from LLVM:
f(testStruct*): # @f(testStruct*)
mov rax, qword ptr [rdi]
mov rcx, qword ptr [rdi + 8]
mov qword ptr [rax + 8], rcx
mov rcx, qword ptr [rdi + 8]
mov qword ptr [rcx], rax
ret</pre>
</div>
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