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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 Store forwarding optimization as the Alias analysis failed to establish "noalias" information."
href="https://bugs.llvm.org/show_bug.cgi?id=44318">44318</a>
</td>
</tr>
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<th>Summary</th>
<td>Missed Store forwarding optimization as the Alias analysis failed to establish "noalias" information.
</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>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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>venkataramanan.kumar.llvm@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Please consider the following test case.
---Snip--
struct B {
int b1;
int b2;
};
struct C {
int b1;
};
struct A {
int a1;
struct C SC;
int a2;
};
int foo1(struct A * Aptr, struct B* Bptr)
{
int *a = &Aptr->SC.b1;
*a=10;
Bptr->b1 = 11;
return *a;
}
int foo2(struct A * Aptr, struct B* Bptr)
{
Aptr->SC.b1=10;
Bptr->b1 = 11;
return Aptr->SC.b1;
}
---Snip--
For the function "foo1" return value is 10 and can be forwarded directly.
The alias analysis is not able establish the fact that memory accessed by "*a"
and "Bptr->b1" donot alias each other.
Hence the optimization is missed. There is no TBAA information formed for
"&Aptr->SC.b1" and we are not able to prove "noalias" using TBAA.
But for the function "foo2", the Alias analysis proved that "Aptr->SC.b1" and
"Bptr->b1" donot alias each other. TBAA has proper metadata for both the memory
accesses in this case. Here the return value is forwarded.</pre>
</div>
</p>
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