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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - missed optimization initializing an aggregate return value by a sequence of stores"
href="https://bugs.llvm.org/show_bug.cgi?id=47884">47884</a>
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<th>Summary</th>
<td>missed optimization initializing an aggregate return value by a sequence of stores
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>richard-llvm@metafoo.co.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Testcase:
struct Foo {
char a;
long b;
char c;
long d;
};
Foo foo1() {
Foo f;
f.a = 0;
f.b = 0;
f.c = 0;
f.d = 0;
return f;
}
We currently emit this as four stores to the sret parameter (1 byte, 8 byte, 1
byte, 8 byte). It would be more efficient to combine some of these stores
together.
In order for that to be correct, we'll need the frontend to tell the middle-end
that certain bytes of the sret parameter are known to be undef on entry to the
function. This seems like a general attribute we could reasonably apply to a
pointer parameter to a function, for example:
define void @foo(%struct.Foo* noalias sret(%struct.Foo) undef(32) align 8
%agg.result) {
... to indicate that the first 32 bytes reachable through %agg.result can be
assumed to be undef on entry to the function.
(We can't in general assume that sret(%T) implies that the full size of %T is
undef; that's not true in C++ due to the possibility of tail padding reuse.)</pre>
</div>
</p>
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