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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 - Multiple returns are not canonicalized to a single return"
href="https://bugs.llvm.org/show_bug.cgi?id=32784">32784</a>
</td>
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<th>Summary</th>
<td>Multiple returns are not canonicalized to a single return
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<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>Transformation Utilities
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>me@manueljacob.de
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The patch for PR8575 stopped canonicalizing in the other direction, but other
than the bug title suggests, a canonicalization from multiple returns to a
single return was never implemented.
There are examples where this canonicalization would enable other
optimizations. For example, this isn't optimized further:
define void @test(i1 %cond, i64* %ptr1, i64* %ptr2, i64 %value) {
br i1 %cond, label %true_block, label %false_block
true_block:
store i64 %value, i64* %ptr1
ret void
false_block:
store i64 %value, i64* %ptr2
ret void
}
Changing the function to have a single return...
define void @test(i1 %cond, i64* %ptr1, i64* %ptr2, i64 %value) {
br i1 %cond, label %true_block, label %false_block
true_block:
store i64 %value, i64* %ptr1
br label %join
false_block:
store i64 %value, i64* %ptr2
br label %join
join:
ret void
}
... enables common instruction sinking:
define void @test(i1 %cond, i64* nocapture %ptr1, i64* nocapture %ptr2, i64
%value) local_unnamed_addr #0 {
join:
%ptr2.sink = select i1 %cond, i64* %ptr1, i64* %ptr2
store i64 %value, i64* %ptr2.sink, align 8
ret void
}</pre>
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