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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Instcombine can't remove a PHI cycle when that cycle should be replaced with a PHI"
href="https://llvm.org/bugs/show_bug.cgi?id=25585">25585</a>
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<th>Summary</th>
<td>Instcombine can't remove a PHI cycle when that cycle should be replaced with a PHI
</td>
</tr>
<tr>
<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>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>silviu.baranga@arm.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=15319" name="attach_15319" title="The reproducer">attachment 15319</a> <a href="attachment.cgi?id=15319&action=edit" title="The reproducer">[details]</a></span>
The reproducer
For the following IR, instcombine should be able to replace %p1 and %p2 with
%p0.
target triple = "aarch64-linux-gnueabi"
define i32 @f(i1 %c) {
entry:
br i1 %c, label %if.else, label %pre
if.else:
br label %pre
pre:
%p0 = phi i32 [ 0, %entry ], [ 1, %if.else ]
br label %loop
loop:
%p1 = phi i32 [ %p2, %loop ], [ %p0, %pre ]
%p2 = phi i32 [ %p0, %loop ] , [ %p1, %pre ]
br i1 %c, label %loop, label %exit
exit:
ret i32 %p2
}
We're already trying to do this, but the current phi cycle elimination
algorithm can't handle the case where the value that we should be replacing
other the PHIs with is another PHI (in this case %p).
When the value that we would replace the PHI cycle with is not a PHI, this
algorithm works as expected:
define i32 @g(i1 %c, i32 %val) {
entry:
br label %pre
pre:
br label %loop
loop:
%p1 = phi i32 [ %p2, %loop ], [ %val, %pre ]
%p2 = phi i32 [ %val, %loop ] , [ %p1, %pre ]
br i1 %c, label %loop, label %exit
exit:
ret i32 %p2
}
Reproduce with: opt -instcombine ind.ll -S -o -</pre>
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