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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 --- - missed loop invariant optimization and compare optimization"
href="https://llvm.org/bugs/show_bug.cgi?id=30906">30906</a>
</td>
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<tr>
<th>Summary</th>
<td>missed loop invariant optimization and compare optimization
</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>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</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>carrot@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Compile the following code with options -m64 -O2
struct C {
inline bool bar(const C& c) const;
unsigned int f;
};
bool C::bar(const C& c) const {
return (f <= c.f) && // if comment out, llvm can optimize foo
((f ^ c.f) >> f == 0);
}
bool foo(int k, C& a, C& b) {
int r = 1;
for (int i = 0; i < k; ++i) {
r += a.bar(b);
}
return r != 0;
}
llvm generates complex instructions to compute the loop of function foo.
Actually a.bar(b) is loop invariant, the loop can be simplified to
r += k * a.bar(b)
And a.bar(b) returns bool value, it is 0 or 1, so the final result of r is
always greater than 0, and function foo should always return 1.
If I simplify the condition in C::bar a little by commenting out the first
line, then llvm can do the expected optimization and return 1 directly.
On the other hand, even if a.bar(b) is not loop invariant, the final value of r
is still >= 1, foo can still returns 1 directly.</pre>
</div>
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