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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - SCEV unable to infer loop max bound for remainder loops"
href="https://llvm.org/bugs/show_bug.cgi?id=31412">31412</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>SCEV unable to infer loop max bound for remainder loops
</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>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mkuper@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, sanjoy@playingwithpointers.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider:
void foo0(int* in, int* out, unsigned k) {
k %= 4;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (-1 + (zext i2 (trunc i32 %k to i2) to
i32))<nsw>
Loop %for.body: max backedge-taken count is -1
After talking to Sanjoy, it seems that the main issue is that
max-backedge-taken computation does not take the fact that "if the backedge is
taken, necessarily k > 0" into account.
Indeed:
void foo2(int* in, int* out, unsigned k) {
k %= 4;
k += 2;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (1 + (zext i2 (trunc i32 %k to i2) to
i32))<nuw><nsw>
Loop %for.body: max backedge-taken count is 4
But I think this is not the only problem.
If it were, adding 1 to k should have been sufficient, but:
void foo1(int* in, int* out, unsigned k) {
k %= 4;
k += 1;
for (int i = 0; i < k; ++i) {
in[i] += out[i];
}
}
$ bin/clang -m32 -c -S -o - -O2 ~/llvm/temp/smallmax.cpp -emit-llvm | bin/opt
-analyze -scalar-evolution
[...]
Loop %for.body: backedge-taken count is (zext i2 (trunc i32 %k to i2) to i32)
Loop %for.body: max backedge-taken count is -1</pre>
</div>
</p>
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