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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 - Missed optimization for (unsigned) log2((unsigned) X)"
href="https://bugs.llvm.org/show_bug.cgi?id=42206">42206</a>
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<th>Summary</th>
<td>Missed optimization for (unsigned) log2((unsigned) X)
</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>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>It seems we could do following transformations:
(T) log2((T) x) -> x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1 -__builtin_clz(x);
(T) floor(log2((T) x)) -> x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1
-__builtin_clz(x);
where T is unsigned integer type.
Test program:
#include <stdlib.h>
#include <math.h>
#include <limits.h>
#include <stdio.h>
#define T unsigned
#define TT unsigned
TT f(T x) {
return log2(x);
}
TT f2(T x) {
return x == 0 ? 0 : sizeof(T) * CHAR_BIT - 1 - __builtin_clz(x);
}
int main(int argc, char **argv) {
for (T i = 0; i < INT_MAX; ++i) {
if (f(i) != f2(i)) {
abort();
}
}
return 0;
}
I see no crash so I think this is valid transformation. Am I wrong?</pre>
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