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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 - Wrong code generation for CTLZ pattern"
href="https://bugs.llvm.org/show_bug.cgi?id=37973">37973</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Wrong code generation for CTLZ pattern
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>ovmold@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>The motivating example:
int lzcnt(int x)
{
int count = 0;
while (x > 0) {
count++;
x = x >> 1;
}
return count;
}
int main()
{
int x = 1;
int y = lzcnt(x);
printf("count = %d\n", y);
return 0;
}
This code is compiled as: clang test.c -O3 -march=core-avx2
The pattern is recognized and the output looks like: "count = 1". This is
wrong since the number of leading zeros for x = 1 is 31.
Inside test.ll we can see:
define dso_local i32 @main() local_unnamed_addr #1 {
entry:
%call1 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds
([13 x i8], [13 x i8]* @.str, i64 0, i64 0),
i32 1)
ret i32 0
}
So, in my opinion, the problem is in the result substitution (i32 1 in
@printf), this is the result of lzcnt (number of leading 1 position), and not
quantity of leading zeros.
The same example but with scanf("%d", &x) in the main function produces the
following output for 1 as input: "count = 32". This output is also wrong.
Inside test.ll we can see:
%1 = load i32, i32* %x, align 4, !tbaa !3
%cmp4.i = icmp sgt i32 %1, 0
%2 = call i32 @llvm.ctlz.i32(i32 %1, i1 false) #5, !range !2
%3 = sub nsw i32 32, %2
Possibly it should be: %3 = sub nsw i32 31, %2</pre>
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