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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Loop reroller fails to reroll loop with variable upper bound on 64-bit targets"
href="http://llvm.org/bugs/show_bug.cgi?id=18374">18374</a>
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<th>Summary</th>
<td>Loop reroller fails to reroll loop with variable upper bound on 64-bit targets
</td>
</tr>
<tr>
<th>Product</th>
<td>tools
</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>opt
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>dpeixott@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
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</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
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<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11813" name="attach_11813" title="test.c">attachment 11813</a> <a href="attachment.cgi?id=11813&action=edit" title="test.c">[details]</a></span>
test.c
Given this loop:
$ cat test.c
void foo(int *A, int *B, int n) {
for (int i = 0; i < n; i+=4) {
A[i+0] = B[i+0] * 4;
A[i+1] = B[i+1] * 4;
A[i+2] = B[i+2] * 4;
A[i+3] = B[i+3] * 4;
}
}
The loop reroller fails to reroll the loop on the 64-bit targets I tested
(x86_64 and AArch64).
$ clang -target x86_64-none-linux test.c -S -o- -O1 -emit-llvm -o test.ll
$ opt -loop-reroll -S -debug-only=loop-reroll -stats
It works correctly on 32-bit targets (tested i386 and armv7).
Looking at the bitcode, I suspect the problem is that the induction variable is
extended to 64-bits, but the loop bound remains 32-bits. The induction variable
is truncated inside the loop body to 32-bits. I think this makes SCEV unable to
compute the number of times the backedge is taken.
If I change the "int n" to a "long n" the loop is successfully rerolled.</pre>
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