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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Loop reroller not able to reroll when the array base address is a derivative of the loop induction variable"
href="http://llvm.org/bugs/show_bug.cgi?id=18646">18646</a>
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<th>Summary</th>
<td>Loop reroller not able to reroll when the array base address is a derivative of the loop induction variable
</td>
</tr>
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<th>Product</th>
<td>tools
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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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>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chrisj@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=11960" name="attach_11960" title="test.c">attachment 11960</a> <a href="attachment.cgi?id=11960&action=edit" title="test.c">[details]</a></span>
test.c
The loop reroller is not able to reroll loops of the form where the array base
address is a derivative of the loop induction variable.
$ cat test.c
void foo(int *Arr, int n) {
for (int i = 0; i < n; i += 3) {
Arr[0]=0;
Arr[1]=0;
Arr[2]=0;
Arr+=3;
}
}
$ clang -target -arm-none-linux-gnueabi reroll.cpp -S -O1 -emit-llvm -o
test.ll
$ opt -loop-reroll -S test.ll -debug-only=loop-reroll -stats
This is probably because as the array accesses are not a function of the loop
induction variable, which is the loop body form the reroller pass requires.
$ cat test.ll
..
..
for.body: ; preds = %entry, %for.body
%i.09 = phi i32 [ %add, %for.body ], [ 0, %entry ]
%Arr.addr.08 = phi i32* [ %add.ptr, %for.body ], [ %Arr, %entry ]
store i32 0, i32* %Arr.addr.08, align 4, !tbaa !1
..
..
The following pattern is successfully rerolled as it satisfies the requirement,
void foo(int *Arr, int n) {
for (int i = 0; i < n; i += 3) {
Arr[i]=0;
Arr[i+1]=0;
Arr[i+2]=0;
}
}</pre>
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