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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Avoid reloads after vectorizer's runtime alias check"
href="https://bugs.llvm.org/show_bug.cgi?id=43111">43111</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Avoid reloads after vectorizer's runtime alias check
</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>enhancement
</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>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider loop:
int foo(int *p, int *q) {
int s = 0;
for (int i = 0; i < 256; ++i) {
int k = p[i];
q[i] = 2*k;
s+= k;
int o = p[i];
s+= o;
}
return s;
}
Flags: clang -fno-unroll-loops -O3 loop.c -mavx2
In entry block there is a runtime check to check whether pointers do not alias.
entry:
%scevgep = getelementptr i32, i32* %q, i64 256
%scevgep23 = getelementptr i32, i32* %p, i64 256
%bound0 = icmp ugt i32* %scevgep23, %q
%bound1 = icmp ugt i32* %scevgep, %p
%found.conflict = and i1 %bound0, %bound1
br i1 %found.conflict, label %for.body, label %vector.body
Then, in loop body, we know there is no aliasing.
vector.body: ; preds = %entry,
%vector.body
%index = phi i64 [ %index.next, %vector.body ], [ 0, %entry ]
%vec.phi = phi <8 x i32> [ %7, %vector.body ], [ zeroinitializer, %entry ]
%0 = getelementptr inbounds i32, i32* %p, i64 %index
%1 = bitcast i32* %0 to <8 x i32>*
%wide.load = load <8 x i32>, <8 x i32>* %1, align 4, !tbaa !2, !alias.scope
!6
%2 = shl nsw <8 x i32> %wide.load, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1,
i32 1, i32 1>
%3 = getelementptr inbounds i32, i32* %q, i64 %index
%4 = bitcast i32* %3 to <8 x i32>*
store <8 x i32> %2, <8 x i32>* %4, align 4, !tbaa !2, !alias.scope !9,
!noalias !6
%5 = add nsw <8 x i32> %wide.load, %vec.phi
%6 = bitcast i32* %0 to <8 x i32>*
%wide.load25 = load <8 x i32>, <8 x i32>* %6, align 4, !tbaa !2, !alias.scope
!6
...
But we still reload p[i] '%wide.load25'..
Expected IR:
vector.body: ; preds = %entry,
%vector.body
%index = phi i64 [ %index.next, %vector.body ], [ 0, %entry ]
%vec.phi = phi <8 x i32> [ %5, %vector.body ], [ zeroinitializer, %entry ]
%0 = getelementptr inbounds i32, i32* %p, i64 %index
%1 = bitcast i32* %0 to <8 x i32>*
%wide.load = load <8 x i32>, <8 x i32>* %1, align 4, !tbaa !2, !alias.scope
!6
%2 = shl <8 x i32> %wide.load, <i32 1, i32 1, i32 1, i32 1, i32 1, i32 1, i32
1, i32 1>
%3 = getelementptr inbounds i32, i32* %q, i64 %index
%4 = bitcast i32* %3 to <8 x i32>*
store <8 x i32> %2, <8 x i32>* %4, align 4, !tbaa !2, !alias.scope !9,
!noalias !6
...</pre>
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