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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [SIMD] Compiler unable to vectorize trivial loops"
href="https://bugs.llvm.org/show_bug.cgi?id=50803">50803</a>
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<th>Summary</th>
<td>[SIMD] Compiler unable to vectorize trivial loops
</td>
</tr>
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<th>Product</th>
<td>libraries
</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>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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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>Backend: WebAssembly
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>clang@evan.coeusgroup.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Here is an example (or on Compiler Explorer: <a href="https://godbolt.org/z/5Was18P7r">https://godbolt.org/z/5Was18P7r</a>):
#include <stdint.h>
typedef int32_t i32x4 __attribute__((__vector_size__(16)));
i32x4 add(i32x4 a, i32x4 b);
i32x4
add(i32x4 a, i32x4 b) {
i32x4 r;
#pragma clang loop vectorize(enable)
for (int i = 0 ; i < 4 ; i++) {
r[i] = a[i] + b[i];
}
return r;
}
Compile with -msimd128 -O3, no vectorization. The scalar version seems pretty
bad, too.
Even with the pragma the compiler won't vectorize the loop. I'm wondering if
this might have something to do with the -Wconditional-uninitialized diagnostic
emitted when returning r (it's not enabled by default, -Weverything or
-Wconditional-uninitialized to see it).</pre>
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