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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Early loop unrolling causes missed SLP vectorization"
href="https://bugs.llvm.org/show_bug.cgi?id=50552">50552</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Early loop unrolling causes missed SLP vectorization
</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>Scalar Optimizations
</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>#include <stdint.h>
#include <cstddef>
#define STRIPE_WIDTH 32
#define STRIPE_MASK (STRIPE_WIDTH - 1)
static int16_t dither_line[2 * STRIPE_WIDTH] = {
#if STRIPE_WIDTH > 8
8, 40, 8, 40, 8, 40, 8, 40, 8, 40, 8, 40, 8, 40, 8, 40,
56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24, 56, 24,
#else
8, 40, 8, 40, 8, 40, 8, 40, 56, 24, 56, 24, 56, 24, 56, 24,
#endif
};
void ass_stripe_pack_c(uint8_t *dst, ptrdiff_t dst_stride, const int16_t *src,
uintptr_t width, uintptr_t height) {
uint8_t *ptr = dst;
for (uintptr_t y = 0; y < height; ++y) {
const int16_t *dither = dither_line + (y & 1) * STRIPE_WIDTH;
for (int k = 0; k < STRIPE_WIDTH; ++k)
ptr[k] = (uint16_t)(src[k] - (src[k] >> 8) + dither[k]) >> 6;
ptr += dst_stride;
src += STRIPE_WIDTH;
}
dst += STRIPE_WIDTH;
}
-O3 -fno-unroll-loops: Vectorized by Loop vectorizer
-O3: No vectorization by SLP
Current codegen:
<a href="https://godbolt.org/z/ocxbjx9ss">https://godbolt.org/z/ocxbjx9ss</a></pre>
</div>
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