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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Loop Vectorizer Generates Unreachable Fast Path"
href="https://bugs.llvm.org/show_bug.cgi?id=47371">47371</a>
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<th>Summary</th>
<td>Loop Vectorizer Generates Unreachable Fast Path
</td>
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<th>Product</th>
<td>tools
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>llc
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>thoren.paulson@gmail.com
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Overview:
I found a simple example that the loop vectorizer tries to optimize, but
generates vector code that's unreachable due to contradicting conditions.
Steps to Reproduce:
C++ Example, compiled with `clang -O2` targeting x86_64, see
<a href="https://godbolt.org/z/5x9WTb">https://godbolt.org/z/5x9WTb</a>
```
#include <cstddef>
void pan(float* samples, size_t len, float coef) {
float c[4] = { 1.0f - coef, coef, 1.0f - coef, coef };
for (size_t i = 0; i < len; i++) {
samples[i] *= c[i % 4];
}
}
```
Actual results:
See <a href="https://godbolt.org/z/5x9WTb">https://godbolt.org/z/5x9WTb</a> or compile the example for assembly output,
but here's the part that seems contradictory:
```
test rsi, rsi
je .LBB0_14 # jump if len == 0
xor eax, eax
cmp rsi, 8
jb .LBB0_3 # jump if len < 8
lea rcx, [rsi - 1]
cmp rcx, 4
jae .LBB0_3 # jump if len >= 5
```
`.LBB0_3` is the non-vectorized clean up path, and `rsi` hold `len`. This code
will always go to the cleanup loop instead of falling through to the vectorized
code, since `len` cannot be greater than 8 but less than 5.
Expected results:
Any `len` greater than 8 (or some other appropriate value) should execute the
`mulps` instructions until the remainder is less than the vectorization width.
Additional information:
I originally discovered this in Rust in this form: <a href="https://godbolt.org/z/6M7Pon">https://godbolt.org/z/6M7Pon</a>
This leads me to believe its a backend issue and not clang and rustc.</pre>
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