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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - LLVM generates terrible x86 code for trivial, fully unrolled loops"
href="https://llvm.org/bugs/show_bug.cgi?id=28090">28090</a>
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<tr>
<th>Summary</th>
<td>LLVM generates terrible x86 code for trivial, fully unrolled loops
</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>
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<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chandlerc@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider this code:
----
struct V {
static constexpr int length = 32;
unsigned short data[32];
};
int reduce(V &v) {
int sum = 0;
for (int i = 0; i < v.length; ++i) {
sum += static_cast<int>(v.data[i]);
}
return sum;
}
----
If the length weren't a constant, LLVM would do a delightful job of vectorizing
the reduction loop. But because it happens to be a constant trip count, we
fully unroll the loop and generate this mess:
----
% ./bin/clang++ -std=c++1z -c -S -o - -O2 -march=haswell x.cpp
.text
.file "x.cpp"
.globl _Z6reduceR1V
.p2align 4, 0x90
.type _Z6reduceR1V,@function
_Z6reduceR1V: # @_Z6reduceR1V
.cfi_startproc
# BB#0: # %entry
movzwl (%rdi), %eax
movzwl 2(%rdi), %ecx
addl %eax, %ecx
movzwl 4(%rdi), %eax
addl %ecx, %eax
....
; repeat OVER AND OVER AGAIN with minor variations in registers...
....
movzwl 60(%rdi), %edx
addl %ecx, %edx
movzwl 62(%rdi), %eax
addl %edx, %eax
retq
----
Ow. This hurts code size as well. =/ I figure we need reduction support in the
SLP vectorizer or some such?</pre>
</div>
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