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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 - Missed vectorization for std::reverse"
href="https://bugs.llvm.org/show_bug.cgi?id=43664">43664</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed vectorization for std::reverse
</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>#include <algorithm>
#include <vector>
#include <cstdint>
void reverse_epi8(std::vector<int32_t>&v) {
std::reverse(v.begin(),v.begin()+32);
}
void reverse_epi32(std::vector<int32_t>& v) {
std::reverse(v.begin(),v.begin()+32);
}
Clang does not vectorize std::reverse with -O3 -march=haswell -std=c++11.
reverse_epi8(std::vector<int, std::allocator<int> >&): #
@reverse_epi8(std::vector<int, std::allocator<int> >&)
mov rax, qword ptr [rdi]
lea rcx, [rax + 124]
add rax, 4
.LBB0_1: # =>This Inner Loop Header: Depth=1
mov edx, dword ptr [rax - 4]
mov esi, dword ptr [rcx]
mov dword ptr [rax - 4], esi
mov dword ptr [rcx], edx
add rcx, -4
cmp rax, rcx
lea rax, [rax + 4]
jb .LBB0_1
ret
GCC 8+ vectorizes it.
reverse_epi8(std::vector<int, std::allocator<int> >&):
mov rax, QWORD PTR [rdi]
vmovdqa ymm0, YMMWORD PTR .LC0[rip]
vmovdqu ymm1, YMMWORD PTR [rax]
vpermd ymm2, ymm0, YMMWORD PTR [rax+96]
vpermd ymm1, ymm0, ymm1
vmovdqu YMMWORD PTR [rax], ymm2
vpermd ymm2, ymm0, YMMWORD PTR [rax+64]
vmovdqu YMMWORD PTR [rax+96], ymm1
vmovdqu ymm1, YMMWORD PTR [rax+32]
vmovdqu YMMWORD PTR [rax+32], ymm2
vpermd ymm0, ymm0, ymm1
vmovdqu YMMWORD PTR [rax+64], ymm0
vzeroupper
ret
<a href="https://godbolt.org/z/gJkt3c">https://godbolt.org/z/gJkt3c</a>
Another issue, with std::reverse(v.begin(), v.begin()+2), there is really bad
codegen:
<a href="https://godbolt.org/z/D-k-SA">https://godbolt.org/z/D-k-SA</a></pre>
</div>
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