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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Possible missed optimization: relaxed stores to narrow atomic types are not coalesced"
href="https://bugs.llvm.org/show_bug.cgi?id=51263">51263</a>
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<th>Summary</th>
<td>Possible missed optimization: relaxed stores to narrow atomic types are not coalesced
</td>
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<tr>
<th>Product</th>
<td>new-bugs
</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>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>eric.mueller1024@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<pre>The following code (<a href="https://godbolt.org/z/fcqexoYMe">https://godbolt.org/z/fcqexoYMe</a>):
#include <cstddef>
#include <atomic>
struct normal_storage
{
uint8_t a[8];
};
struct atomic_storage
{
std::atomic<uint8_t> a[8];
};
void f(normal_storage * ns, atomic_storage * as)
{
for (size_t i = 0; i < 8; ++i) {
as->a[i].store(ns->a[i], std::memory_order_relaxed);
}
}
generates 8 single-byte loads and stores when compiled for x86_64 with -O3 with
clang trunk.
f(normal_storage*, atomic_storage*): # @f(normal_storage*, atomic_storage*)
mov al, byte ptr [rdi]
mov byte ptr [rsi], al
mov al, byte ptr [rdi + 1]
mov byte ptr [rsi + 1], al
mov al, byte ptr [rdi + 2]
mov byte ptr [rsi + 2], al
mov al, byte ptr [rdi + 3]
mov byte ptr [rsi + 3], al
mov al, byte ptr [rdi + 4]
mov byte ptr [rsi + 4], al
mov al, byte ptr [rdi + 5]
mov byte ptr [rsi + 5], al
mov al, byte ptr [rdi + 6]
mov byte ptr [rsi + 6], al
mov al, byte ptr [rdi + 7]
mov byte ptr [rsi + 7], al
ret
Given that relaxed atomics provide no guarantees about ordering, it seems to me
like it may be legal to coalesce this into a single 8-byte store instead.
Is this a missed optimization? Or is there some specific language in the
standard that makes such an optimization illegal?
For what it's worth, from what I can tell it looks like none of GCC, ICC, nor
MSVC make this optimization.
Clearly the above code could be changed to use a single 8-byte atomic instead,
but that's somewhat besides the point. I can provide more context on why this
is useful if it helps.</pre>
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