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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 - Missed optimization: store coalescing"
href="https://bugs.llvm.org/show_bug.cgi?id=41761">41761</a>
</td>
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<th>Summary</th>
<td>Missed optimization: store coalescing
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<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>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>costan@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>LLVM coalesces consecutive byte loads into larger loads when appropriate, but
doesn't do so for stores. This is unfortunate -- having these optimizations
would allow me to write platform-independent code for loading/storing
{little,big}-endian integers. Without the optimizations, I need to write an
explicit fast path for little-endian platforms.
The details below are also in the Godbolt example at
<a href="https://godbolt.org/z/45S0ID">https://godbolt.org/z/45S0ID</a>
The following functions get optimized to single mov instructions on x864_64.
uint32_t DecodeFixed32(const char* ptr) noexcept {
const uint8_t* buffer = reinterpret_cast<const uint8_t*>(ptr);
return ((static_cast<uint32_t>(buffer[0])) |
(static_cast<uint32_t>(buffer[1]) << 8) |
(static_cast<uint32_t>(buffer[2]) << 16) |
(static_cast<uint32_t>(buffer[3]) << 24));
}
uint64_t DecodeFixed64(const char* ptr) noexcept {
const uint8_t* buffer = reinterpret_cast<const uint8_t*>(ptr);
return ((static_cast<uint64_t>(buffer[0])) |
(static_cast<uint64_t>(buffer[1]) << 8) |
(static_cast<uint64_t>(buffer[2]) << 16) |
(static_cast<uint64_t>(buffer[3]) << 24) |
(static_cast<uint64_t>(buffer[4]) << 32) |
(static_cast<uint64_t>(buffer[5]) << 40) |
(static_cast<uint64_t>(buffer[6]) << 48) |
(static_cast<uint64_t>(buffer[7]) << 56));
}
However, the following functions do not get optimized to mov instructions.
void EncodeFixed32(char* dst, uint32_t value) noexcept {
uint8_t* buffer = reinterpret_cast<uint8_t*>(dst);
buffer[0] = static_cast<uint8_t>(value);
buffer[1] = static_cast<uint8_t>(value >> 8);
buffer[2] = static_cast<uint8_t>(value >> 16);
buffer[3] = static_cast<uint8_t>(value >> 24);
}
void EncodeFixed64(char* dst, uint64_t value) noexcept {
uint8_t* buffer = reinterpret_cast<uint8_t*>(dst);
buffer[0] = static_cast<uint8_t>(value);
buffer[1] = static_cast<uint8_t>(value >> 8);
buffer[2] = static_cast<uint8_t>(value >> 16);
buffer[3] = static_cast<uint8_t>(value >> 24);
buffer[4] = static_cast<uint8_t>(value >> 32);
buffer[5] = static_cast<uint8_t>(value >> 40);
buffer[6] = static_cast<uint8_t>(value >> 48);
buffer[7] = static_cast<uint8_t>(value >> 56);
}
GCC versions 8 and above optimize stores. Lower versions only optimize loads.
MSVC does not optimize loads or stores.</pre>
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