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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 - Missing optimization opportunity: non-power-of-two integer loading with fewer movs"
href="https://bugs.llvm.org/show_bug.cgi?id=41216">41216</a>
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<th>Summary</th>
<td>Missing optimization opportunity: non-power-of-two integer loading with fewer movs
</td>
</tr>
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<th>Product</th>
<td>clang
</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>Windows NT
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Keywords</th>
<td>code-quality
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>nok.raven@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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</tr></table>
<p>
<div>
<pre>Currently non-power-of-two integers loads are done byte-per-byte:
#include <cstdint>
// could be two loads instead of three: (u16 << 8) | u8
std::uint32_t foo_24(unsigned char const* p)
{
return static_cast<std::uint32_t>(p[0])
| (static_cast<std::uint32_t>(p[1]) << 8)
| (static_cast<std::uint32_t>(p[2]) << 16)
;
}
// could be two loads instead of five: (u32 << 8) | u8
std::uint64_t foo_40(unsigned char const* p)
{
return static_cast<std::uint64_t>(p[0])
| (static_cast<std::uint64_t>(p[1]) << 8)
| (static_cast<std::uint64_t>(p[2]) << 16)
| (static_cast<std::uint64_t>(p[3]) << 24)
| (static_cast<std::uint64_t>(p[4]) << 32)
;
}
// could be two loads instead of six: (u32 << 16) | u16
std::uint64_t foo_48(unsigned char const* p)
{
return static_cast<std::uint64_t>(p[0])
| (static_cast<std::uint64_t>(p[1]) << 8)
| (static_cast<std::uint64_t>(p[2]) << 16)
| (static_cast<std::uint64_t>(p[3]) << 24)
| (static_cast<std::uint64_t>(p[4]) << 32)
| (static_cast<std::uint64_t>(p[5]) << 40)
;
}
// could be three loads instead of seven: (u32 << 24) | (u16 << 8) | u8
std::uint64_t foo_56(unsigned char const* p)
{
return static_cast<std::uint64_t>(p[0])
| (static_cast<std::uint64_t>(p[1]) << 8)
| (static_cast<std::uint64_t>(p[2]) << 16)
| (static_cast<std::uint64_t>(p[3]) << 24)
| (static_cast<std::uint64_t>(p[4]) << 32)
| (static_cast<std::uint64_t>(p[5]) << 40)
| (static_cast<std::uint64_t>(p[6]) << 48)
;
}
<a href="https://godbolt.org/z/Re7dWL">https://godbolt.org/z/Re7dWL</a>
GCC produces better code (however currently it optimizes only 32bit loads
<a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89809">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89809</a>)</pre>
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