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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Convert mov and shr to shrx in loops constrained by retirement rate"
href="https://bugs.llvm.org/show_bug.cgi?id=51288">51288</a>
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<th>Summary</th>
<td>Convert mov and shr to shrx in loops constrained by retirement rate
</td>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>12.0
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</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>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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</tr>
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<th>Reporter</th>
<td>todd@lipcon.org
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>This input file:
#include <stdint.h>
#include <utility>
struct Foo {
uint64_t v;
std::pair<uint32_t, uint32_t> Get() { return {v & 0xffffffff, v >> 32}; }
};
void Process(Foo* f, uint32_t* dst, int n) {
#pragma unroll
for (int i = 0; i < n; i++) {
auto [mask, idx] = f[i].Get();
dst[idx] |= mask;
}
}
Generates some assembly where the core of the loop has the following sequence:
movq 24(%rdi,%rax,8), %r9
movq %r9, %rcx
shrq $32, %rcx
orl %r9d, (%rsi,%rcx,4)
When compiling with bmi2 support, it would instead be slightly faster to store
the constant 32 into a register and use shrx to combine the copy of %r9 into
%rcx with a shift.
Generated version:
<a href="https://bit.ly/2WzH8Pj">https://bit.ly/2WzH8Pj</a>
Preferred version (~saving half a cycle per unrolled-by-4 loop):
<a href="https://bit.ly/3jaXBBh">https://bit.ly/3jaXBBh</a></pre>
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