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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 - [SIMD opt of checksum func] Smart, but could be smarter"
href="https://bugs.llvm.org/show_bug.cgi?id=42674">42674</a>
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<th>Summary</th>
<td>[SIMD opt of checksum func] Smart, but could be smarter
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</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>LLVM Codegen
</td>
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
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<th>Reporter</th>
<td>bisqwit@iki.fi
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>#include <string.h>
unsigned char calculate_checksum(const void* ptr)
{
unsigned char bytes[16], result = 0;
memcpy(bytes, ptr, 16); // Endianess does not matter.
for(unsigned n=0; n<16; ++n) result += bytes[n];
return result;
}
For this code, Clang (generates the following SIMD code, which is not too bad.
vmovdqu xmm0, xmmword ptr [rdi]
vmovdqa xmmword ptr [rsp - 24], xmm0
vpshufd xmm1, xmm0, 78 # xmm1 = xmm0[2,3,0,1]
vpaddb xmm0, xmm0, xmm1
vpshufd xmm1, xmm0, 229 # xmm1 = xmm0[1,1,2,3]
vpaddb xmm0, xmm0, xmm1
vpsrld xmm1, xmm0, 16
vpaddb xmm0, xmm0, xmm1
vpsrlw xmm1, xmm0, 8
vpaddb xmm0, xmm0, xmm1
vpextrb eax, xmm0, 0
ret
But here is what ICC (version 18 or newer) generates for it.
vmovups xmm0, XMMWORD PTR [rdi] #5.9
vpxor xmm2, xmm2, xmm2 #4.41
vpaddb xmm0, xmm2, xmm0 #4.41
vpsrldq xmm1, xmm0, 8 #4.41
vpaddb xmm3, xmm0, xmm1 #4.41
vpsadbw xmm4, xmm2, xmm3 #4.41
vmovd eax, xmm4 #4.41
movsx rax, al #4.41
ret #7.16
Hence the title of this report: Smart, but could be smarter.
For the record, both GCC and MSVC generates almost identical code as Clang for
this testcase. They use VPSRLDQ instead of VPSHUFD rthough. Compiler Explorer
link: <a href="https://godbolt.org/z/hA4J94">https://godbolt.org/z/hA4J94</a></pre>
</div>
</p>
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