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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - loop unrolling fails to properly optimize the remainder"
href="https://bugs.llvm.org/show_bug.cgi?id=42900">42900</a>
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<th>Summary</th>
<td>loop unrolling fails to properly optimize the remainder
</td>
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<th>Product</th>
<td>clang
</td>
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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>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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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
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<th>Reporter</th>
<td>mschwar42@gmail.com
</td>
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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>
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<pre>when telling clang to unroll a loop using the pragma unroll, one would expect
the remainder (i.e. N % unroll_factor) to be taken care of outside of the
unrolled loop. Instead clang decides to include this check in each chunk of the
unrolled loop.
int cmp(const char *s1, const char *s2, size_t n){
unsigned char c1 = '\0';
unsigned char c2 = '\0';
#pragma unroll 4
for(size_t i = 0; i < n; i++){
c1 = (unsigned char) *s1++;
c2 = (unsigned char) *s2++;
if (c1 == '\0' || c1 != c2) return c1 - c2;
}
return c1 - c2;
}
produces chunks like this:
movzx r11d, byte ptr [rdi + rax]
movzx ecx, byte ptr [rsi + rax]
test r11b, r11b
je .LBB0_10
cmp r11b, cl
jne .LBB0_10
cmp r10, rax
je .LBB0_6
instead of this:
movzx eax, BYTE PTR [rdi+rcx]
movzx r8d, BYTE PTR [rsi+rcx]
test al, al
je .L32
cmp al, r8b
jne .L32
comparison between gcc and clang:
<a href="https://godbolt.org/z/ZTADbh">https://godbolt.org/z/ZTADbh</a></pre>
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