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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - suboptimal loop codegen?"
href="https://bugs.llvm.org/show_bug.cgi?id=35016">35016</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>suboptimal loop codegen?
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>trass3r@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <stdint.h>
uint8_t data[32] = { 1,2,3,4 };
int main(int argc, const char* argv[])
{
int sum = 0;
for (int i=0; i < argc; ++i)
#if 0
sum += ((uint32_t*)data)[2 + i];
#else
sum += *(uint32_t*)&data[8 + i * 4];
#endif
return sum;
}
<a href="https://godbolt.org/g/sJTGjz">https://godbolt.org/g/sJTGjz</a>
$ clang -c -Oz
main: # @main
movsxd rcx, edi
xor edx, edx
push 8
pop rsi
xor eax, eax
jmp .LBB0_1
.LBB0_3: # in Loop: Header=BB0_1 Depth=1
movsxd rsi, esi
add eax, dword ptr [rsi + data]
inc rdx
add esi, 4
.LBB0_1: # =>This Inner Loop Header: Depth=1
cmp rdx, rcx
jl .LBB0_3
ret
data:
.asciz
"\001\002\003\004\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000\000"
$ g++ -c -Os
main:
xor edx, edx
xor eax, eax
.L3:
cmp edi, edx
jle .L1
add eax, DWORD PTR data[8+rdx*4]
inc rdx
jmp .L3
.L1:
ret
data:
.byte 1
.byte 2
.byte 3
.byte 4
.zero 28</pre>
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