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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Optimization for removing same variable comparisons in loop: while(it != end1 && it != end2)"
href="https://bugs.llvm.org/show_bug.cgi?id=33813">33813</a>
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<th>Summary</th>
<td>Optimization for removing same variable comparisons in loop: while(it != end1 && it != end2)
</td>
</tr>
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<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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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>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<p>
<div>
<pre>Simple iteration by std::deque elements produces suboptimal code. For example
#include <deque>
unsigned sum(std::deque<unsigned> cont) {
unsigned sum = 0;
for (unsigned v : cont)
sum += v;
return sum;
}
produces the following loop:
.LBB0_1: # =>This Inner Loop Header: Depth=1
cmp rsi, rcx
je .LBB0_4
add eax, dword ptr [rcx]
add rcx, 4
cmp rcx, rdx
jne .LBB0_1
jmp .LBB0_3
The loop has two comparisons in it and behaves close to the following C code:
unsigned sum_like_deque_does(unsigned** chunks, unsigned* end) {
unsigned sum = 0;
for (unsigned* it = *chunks; it != end; it = *(++chunks)) {
for (;it != end && it != *chunks + 128; ++it) {
sum += *it;
}
}
return sum;
}
Note the `it != end && it != *chunks + 128` condition. It could be simplified:
if `end` belongs to `[it, *chunks + 128]` change the condition to `it != end`
and use the condition `it != *chunks + 128` otherwise. Such optimization
removes the cmp from the loop and produces a much more faster loop:
.LBB2_3: # Parent Loop BB2_2 Depth=1
add eax, dword ptr [rcx]
add rcx, 4
cmp rdx, rcx
jne .LBB2_3
Synthetic tests show up to 2 times better performance. Assembly outputs:
<a href="https://godbolt.org/g/vGs2qs">https://godbolt.org/g/vGs2qs</a></pre>
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