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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Failure to optimize range loops"
href="https://bugs.llvm.org/show_bug.cgi?id=34538">34538</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Failure to optimize range loops
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>gonzalobg88@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Given the following modern C++ range-like code using optionals (see it here in
action: <a href="https://godbolt.org/g/Lz1xNK">https://godbolt.org/g/Lz1xNK</a>):
struct range { int start; int end; };
struct option { int ret; int tag; };
inline
option next(range *range) {
if (range->start < range->end) {
int n = range->start;
range->start++;
option o{n, 1};
return o;
} else {
option o{0, 0};
return o;
}
}
int loop() {
range rng = {0, 100};
for(;;) {
option o = next(&rng);
if (o.tag) {
// nothing
} else {
break;
}
}
return 0;
}
GCC generates the following assembly:
loop():
xor eax, eax
ret
while clang fails to optimize the loop, generating:
loop(): # @loop()
xor eax, eax
.LBB0_1: # =>This Inner Loop Header: Depth=1
xor ecx, ecx
cmp eax, 100
setl cl
add ecx, eax
cmp eax, 100
mov eax, ecx
jl .LBB0_1
xor eax, eax
ret</pre>
</div>
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