<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missing optimization: suboptimal code without constexpr"
href="https://bugs.llvm.org/show_bug.cgi?id=37415">37415</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missing optimization: suboptimal code without constexpr
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</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>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>antoshkka@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following code snippet:
// Bubble-like sort. Anything complex enough will work
template <class It>
constexpr void sort(It first, It last) {
for (;first != last; ++first) {
auto it = first;
++it;
for (; it != last; ++it) {
if (*it < *first) {
auto tmp = *it;
*it = *first;
*first = tmp;
}
}
}
}
constexpr int generate() {
int a[7] = {3, 7, 4, 2, 8, 0, 1};
sort(a + 0, a + 7);
return a[0] + a[6];
}
int no_constexpr() {
return generate();
}
int with_constexpr() {
constexpr auto res = generate();
return res;
}
Above code generates ~150 assembly instructions for `no_constexpr()` function
and just 2 instructions for `with_constexpr()`:
with_constexpr(): # @with_constexpr()
mov eax, 8
ret
Could the compiler detect that `a[7]` holds values known at compile time and
force the constexpr on `sort(a + 0, a + 7);` and `generate()` evaluations?</pre>
</div>
</p>
<hr>
<span>You are receiving this mail because:</span>
<ul>
<li>You are on the CC list for the bug.</li>
</ul>
</body>
</html>