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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 - Exorbitant compile times"
href="https://bugs.llvm.org/show_bug.cgi?id=39353">39353</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Exorbitant compile times
</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>lumosimann@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>dgregor@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I was just experimenting a bit and realized that cache might miss some cache
opportunities:
```
#include <iostream>
#include <tuple>
#include <type_traits>
template <size_t I0, size_t... I>
struct Recursion {
using type = std::tuple<
typename Recursion<I0 - 1, I - 1, I - 1, I - 1, I - 1>::type...>;
};
template <size_t... I>
struct Recursion<0, I...> {
using type = std::integral_constant<int, 0>;
};
template <std::size_t I, typename T>
struct SomeWrapper {
using type = std::tuple_element_t<0, T>;
};
template <size_t I>
using ComplexType = typename Recursion<I, I, I, I, I>::type;
struct SimpleType {
using type = int;
};
template <typename T>
struct Concept : std::bool_constant<std::tuple_size_v<typename T::type> != 4> {
};
template <typename T>
std::enable_if_t<Concept<T>::value, int> g(T) {
return 3;
}
template <typename T>
std::enable_if_t<!Concept<T>::value, int> g(T) {
return 3;
}
template <std::size_t... Is>
int f(std::index_sequence<Is...>) {
return (... + g(SomeWrapper<Is, ComplexType<9>>{}));
}
int main() {
std::cout << f(std::make_index_sequence<200>()) << std::endl;
return 0;
}
```
Compilation time for this code scales linearly with Clang (linear in terms
ofthe number in `make_index_sequence`), but remains constant for gcc.
The whole thing is just setting up a huge type that is used in SFiNAE for
evaluating g. It looks like Concept<T> is not cached during consecutive calls
to g.</pre>
</div>
</p>
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