https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67565
Bug ID: 67565
Summary: [concepts] Very slow compile time and high memory
usage with complex concept definitions, even if unused
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: adrian.wielgosik at gmail dot com
Target Milestone: ---
Example:
template <class T>
concept bool Constructible() {
return false ||
requires (T t) {
{ t.~T() } noexcept;
};
}
template <class T>
concept bool Semiregular() {
return
Constructible<T>() &&
Constructible<T>() &&
Constructible<T>() &&
Constructible<T>() &&
Constructible<T>() &&
Constructible<T>() &&
Constructible<T>();
}
template <class T, class I>
concept bool Sentinel() {
return Semiregular<T>() && Semiregular<I>();
}
template<Semiregular I, Sentinel<I> S>
void func(I first, S last) { }
int main(){}
time ./g++trunk -std=c++1z main.cpp
real 0m8.855s
user 0m7.887s
sys 0m0.896s
It also consumed over 2GB of RAM at peak. Note that the function which checks
for concepts wasn't even called, so no actual concept checks were done.
Simplifying this case any more, like removing "false ||", fixes the problem.
A big chain of "Constructible<T>() &&" may look strange, but it's just a
simplified case - originally I tried to rewrite concepts from Eric Niebler's
Ranges proposal, which could check the same concept multiple times.
