https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67579
Bug ID: 67579
Summary: [concepts] Memoization for constraint expressions
Product: gcc
Version: 6.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: Casey at Carter dot net
Target Milestone: ---
While implementing the Ranges TS I've found that the implementation of concepts
in GCC scales very poorly to larger and more complex systems. I've been able to
achieve 1-2 order of magnitude improvements in compile time and memory usage by
hand coding memoization for some concepts with constexpr variable templates.
E.g., replacing
template </* parameters */>
concept bool Foo = // requirements
with
template </* parameters */>
constexpr bool Foo_ = false;
template </* parameters */>
requires // requirements
constexpr bool Foo_</* parameter names */> = true;
template </* parameters */>
concept bool Foo = Foo_</* parameter names */>;
which is fine when Foo need not participate in subsumption relationships. If
Foo *does* need to participate in subsumption relationships then performing
this transformation by hand is not possible since it hides the relationship
between Foo and "requirements" from the compiler's view.
If concepts & constraint expressions are to be generally applicable the
implementation must provide some means of reducing the cost of repeated
evaluation.
