https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102780
Bug ID: 102780
Summary: Checking constraints using large fold expression is
slow
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Keywords: compile-time-hog
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
template<int I> struct S { };
template<typename T, T...> struct integer_sequence { };
template<typename T, T N>
using make_integer_sequence
#if __has_builtin(__make_integer_seq)
= __make_integer_seq<integer_sequence, T, N>;
#else
= integer_sequence<T, __integer_pack(N)...>;
#endif
template<typename... _Types>
concept trivially_destructible
= (__has_trivial_destructor(_Types) && ...);
template<typename...> union variadic_union { static constexpr int size = 0; };
template<typename T, typename... U>
union variadic_union<T, U...>
{
~variadic_union() = default;
#ifndef TRIVIAL_ONLY
// Conditionally non-trivial dtor, if required.
constexpr ~variadic_union() requires (!trivially_destructible<T, U...>)
{ }
#endif
T first;
variadic_union<U...> rest;
static constexpr int size = variadic_union<U...>::size + 1;
};
template <int... Is>
void f_impl(integer_sequence<int, Is...>)
{
using V = variadic_union<S<Is>...>;
// cause instantiation of V:
static_assert( V::size == sizeof...(Is) );
}
template <int I>
void f()
{
f_impl(make_integer_sequence<int, I>());
}
int main()
{
f<254>();
f<255>();
f<256>();
}
Compiled with -std=gnu++20 -ftime-report I get:
TOTAL : 10.46 3.58 14.08 78M
Adding -fno-checking helps a little:
TOTAL : 7.71 3.70 11.43 78M
Clang compiles this in under a second.
Full details:
> Time variable usr sys
> wall GGC
> phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.01 (
> 0%) 1589k ( 2%)
> phase lang. deferred : 7.88 ( 75%) 3.68 (100%) 11.62 (
> 82%) 69M ( 88%)
> phase opt and generate : 2.59 ( 25%) 0.00 ( 0%) 2.60 (
> 18%) 7297k ( 9%)
> |name lookup : 0.05 ( 0%) 0.02 ( 1%) 0.02 (
> 0%) 670k ( 1%)
> |overload resolution : 7.35 ( 70%) 3.51 ( 95%) 10.91 (
> 77%) 10M ( 13%)
> garbage collection : 0.04 ( 0%) 0.00 ( 0%) 0.04 (
> 0%) 0 ( 0%)
> callgraph construction : 2.56 ( 24%) 0.00 ( 0%) 2.58 (
> 18%) 7016k ( 9%)
> callgraph ipa passes : 0.02 ( 0%) 0.00 ( 0%) 0.01 (
> 0%) 15k ( 0%)
> CFG verifier : 0.01 ( 0%) 0.00 ( 0%) 0.00 (
> 0%) 0 ( 0%)
> template instantiation : 0.25 ( 2%) 0.11 ( 3%) 0.37 (
> 3%) 43M ( 56%)
> constant expression evaluation : 4.36 ( 42%) 2.02 ( 55%) 6.61 (
> 46%) 72 ( 0%)
> constraint satisfaction : 3.20 ( 31%) 1.55 ( 42%) 4.57 (
> 32%) 20M ( 25%)
> symout : 0.05 ( 0%) 0.00 ( 0%) 0.04 (
> 0%) 5726k ( 7%)
> initialize rtl : 0.00 ( 0%) 0.00 ( 0%) 0.01 (
> 0%) 12k ( 0%)
> TOTAL : 10.47 3.68 14.23
> 78M
> Extra diagnostic checks enabled; compiler may run slowly.
> Configure with --enable-checking=release to disable checks.
Defining -DTRIVIAL_ONLY makes it compile in under 2s, and 0.4s with
-DTRIVIAL_ONLY -fno-checking (almost as fast as clang)
The constraint satisfaction seems to be the problem, even though "template
instantiation" shows up as a larger percentage in the time report.
