https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109678
Bug ID: 109678
Summary: exponential time behavior on std::variant
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: quentin.sabah at gmail dot com
Target Milestone: ---
Hi,
I observe an exponential time behavior while compiling an std::variant with an
increasing number of alternatives (see testcase below).
It seems to be a regression in gcc 13 of the constant expression evaluation
while compiling "constexpr std::_Enable_default_constructor".
Found with gcc 13 shipped with Fedora 38.
Reproduced on Compiler Explorer:
Compiles in 20 seconds with gcc 13.1: https://godbolt.org/z/hhWx51sds
Compiles in a hundreds of milliseconds with gcc 12.2 and clang 16.
Testcase:
//=======================
#include <variant>
struct A {};
struct B {};
struct C {};
struct D {};
struct E {};
struct F {};
struct G {};
struct H {};
struct I {};
struct J {};
struct K {};
struct L {};
struct M {};
struct N {};
struct O {};
struct P {};
struct Q {};
struct R {};
struct S {};
struct T {};
struct U {};
struct V {};
struct W {
// gcc13 + compiler explorer = 20000ms
// gcc12.2 + compiler explorer = 400ms
int i;
};
struct X {};
struct Y {};
struct Z {};
using Foo = std::variant<A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R,
S, T, U, V, W, X, Y, Z>;
struct Bar {
Foo f;
static Bar dummy() {
// issue is triggered by this initialization
return {Z{}};
// return {A{}}; // would be very quick
}
};
//=======================
Compilation time reported on my machine (Intel(R) Core(TM) i7-4790 CPU @
3.60GHz)
$ time g++ -v -std=c++17 -Wall -Wextra -c test.cpp
...
GNU C++17 (GCC) version 13.0.1 20230401 (Red Hat 13.0.1-0)
(x86_64-redhat-linux)
compiled by GNU C version 13.0.1 20230401 (Red Hat 13.0.1-0), GMP
version 6.2.1, MPFR version 4.1.1-p1, MPC version 1.3.1, isl version
isl-0.24-GMP
...
________________________________________________________
Executed in 131.43 secs fish external
usr time 125.50 secs 659.00 micros 125.50 secs
sys time 5.53 secs 109.00 micros 5.53 secs
perf report:
29.51% cc1plus cc1plus [.] cxx_eval_constant_expression
20.46% cc1plus cc1plus [.] cxx_fold_indirect_ref_1
4.17% cc1plus cc1plus [.] is_overloaded_fn
g++ -Q show that most time is spent in:
constexpr std::_Enable_default_constructor<_Switch,
_Tag>::_Enable_default_constructor(std::_Enable_default_constructor_tag) [with
bool _Switch = true; _Tag = std::variant<A, B, C, D, E, F, G, H, I, J, K, L, M,
N, O, P, Q, R, S, T, U, V, W, X, Y, Z>]
g++ -ftime-report-details:
Time variable usr sys wall
GGC
phase setup : 0.00 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
1752k ( 0%)
phase parsing : 127.30 (100%) 4.76 (100%) 132.47 (100%)
16911M (100%)
phase lang. deferred : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
19k ( 0%)
phase opt and generate : 0.03 ( 0%) 0.00 ( 0%) 0.03 ( 0%)
103k ( 0%)
|name lookup : 0.02 ( 0%) 0.01 ( 0%) 0.04 ( 0%)
963k ( 0%)
|overload resolution : 0.02 ( 0%) 0.01 ( 0%) 0.04 ( 0%)
3018k ( 0%)
garbage collection : 0.12 ( 0%) 0.00 ( 0%) 0.12 ( 0%)
0 ( 0%)
callgraph construction : 0.03 ( 0%) 0.00 ( 0%) 0.03 ( 0%)
101k ( 0%)
preprocessing : 0.02 ( 0%) 0.00 ( 0%) 0.04 ( 0%)
361k ( 0%)
parser (global) : 0.01 ( 0%) 0.02 ( 0%) 0.05 ( 0%)
2957k ( 0%)
`- parser struct body : 0.01 ( 0%) 0.00 ( 0%) 0.02 ( 0%)
669k ( 0%)
`- template instantiation : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
72k ( 0%)
`- preprocessing : 0.02 ( 0%) 0.00 ( 0%) 0.04 ( 0%)
361k ( 0%)
`- parser inl. func. body : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
263k ( 0%)
`- varconst : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
0 ( 0%)
parser struct body : 0.02 ( 0%) 0.02 ( 0%) 0.04 ( 0%)
2568k ( 0%)
parser inl. func. body : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
362k ( 0%)
parser inl. meth. body : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
654k ( 0%)
`- garbage collection : 0.12 ( 0%) 0.00 ( 0%) 0.12 ( 0%)
0 ( 0%)
`- template instantiation : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
13k ( 0%)
`- constant expression evaluation : 127.02 (100%) 4.68 ( 98%) 132.10 (100%)
16896M (100%)
template instantiation : 0.06 ( 0%) 0.04 ( 1%) 0.12 ( 0%)
9062k ( 0%)
`- template instantiation : 0.02 ( 0%) 0.01 ( 0%) 0.05 ( 0%)
1618k ( 0%)
`- symout : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
0 ( 0%)
`- constant expression evaluation : 0.02 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
39k ( 0%)
constant expression evaluation : 127.04 (100%) 4.68 ( 98%) 132.10 (100%)
16896M (100%)
`- template instantiation : 0.01 ( 0%) 0.01 ( 0%) 0.00 ( 0%)
345k ( 0%)
varconst : 0.00 ( 0%) 0.00 ( 0%) 0.01 ( 0%)
4376 ( 0%)
symout : 0.01 ( 0%) 0.00 ( 0%) 0.00 ( 0%)
0 ( 0%)
TOTAL : 127.33 4.76 132.51
16913M
