[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #7 from Jonathan Wakely  ---
For some value of 1024

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #6 from Jonathan Wakely  ---
The bottom line, however, is that this use of std::to_array is not really
reasonable.

You're asking the compiler to create a variadic pack of 262144 integers, and
then create a pack expansion for std::array{arr[i]...} where i...
has 262144 elements in the pack.

That's going to be insanely large.

Maybe we can do this though:

  template
[[nodiscard]]
constexpr array, _Nm>
to_array(_Tp (&__a)[_Nm])
noexcept(is_nothrow_constructible_v<_Tp, _Tp&>)
{
  static_assert(!is_array_v<_Tp>);
  static_assert(is_constructible_v<_Tp, _Tp&>);
  if constexpr (_Nm > 1024 && is_trivially_default_constructible_v<_Tp>
  && is_trivially_assignable_v<_Tp&, _Tp&>)
{
  array<_Tp, _Nm> __arr;
  for (size_t __i = 0; __i < _Nm; ++i)
__arr[__i] = __a[__i];
  return __arr;
}
  else if constexpr (is_constructible_v<_Tp, _Tp&>)
return [&__a](index_sequence<_Idx...>) {
  return array, _Nm>{{ __a[_Idx]... }};
}(make_index_sequence<_Nm>{});
  __builtin_unreachable(); // FIXME: see PR c++/91388
}

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #5 from Jonathan Wakely  ---
It's hard to bisect where it got slower between 10 and 11, and between 11 and
12, because for --enable-checking builds the extra assertions slow everything
down and the difference between "fast with slow assertions" and "slow with slow
assertions" is much less significant than "fast" and "slow".

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #4 from Jonathan Wakely  ---
(In reply to Jonathan Wakely from comment #2)
> The "obvious" alternative impl using a lambda expression is even slower:

Actually for GCC 13 it's faster:

$ time ~/gcc/13/bin/g++ 110167.cc -c -O3 

real0m9.422s
user0m9.232s
sys 0m0.170s

$ time ~/gcc/13/bin/g++ 110167.cc -c -O3  -DUSE_LAMBDA

real0m6.403s
user0m6.342s
sys 0m0.048s

So that's nice.

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #3 from Jonathan Wakely  ---
$ time ~/gcc/9/bin/g++ 110167.cc -c -O3 

real0m5.568s
user0m5.497s
sys 0m0.035s

$ time ~/gcc/10/bin/g++ 110167.cc -c -O3 

real0m5.393s
user0m5.355s
sys 0m0.028s

$ time ~/gcc/11/bin/g++ 110167.cc -c -O3 

real0m10.543s
user0m10.205s
sys 0m0.317s

$ time ~/gcc/12/bin/g++ 110167.cc -c -O3 

real0m14.042s
user0m13.747s
sys 0m0.270s

$ time ~/gcc/13/bin/g++ 110167.cc -c -O3 

real0m9.422s
user0m9.232s
sys 0m0.170s

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

--- Comment #2 from Jonathan Wakely  ---
The "obvious" alternative impl using a lambda expression is even slower:

template
struct array
{
  int data[N];
};

template struct seq { };

#ifndef USE_LAMBDA
template
array
to_array_impl(int ()[sizeof...(Idx)], seq)
{
  return {{a[Idx]...}};
}
#endif

template
array
to_array(int ()[N])
{
#ifndef USE_LAMBDA
  return to_array_impl(a, seq<__integer_pack(N)...>{});
#else
  return [](seq) {
return array{{ a[I]... }};
  }(seq<__integer_pack(N)...>{});
#endif
}

constexpr int S = 2000;
int f[S];

array g(void) { return to_array(f); }

[Bug c++/110167] excessive compile time when optimizing std::to_array

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110167

Jonathan Wakely  changed:

   What|Removed |Added

 Ever confirmed|0   |1
   Last reconfirmed||2023-06-08
   Keywords||compile-time-hog
 Status|UNCONFIRMED |NEW
  Component|libstdc++   |c++

--- Comment #1 from Jonathan Wakely  ---
Reduced:

template
struct array
{
  int data[N];
};

template struct seq { };


template
array
to_array_impl(int ()[sizeof...(Idx)], seq)
{
  return {{a[Idx]...}};
}


template
array
to_array(int ()[N])
{
  return to_array_impl(a, seq<__integer_pack(N)...>{});
}

int f[262144];

auto g(void) { return to_array(f); }