Re: [boost] [outcome] High level summary of review feedback accepted so far

Subject: Re: [boost] [outcome] High level summary of review feedback accepted so far
From: Niall Douglas (s_sourceforge_at_[hidden])
Date: 2017-06-01 12:16:21

>> Well, apart from poor quality of implementation of std::variant
>> sometimes. For example, libstdc++'s for GCC 7.1 appears to always use
>> type erased memory allocation. Which, frankly, is daft in C++ 14 when
>> you are storing either an int or a std::error_code. I would also assume
>> that std::bad_alloc can be thrown from libstdc++'s std::variant<> :(
> 
> {{citation needed}}.

See what you make of my toy std::variant<> based Outcome at:

https://tinyurl.com/ybxwlewh

> libstdc++'s <variant> may have problems, but dynamic allocation (which
> would be completely nonconforming) is not one of them.

I think I was wrong about the dynamic memory allocation maybe, but
something very odd is being done with type erasure. libstdc++'s variant
certainly uses a vtable which Dinkumware's does not. Operator=() vectors
via that vtable, and that wrecks code folding by the optimiser such that
the main() function fully expands into all the checks of state every
single operation with potential exception throws, rather than being
pruned into a minimum set of assembler.

Contrast that libstdc++ spew with what Dinkumware STL outputs which is
short enough to paste here:

int main(void) {
00007FF7DEED2200  sub         rsp,88h
00007FF7DEED2207  mov         qword ptr [rsp+20h],0FFFFFFFFFFFFFFFEh
  {
    static_checked_result<int> a(5), b(6);
    std::cout << a.value() << std::endl;
00007FF7DEED2210  mov         edx,5
00007FF7DEED2215  mov         rcx,qword ptr [__imp_std::cout
(07FF7DEED4118h)]
00007FF7DEED221C  call        qword ptr
[__imp_std::basic_ostream<char,std::char_traits<char> >::operator<<
(07FF7DEED40D0h)]
00007FF7DEED2222  mov         rcx,rax
00007FF7DEED2225  call        std::endl<char,std::char_traits<char> >
(07FF7DEED1680h)
    a = b;
    std::cout << a.value() << std::endl;
00007FF7DEED222A  mov         edx,6
00007FF7DEED222F  mov         rcx,qword ptr [__imp_std::cout
(07FF7DEED4118h)]
00007FF7DEED2236  call        qword ptr
[__imp_std::basic_ostream<char,std::char_traits<char> >::operator<<
(07FF7DEED40D0h)]
00007FF7DEED223C  mov         rcx,rax
00007FF7DEED223F  call        std::endl<char,std::char_traits<char> >
(07FF7DEED1680h)
    static_checked_result<int>
c(std::make_error_code(std::errc::timed_out));
00007FF7DEED2244  call
std::_Immortalize<std::_Generic_error_category> (07FF7DEED15F0h)
00007FF7DEED2249  nop
    // std::cout << c.value() << std::endl;  // segfaults, as is correct
  }
  try {
    runtime_checked_result<int> a(5), b(6);
    std::cout << a.value() << std::endl;
00007FF7DEED224A  mov         edx,5
00007FF7DEED224F  mov         rcx,qword ptr [__imp_std::cout
(07FF7DEED4118h)]
00007FF7DEED2256  call        qword ptr
[__imp_std::basic_ostream<char,std::char_traits<char> >::operator<<
(07FF7DEED40D0h)]
00007FF7DEED225C  mov         rcx,rax
00007FF7DEED225F  call        std::endl<char,std::char_traits<char> >
(07FF7DEED1680h)
    a = b;
    std::cout << a.value() << std::endl;
00007FF7DEED2264  mov         edx,6
00007FF7DEED2269  mov         rcx,qword ptr [__imp_std::cout
(07FF7DEED4118h)]
00007FF7DEED2270  call        qword ptr
[__imp_std::basic_ostream<char,std::char_traits<char> >::operator<<
(07FF7DEED40D0h)]
00007FF7DEED2276  mov         rcx,rax
00007FF7DEED2279  call        std::endl<char,std::char_traits<char> >
(07FF7DEED1680h)
    runtime_checked_result<int>
c(std::make_error_code(std::errc::timed_out));
00007FF7DEED227E  call
std::_Immortalize<std::_Generic_error_category> (07FF7DEED15F0h)
00007FF7DEED2283  mov         dword ptr [rsp+30h],8Ah
00007FF7DEED228B  mov         qword ptr [rsp+38h],rax
00007FF7DEED2290  movaps      xmm0,xmmword ptr [rsp+30h]
00007FF7DEED2295  movups      xmmword ptr [rsp+40h],xmm0
00007FF7DEED229A  mov         byte ptr [rsp+50h],2
    std::cout << c.value() << std::endl; // throws
00007FF7DEED229F  movdqa      xmmword ptr [rsp+30h],xmm0
00007FF7DEED22A5  lea         rdx,[rsp+30h]
00007FF7DEED22AA  lea         rcx,[rsp+58h]
00007FF7DEED22AF  call        std::system_error::system_error
(07FF7DEED17C0h)
00007FF7DEED22B4  lea         rdx,[_TI4?AVsystem_error_at_std@@
(07FF7DEED5048h)]
00007FF7DEED22BB  lea         rcx,[rsp+58h]
00007FF7DEED22C0  call        _CxxThrowException (07FF7DEED32C0h)
00007FF7DEED22C5  mov         edx,dword ptr [0]
00007FF7DEED22CC  mov         rcx,qword ptr [__imp_std::cout
(07FF7DEED4118h)]
00007FF7DEED22D3  call        qword ptr
[__imp_std::basic_ostream<char,std::char_traits<char> >::operator<<
(07FF7DEED40D0h)]
00007FF7DEED22D9  mov         rcx,rax
00007FF7DEED22DC  call        std::endl<char,std::char_traits<char> >
(07FF7DEED1680h)
00007FF7DEED22E1  nop
  } catch (const std::exception &e) {
    std::cerr << "Exception thrown: " << e.what() << std::endl;
  }
  return 0;
00007FF7DEED22E2  xor         eax,eax
  } catch (const std::exception &e) {
    std::cerr << "Exception thrown: " << e.what() << std::endl;
  }
  return 0;
00007FF7DEED22E4  add         rsp,88h
00007FF7DEED22EB  ret

I reckon Dinkumware's variant implementation to be pretty much optimal,
lovely. Nice job Microsoft & Dinkumware!

Niall

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