I wonder whether the compiler would really "do the work twice" in a simplified version

Simplified (?) static_views about tools HOT 3 CLOSED

Tradias commented on July 30, 2024

Simplified (?) static_views

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Comments (3)

Tradias commented on July 30, 2024

I went ahead a tried to get some data.

For MSVC 14.31.31103 (VS 2022 17.1) using /Bt+ and given the following code:

constexpr auto repeat(std::string_view str, size_t count)
{
    std::string result;
    for (size_t i = 0; i < count; ++i)
    {
        result += str;
    }
    return result;
}

int main()
{
    static constexpr auto TEST = to_string_view(
        []
        {
            return repeat(
                "Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do eiusmod tempor incididunt ut labore "
                "et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut "
                "aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse "
                "cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa "
                "qui officia deserunt mollit anim id est laborum.",
                15);
        });
    fmt::print("{}", TEST);
}

My version (c1xx.dll is compiler frontend and c2.dll backend):

Debug
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c1xx.dll)=1.40770s < 262926248496 - 262940325518 > BB
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c2.dll)=0.00985s < 262940328476 - 262940426948 > BB

Release
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c1xx.dll)=1.38381s < 262677700111 - 262691538237 > BB
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c2.dll)=0.01516s < 262691539825 - 262691691389 > BB

and your version:

Debug
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c1xx.dll)=1.04090s < 261748439353 - 261758848310 > BB
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c2.dll)=0.00958s < 261758851469 - 261758947317 > BB

Release
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c1xx.dll)=1.01833s < 262397415096 - 262407598432 > BB
[build] time(C:\PROGRA~1\MICROS~1\2022\COMMUN~1\VC\Tools\MSVC\1431~1.311\bin\Hostx64\x64\c2.dll)=0.01552s < 262407599893 - 262407755112 > BB

So it seems that there isn't as much memoization as I was expecting, at least in the case of MSVC.

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lefticus commented on July 30, 2024

I expect no memoization unless it is passed as a template parameter, which is why I implemented this solution.

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Tradias commented on July 30, 2024

With the advent of more and more constexpr evaluated code, compilers should probably start to add some optimization passes to such code. In my example above it seems that the call to repeat takes almost 400ms, quite a lot. Even in Debug mode, the same code evaluated at runtime would probably not take more than 10ms.

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Simplified (?) static_views about tools HOT 3 CLOSED

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