Optimizing the unoptimizable: a journey to faster C++ compile times

Re:Optimizing the unoptimizable: a journey to faster C++ com... by widdershins on Sunday January 07, 2024 @04:31AM (#38899817)

So why on earth is std::string so slow to compile, if it's possible to compile this full-featured formatting library in 1/4 of the time?

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by vitaut on Sunday January 07, 2024 @10:48AM (#38902177)

AFAICS the main problem is that <string> pulls in a lot of dependencies.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by mkoubaa on Sunday January 07, 2024 @03:51PM (#38905007)

Surely this is an implementation detail that vendors can change

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by vitaut on Sunday January 07, 2024 @05:03PM (#38905622)

Implementations can be improved somewhat but there is a certain set of dependencies that must be included according to the standard and for string it's pretty big.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by pjmlp on Sunday January 07, 2024 @03:19PM (#38904758)

It isn't when using modules, import std brings in the whole standard C++ faster than that #include.

EDIT:

This compiles in 1 second on an i7 laptop.

    import std;

    int main()
    {
        std::cout << "Hello World!\n";
    }

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by vitaut on Sunday January 07, 2024 @05:35PM (#38905916)

Unfortunately the std module alone doesn't help much because 1 second is 3x slower than before the optimization described in the post but maybe more fine-grained modules will (or if std module import made faster).

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by pjmlp on Monday January 08, 2024 @02:02AM (#38909009)

For the whole library, everything.

Also Microsof Office team has been migrating to modules, with great build improvements.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by za3faran on Monday January 08, 2024 @09:36AM (#38912487)

Any place where I can read up more about this?

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by pjmlp on Monday January 08, 2024 @09:57AM (#38912822)

https://devblogs.microsoft.com/cppblog/integrating-c-header-...

Key takeway

"Fortunately, we were able to show a build performance improvement great enough that the team agreed to adopt header units into the Office production build system alongside msvc 17.6.6!"

Unfortunely the performance findings blogpost is still WIP.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by OskarS on Monday January 08, 2024 @03:37AM (#38909500)

Not to be "that guy", but... one second is a VERY VERY long time to compile a hello world program.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by pjmlp on Monday January 08, 2024 @03:46AM (#38909556)

Including compiling from scratch the complete C++ standard library on first use, which I should probably have mentioned.

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Re:Optimizing the unoptimizable: a journey to faster C++ com... by OskarS on Tuesday January 09, 2024 @11:32AM (#38928062)

What's the incremental compile time? If you just change that "Hello World" string to something else, does it compiler faster the second time?

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