5 ms·

I knew C++ compilation was slow but 30x slower? I'm sure the compile-time memory usage will also show a similar trend. It would be interesting if somebody could

I knew C++ compilation was slow but 30x slower? I'm sure the compile-time memory usage will also show a similar trend. It would be interesting if somebody could explain the reason for this disparity.

dsign 9y ago
It's a well known fact. Just by including some headers from the standard library the compiler has to go over huge chunks of library template code, and it usually needs quite a few complicated phases to slowly morph those templates into executable code.
TheCoelacanth 9y ago
Not exactly a fair comparison when the C program is using 1.5MB of pre-compiled dependencies while the C++ program reimplements all of the functionality from the dependencies in the code being compiled.
astrodust 9y ago
The problem, by and large, is that C++ is heavily dependent on header files to implement the Standard Library. It's largely templated, which means there's no way to make a pre-compiled version, the code generated varies wildly depending on the types involved. C has relatively simple header files, they usually contain structs, function signatures, and a bunch of macros. They're easy to parse and apply by comparison, plus don't tend to be as deeply nested. If C++ ever adopts the Pascal-style "module" extensions that have been kicking around in various proposals compile times could shrink by several orders of magnitude.
- aewnjfksd 9y ago
  > If C++ ever adopts the Pascal-style "module" extensions that have been kicking around in various proposals compile times could shrink by several orders of magnitude. I'm skeptical. Modules don't avoid the need for template instantiation.
  - nly 9y ago
    Template instantiation surely only requires type substitution and re-running some analysis though. What makes C++ compilation slow is reparsing headers again and again and again because the C the preprocessor means that every time they are encountered they may have new semantics. The motivation for modules in C++ is similar to that of developing a Binary AST for Javascript, discussed on HN recently.
    - astrodust 9y ago
      If modules ever happen, wow, C++ is going to feel like a whole new language. I remember large Pascal codebases compiling in as little time as it took to press the key, and this was in the era of computers with mere megabytes of memory. Turbo C++ was never as "turbo" as Turbo Pascal.
      - pjmlp 9y ago
        They already happened, kind of. You can get Visual C++ with support for them. clang has support, but the old style ones Currently they are already a TS targeted for C++20.
      - fasquoika 9y ago
        That's probably not just modules though. Pascal, being one of Niklaus Wirth's languages, was specifically designed to be easy to compile, generally not even requiring building an AST (though a particular compiler still might, especially of it added extensions to the language).
    - dozzie 9y ago
      > What makes C++ compilation slow is reparsing headers again and again and again because the C the preprocessor means that every time they are encountered they may have new semantics. Really? And I thought that this is why C and C++ headers are typically wrapped in #ifndef-#define-#endif block, so they only produce whitespace after preprocessing on second inclusion.
      - 9y ago
Matthias247 9y ago
Based on your dependencies and programming style it might be less or more, but it sounds not unreasonable. I wrote the same program in two environments once: C++ with stdlib and boost, and C++ purely using QT abstractions. The second one already compiled in 1/5th of the time. I guess that's because with QT the headers are small because most implementations are hidden behind pointers (PIMPL), while with boost you often pull in lots of code through headers and compile dozens of specializations of similar types to avoid indirection costs.
pandaman 9y ago
In this case it's likely headers size. In native C++ programs you usually have more complex syntax (e.g. namespace is not global any more and each id has to be properly located in a hierarchy of namespaces) and, sometimes, compile-time evaluation via templates.

I knew C++ compilation was slow but 30x slower? I'm sure the compile-time memory usage will also show a similar trend. It would be interesting if somebody could explain the reason for this disparity.

dsign 9y ago

It's a well known fact. Just by including some headers from the standard library the compiler has to go over huge chunks of library template code, and it usually needs quite a few complicated phases to slowly morph those templates into executable code.

TheCoelacanth 9y ago

Not exactly a fair comparison when the C program is using 1.5MB of pre-compiled dependencies while the C++ program reimplements all of the functionality from the dependencies in the code being compiled.

astrodust 9y ago

The problem, by and large, is that C++ is heavily dependent on header files to implement the Standard Library. It's largely templated, which means there's no way to make a pre-compiled version, the code generated varies wildly depending on the types involved. C has relatively simple header files, they usually contain structs, function signatures, and a bunch of macros. They're easy to parse and apply by comparison, plus don't tend to be as deeply nested. If C++ ever adopts the Pascal-style "module" extensions that have been kicking around in various proposals compile times could shrink by several orders of magnitude.

aewnjfksd 9y ago
> If C++ ever adopts the Pascal-style "module" extensions that have been kicking around in various proposals compile times could shrink by several orders of magnitude. I'm skeptical. Modules don't avoid the need for template instantiation.
- nly 9y ago
  Template instantiation surely only requires type substitution and re-running some analysis though. What makes C++ compilation slow is reparsing headers again and again and again because the C the preprocessor means that every time they are encountered they may have new semantics. The motivation for modules in C++ is similar to that of developing a Binary AST for Javascript, discussed on HN recently.
  - astrodust 9y ago
    If modules ever happen, wow, C++ is going to feel like a whole new language. I remember large Pascal codebases compiling in as little time as it took to press the key, and this was in the era of computers with mere megabytes of memory. Turbo C++ was never as "turbo" as Turbo Pascal.
    - pjmlp 9y ago
      They already happened, kind of. You can get Visual C++ with support for them. clang has support, but the old style ones Currently they are already a TS targeted for C++20.
    - fasquoika 9y ago
      That's probably not just modules though. Pascal, being one of Niklaus Wirth's languages, was specifically designed to be easy to compile, generally not even requiring building an AST (though a particular compiler still might, especially of it added extensions to the language).
  - dozzie 9y ago
    > What makes C++ compilation slow is reparsing headers again and again and again because the C the preprocessor means that every time they are encountered they may have new semantics. Really? And I thought that this is why C and C++ headers are typically wrapped in #ifndef-#define-#endif block, so they only produce whitespace after preprocessing on second inclusion.
    - 9y ago

Matthias247 9y ago

Based on your dependencies and programming style it might be less or more, but it sounds not unreasonable. I wrote the same program in two environments once: C++ with stdlib and boost, and C++ purely using QT abstractions. The second one already compiled in 1/5th of the time. I guess that's because with QT the headers are small because most implementations are hidden behind pointers (PIMPL), while with boost you often pull in lots of code through headers and compile dozens of specializations of similar types to avoid indirection costs.

pandaman 9y ago

In this case it's likely headers size. In native C++ programs you usually have more complex syntax (e.g. namespace is not global any more and each id has to be properly located in a hierarchy of namespaces) and, sometimes, compile-time evaluation via templates.