C++ __builtin_* functions source codes

Question

I want to learn how can I find source codes of __builtin functions like __builtin_cbrt() in C++. If you know, Can you help me?

Functions that start with `__` are special implementation-specific functions. They aren't necessarily implemented by C++ source code. They can be implemented as assembly or just treated uniquely by the compiler. — François Andrieux, Feb 16 '21 at 19:14
You would need to check the source code for the compiler you are using. — Richard Critten, Feb 16 '21 at 19:15
By definition, platform-specific functions `__builtin_*` are _builtin_ in the compiler. They are not implemented in C++ in a library. Of course, you can look in the source code of _Clang_ or _GCC_ themselves, but it will be hard to spot precisely where those functions are handled. — prapin, Feb 16 '21 at 19:26
what research did you do on your own? Have you at least found out your compiler sources? — KamilCuk, Feb 16 '21 at 19:40
@KamilCuk I was trying to find source code of ```std::cbrt()```, i find it in STL library on local, it calss ```__builtin_cbrt()``` but I couldn't find it's source code... — Halit, Feb 17 '21 at 14:05

score 1 · Accepted Answer · answered Feb 16 '21 at 19:56

I want to learn how can I find

First become acquainted with the language you are working with - learn C and C++ programming languages. Learn about the tools like git and autotools and the environment around these programming languages. Become familiar with the tool set needed browsing files - at least grep, but I recommend it's (way) faster alternatives - "the silver searcher" ag or ack, but be aware of tools like ctags or GNU Global.

Then research. GNU projects are available open source - it's very easy to find source code of GNU projects, nowadays they are even mirrored on github.

Then it's just a "feeling" or "experience". Surely a project will have builtins functions in a file named "builtins.c" or something similar. Be curious, reasonable and inventive. If you would want to add a builtin function to a codebase, where would you put it? Become familiar with the project structure you are working with. And expect big and old projects to have stuff scattered all over the place.

First I find gcc sources with builtins.def (BUILT_IN_CBRT, "cbrt", and some references of BUILT_IN_CBRT in builtins.c.

After cloning the gcc repository I scan for BUILT_IN_CBRT macro name. Browsing the code leads me to CASE_CFN_CBRT macro name, which leads me to fold-const-call.c:

CASE_CFN_CBRT:
  return do_mpfr_arg1 (result, mpfr_cbrt, arg, format);

By the name of the file fold-const-call.c I suspect this part of code is taken only when folding a constant call.

From there I can browse google about mpfr_cbrt symbol, which leads me to GNU MPFR library. I find clone of MPRF library on github and search for a file named cbrt, I find cbrt.c with mpfr_cbrt() sources with the source of cbrt within MPRF library. This is the code that will be called and will compute cbrt of a number when __builtin_cbrt is folded inside a constant expression, I suspect.

When not in constnat expression, I suspect that [fold_const_call_ss]https://code.woboq.org/gcc/gcc/fold-const-call.c.html#_ZL18fold_const_call_ssP10real_value11combined_fnPKS_PK11real_format) is not called at all, instead some fold_const_* function returns to gcc that the expression cannot be constantly folded so a call to the actual cbrt() standard function is generated.

C++ __builtin_* functions source codes

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