Use a single check for both cases when we have includes and when we
don't. This way we ensure three things:
1. Built-in checks are 100% reliable with clang and on macOS since clang
implements __has_builtin
2. When the #include is present, this ensures that __builtin_func is not
checked for (because of MSYS, and because it is faster)
3. We fallback to checking __builtin_func when all else fails
Our "43 has function" test should also work with clang and icc on Linux,
so enable them. Also detect builtins with __has_builtin if available,
which is much faster on clang.
There is a feature request for the same with GCC too:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66970
We were checking for builtins explicitly like this because the ordinary
checks don't work for builtins at all. We do exactly the same check as
Autoconf and it doesn't work with Autoconf either (Autoconf is broken!)
So now we check for it in two ways: if there's no #include in prefix, we
check if `__builtin_symbol` exists (has_function allows checking for
functions without providing includes). If there's a #include, we check
if `symbol` exists.
The old method was causing problems with some buggy toolchains such as
MSYS2 which define some builtins in the C library but don't expose them
via headers which meant that `__builtin_symbol` would be found even
though `symbol` is not available.
Doing this allows people to always get the correct answer as long as
they specify the includes that are required to find a function while
also not forcing people to always specify includes to find a function
which is cumbersome.
Closes#1083
We want compiler check arguments (-O0, -fpermissive, etc) to override
all other arguments, and we want extra_args passed in by the build file
to always override everything.
To do this properly, we must split include arguments out, append them
first, append all other arguments as usual, and then append the rest.
As part of this, we also add the compiler check flags to the
cc.compiles() and cc.links() helper functions since they also most
likely need them.
Also includes a unit test for all this.
We now use .links() to detect if a C compiler function is available
or not, that way the user doesn't need to specify all the possible
includes for the check, which simplifies things considerably.
Also detect glibc stub functions that will never work and return
false for them.
Closes#437
Jussi Pakkanen
Nirbheek Chauhan
Nirbheek Chauhan
Tim-Philipp Müller