CLANG SUPPORT

Circle is normally build using a GNU-C/C++-based toolchain. For some reasons
(e.g. better generated code, more strict error checking) it may be interesting
to build it using LLVM with the Clang frontend. To allow this, Circle provides
some experimental support, which is described here.

Please note that this support has been tested with Clang 15 on Linux hosts only.
It cannot be guaranteed, that all code in the Circle repository will work, when
built with Clang/LLVM, because the release tests will be normally run with GNU
C/C++ builds. That's why you have to test your own application very carefully,
if you are using Clang/LLVM to build it.

To build Circle with Clang, you need a special build configuration in the file
Config.mk, which should look like this:

	CLANG = 1
	AARCH = 64
	RASPPI = 3

"CLANG = 1" enables the Clang build support. The other settings are similar to
GNU C/C++ builds and are described in the main Circle README.md file.

The Clang/LLVM tools must be found via the PATH environment variable. If your
Clang/LLVM toolchain command names need a specific suffix (e.g. "-11"), it can
be given like that:

	SUFFIX = -11

By default Clang builds will be done with the setting "STDLIB_SUPPORT = 0". That
means that external functions (e.g. from <math.h>) cannot be used. This is
caused by the reason that available LLVM/Clang toolchains do not provide
libraries for Raspberry Pi targets.

There is a solution to provide additional libraries, which come with the
recommended standard GNU-C/C++-based toolchains, which are referenced in the
main Circle README.md file. This will rise the STDLIB_SUPPORT level to 1.

To enable this you need the recommended GNU-C/C++-based toolchain installed on
your host computer, along with Clang/LLVM. Then add the following lines to your
Config.mk configuration file:

	STDLIB_SUPPORT = 1
	PREFIX64 = aarch64-none-elf-
	EXTRAINCLUDE += -I /absolute/path/to/toolchain/aarch64-none-elf/include

This is for "AARCH = 64" builds and sets the prefix of the GNU-C/C++-based
toolchain command names. In the given case the commands will be found using the
PATH variable. Alternatively you can provide an absolute path to the GNU C/C++
toolchain commands in the PREFIX64 variable. For "AARCH = 32" builds this
variable has the name PREFIX.

The variable EXTRAINCLUDE provides the absolute path to the standard include
files, which come with the GNU-C/C++-based toolchain. This path can be normally
determined by entering:

	$ cd /absolute/path/to/toolchain
	$ find -name math.h

You have to remove the suffix "/math.h" from the reported directory path.


KNOWN ISSUES

* Unlike GNU C/C++ builds the order of libraries, given in the LIBS variable,
  does matter with Clang/LLVM.

* The Linker may fail with arm-none-eabi target (AArch32) and "STDLIB_SUPPORT =
  1", because of missing __aeabi_*() functions in the provided toolchain
  libraries.

* There have been reports, that currently there are problems on Macs, especially
  with the Linker. If somebody is able to sort these out and would contribute the
  necessary modifications, this would be highly appreciated.