[Picolibc] Integrating Picolibc into a GCC toolchain

[R. Diez]

Hi Keith:

I have been using the following makefile for years to build a cross-compiler GCC toolchain with Newlib:

https://github.com/rdiez/JtagDue/blob/master/Toolchain/Makefile

I would like to replace Newlib with Picolibc.

The main problem I am facing is that I need to carefully control the compiler flags. This is the file where the flags for my target are specified:

scripts/cross-arm-none-eabi.txt

I need to replace this line:

c_args = [ '-nostdlib', '-fno-common']


with something like this:

c_args = [ '-nostdlib', '-fno-common',
            '-g', '-O2',
             '-mcpu=cortex-m3', '-mthumb',
            '-fno-omit-frame-pointer',
            '-fdata-sections', '-ffunction-sections',
            '-DHAVE_ASSERT_FUNC'
          ]

Or whatever the flags are for today's toolchain.

Newlib's configuration script honours CFLAGS_FOR_TARGET and CXXFLAGS_FOR_TARGET to that effect, but Picolibc uses Meson which has known shortcomings in this area when cross-compiling.

I heard that some users generate scripts/cross-arm-none-eabi.txt with the right flags. However, I would rather not change anything else in that file, only the c_args line, in case the next version of Picolibc ships with different settings before or after c_args unrelated to the compiler flags.

What are my options?

Is there a ready-made example somewhere of a script of makefile that builds a complete cross-compilation toolchain with Picolibc instead of Newlib?

There are ways to make a standard GCC toolchain use a different libc than the one built-in, but
creating a GCC toolchain with an integrated Picolib has the following advantages:

1) It is convenient. No need to mess around with some weird --specs=picolibc.specs argument or the like.

2) No need to worry about the C++ support library. Normally, GCC comes with an integrated C++ support library that has been built against the integrated libc (such as Newlib). I haven't figured out yet how to rebuild that C++ support library against Picolibc and tell GCC it should be using another version when it is using a different libc. My firmware is actually written in C++.

Thanks in advance,
   rdiez

-- 
rdiez