Add newlib-nano as multilib

[stevew817]

See ARM-software/LLVM-embedded-toolchain-for-Arm#628 for context.

This enables newlib-nano as a multilib. I didn't find any release scripts, so I don't know how to add it to the CI such that it also generates an overlay package for newlib-nano on release.

Note: this also fixes an issue with the current newlib builds where they don't get built correctly for anything above ARMv6 as the -march, -mfloat-abi and -mfpu arguments weren't being passed on to the newlib compilation configuration. A little unsure why this hasn't popped up before...

[voltur01]

Hi Stephen,

Thank you for working on this and posting the patch!

I was playing with the changes, I can confirm that with this patch I can build the newlib-nano overlay package which works with the C sample https://github.com/arm/arm-toolchain/tree/arm-software/arm-software/embedded/newlib-samples/src/baremetal-semihosting-newlib (after adjusting the config file name in the Makefile), however the C++ sample https://github.com/arm/arm-toolchain/tree/arm-software/arm-software/embedded/newlib-samples/src/cpp-baremetal-semihosting-newlib reports missing symbols: ungetwc, fgetwc, swprintf. Is this expected for the nano variant?

Surprisingly, I am not able to build the normal newlib package anymore: newlib/libm/common/math_config.h gives multiple errors: couldn't allocate output register for constraint 'w' in __asm__ __volatile__ ("", : "+w" (x))

[stevew817]

Thanks for the swift feedback!

Is this expected for the nano variant?

I don't think so - according to the readme for newlib, enabling newlib-nano-formatted-io shouldn't impact any of the wchar-related functionality. But, when looking at what the newlib makefile is doing, it is actually dropping all wide-char-related functions from the library build when compiling with nano-formatted IO set. See here, line 298 - 335.
I guess that means the C++ libs accompanying nanolib need to be built without wchar too in order for that to not show? Does libstdc++ somehow account for that without a specific flag, and this is popping up here since we're using libc++ here? Or am I just not finding it in the ARMGCC build script over here? I see that picolibc is also compiling the C++ libs without wchar support, so that's probably the 'fix'.

Surprisingly, I am not able to build the normal newlib package anymore: newlib/libm/common/math_config.h gives multiple errors: couldn't allocate output register for constraint 'w' in asm volatile ("", : "+w" (x))

I'll admit I didn't retry building the regular newlib locally (as each build run takes about four hours for me... I do need a new machine). Since I didn't change any of the newlib config flags, it's probably due to now passing the actual architecture and architectural option flags to the compiler? Before I spend hours rebuilding newlib from scratch, so you have any more information on the failure (such as line number etc)?

[voltur01]

Please see the newlib build log for one of the variants - looks like there is the same pattern of errors in multiple variants.

2025-01-27_newlib_buildlog.txt

[voltur01]

LLVM libc++ seems to have this option to turn wide characters support on/off: https://github.com/llvm/llvm-project/blob/83433d936195c612a51b54397f82ab0d97369d86/libcxx/CMakeLists.txt#L98

[stevew817]

Please see the newlib build log for one of the variants - looks like there is the same pattern of errors in multiple variants.

Thanks! Is it only on AArch64 or does it also appear on ARM?

[voltur01]

I rerun the build, apparently it tries to build the first configuration which is aarch64a_soft_nofp_exn_rtti, fails and stops.

When I try to build armv6m and armv8.m-main separately, the build succeeds, so likely the above is an AArch64 specific issue.