note for ATfE (clang) / picolibc

[FabKlein]

When using Arm Toolchain for Embedded (ATfE) with picolibc, it would be useful
to document the following linker-script requirement to avoid not-so-easy-to-debug
issues with CMSIS-Compiler retargeting.

If stdin/stdout are initialized from a registered constructor, e.g:

__attribute__((constructor(255)))
void platform_init(void) {
#ifdef RTE_CMSIS_Compiler_STDOUT_Custom
  extern void stdout_init(void);
  stdout_init();
#endif
} 

the Clang linker script needs to provide the picolibc _bothinit_array*
symbols around the preinit/init-array region:

PROVIDE_HIDDEN(__bothinit_array_start = .);

PROVIDE_HIDDEN(__preinit_array_start = .);
KEEP(*(.preinit_array))
PROVIDE_HIDDEN(__preinit_array_end = .);

PROVIDE_HIDDEN(__init_array_start = .);
KEEP(*(SORT_BY_INIT_PRIORITY(.init_array.*) SORT_BY_INIT_PRIORITY(.ctors.*)))
KEEP(*(.init_array .ctors))
PROVIDE_HIDDEN(__init_array_end = .);

PROVIDE_HIDDEN(__bothinit_array_end = .);

Could the CMSIS-Compiler documentation add a note for CLANG/picolibc users that
linker scripts must provide __bothinit_array_start and
__bothinit_array_end when constructor/init-array based initialization is used?

This is easy to miss because the CMSIS-Compiler retarget example calls
stdout_init() directly from main(), so the example does not expose the
problem. Projects that initialize CMSIS-Compiler streams from platform
constructors can fail silently.

ref:
https://developer.arm.com/documentation/107976/22-1-0/C-and-C---libraries/picolibc-C-runtime

AC6/GCC are more immune because:

• AC6 uses the Arm runtime initialization conventions.
• GCC/newlib uses the usual _preinit_array* and _init_array* symbols,
  which were already present in default linker scripts.

[VladimirUmek]

This was introduced in Picolib included in CLANG 21.x.x (more info also here).
I didn't yet investigate all implications since there will be more focus on better support for LLVM-libc.