casaroli commented on issue #19509:
URL: https://github.com/apache/nuttx/issues/19509#issuecomment-6068717110
Status, to give a starting point:
1. Contradictory register flags: fixed. #19600 moved the PIC base register
to _r9_, and the `elseif(CONFIG_PIC)` branch of
**arch/arm/src/cmake/elf.cmake** now gives ELF applications only
`-mpic-register=r9` (no `--fixed-r10`). The title still says r10; since #19600
the register is `r9`.
Two parts are still open, both in the CMake build only. The Make build
(**arch/arm/src/common/Toolchain.defs**) already does both and is the reference.
2. ELF applications get no `-fpic`. The `elseif(CONFIG_PIC)` branch in
**elf.cmake** passes only `-mpic-register=r9`, so GCC warns "-mpic-register= is
useless without -fpic". Make adds `$(PICFLAGS) -mpic-register=r9` to
`CELFFLAGS`/`CXXELFFLAGS`. `PICFLAGS` (`-fpic -fPIE
-mno-pic-data-is-text-relative -msingle-pic-base`) is already defined in
**arch/arm/src/cmake/gcc.cmake**, so the fix there is probably
`nuttx_elf_compile_options(${PICFLAGS} -mpic-register=r9)`.
3. The base firmware does not reserve _r9_. Make adds `--fixed-r9` to
`ARCHCFLAGS` under `CONFIG_PIC` and filters it back out of the ELF and module
flags. CMake has nothing like this for a flat build:
`NUTTX_KERNEL_COMPILE_OPTIONS` is applied only to split kernel libraries
(**cmake/nuttx_add_library.cmake**). The flag must reach the firmware and libc
but not the ELF applications, otherwise GCC rejects the pair again ("unable to
use 'r9' for PIC register"). One possible way is a generator expression keyed
on a property that only ELF targets have. `ELF_BINARY` in
**cmake/nuttx_add_application.cmake** may work, but check that every ELF path
sets it. This part matters when firmware code calls back into module code, for
example `qsort()` with a comparator in the module.
To test: `cmake -B build -DBOARD_CONFIG=mps3-an547:picostest -GNinja
-DCMAKE_EXPORT_COMPILE_COMMANDS=1`. `picostest` is the only defconfig with
`CONFIG_PIC=y`. In **compile_commands.json**, firmware objects should have
`--fixed-r9`, and ELF application objects should have the `PICFLAGS`,
`-mpic-register=r9` and no `--fixed-r9`. The build should not print the
"useless without -fpic" warning.
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