Why doesn't gcc always emit sync builtins for MIPS?

Why doesn't gcc always emit sync builtins for MIPS?

[Dani T]

Using crosstool-ng, I built a GCC 10.2 cross toolchain for
mips-unknown-linux-uclibc. It works flawlessly for the legacy app I am
compiling. I recently tried to compile the same application for an embedded
device running an RTOS using a similar GCC cross toolchain built using
crosstool-ng. The main differences are that I configured it to use newlib
instead of uclibc and set it up for bare metal instead of linux. The
toolchain mostly works but I get the following errors when linking:

undefined reference to `__sync_val_compare_and_swap_4'
undefined reference to `__sync_add_and_fetch_4'

A quick look at libgcc.a shows that these symbols exist in the
mips-unknown-linux-uclibc toolchain

dani@ws:~/x-tools$ nm
mips-unknown-linux-uclibc/lib/gcc/mips-unknown-linux-uclibc/10.2.0/libgcc.a
| grep sync_add
sync_add_and_fetch_1.o:00000000 T __sync_add_and_fetch_1
sync_add_and_fetch_2.o:00000000 T __sync_add_and_fetch_2
sync_add_and_fetch_4.o:00000000 T __sync_add_and_fetch_4
sync_add_and_fetch_8.o:
sync_add_and_fetch_16.o:

And not in the mips-unknown-elf toolchain

dani@ws:~/x-tools$ nm
mips-unknown-elf/lib/gcc/mips-unknown-elf/10.2.0/libgcc.a | grep
sync_add
sync_add_and_fetch_1.o:
sync_add_and_fetch_2.o:
sync_add_and_fetch_4.o:
sync_add_and_fetch_8.o:
sync_add_and_fetch_16.o:
dani@ws:~/x-tools$

Why does the GCC compiler not emit the sync builtins when linux is not the
target? How do I configure it so that it does? My current hack is to create
my own assembly implementations that are identical to the ones that GCC
emits in the mips-unknown-linux-uclibc toolchain but would rather
understand why they are not emitted in the first place.


Thanks,


Dan