rustc emits calls to __sync_ when using load/store with +forced-atomics #101300
Description
I went to start a new branch for the gba crate and things were going well until LLVM demanded that I add a bunch of definitions for atomic intrinsics:
= note: arm-none-eabi-ld: D:\dev\gba\target\thumbv4t-none-eabi\debug\deps\libgba-e42d0bb0e1acbf2b.rlib(gba-e42d0bb0e1acbf2b.8ymripsqk83bpm.rcgu.o): in function `core::sync::atomic::atomic_store':
C:\Users\Daniel\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\sync/atomic.rs:2948: undefined reference to `__sync_lock_test_and_set_4'
arm-none-eabi-ld: D:\dev\gba\target\thumbv4t-none-eabi\debug\examples\hello-4b3a62c592de993e.5c84v82b0il7lif5.rcgu.o: in function `core::sync::atomic::atomic_store':
C:\Users\Daniel\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\sync/atomic.rs:2948: undefined reference to `__sync_lock_test_and_set_2'
arm-none-eabi-ld: D:\dev\gba\target\thumbv4t-none-eabi\debug\examples\hello-4b3a62c592de993e.5c84v82b0il7lif5.rcgu.o: in function `core::sync::atomic::atomic_load':
C:\Users\Daniel\.rustup\toolchains\nightly-x86_64-pc-windows-msvc\lib\rustlib\src\rust\library\core\src\sync/atomic.rs:2963: undefined reference to `__sync_val_compare_and_swap_2'
These are all part of LLVM's sync function set, and they are probably caused by the atomic accesses I'm using:
Now the GBA is using the ARMv4T architecture, which does not have atomic instructions at the CPU level, but the +forced-atomic feature was recently added to many target definitions for older targets. My understanding of how this was supposed to work is that an aligned atomic load/store will just use the normal load/store instruction, and then the "advanced" atomic actions like fetch_add would call a function. However, it seems to be the cast that (using the thumbv4t-none-eabi target) LLVM still wants to call functions for plain load/store.