Created attachment 30966 [details]
libgcc patch to correct operation with non-interworking thumb builds

I have included a multilib in my GCC for ARM to generate thumb code with interworking disabled, intended for an ARMv4T CPU (e.g. ARM7T or ARM9). In other words, -mcpu=arm9 -mthumb -mno-thumb-interwork.

However this fails to work because of a problem in libgcc making it incorrect for non-interworking operation. A specific example is aeabi_uldivmod in bpabi.S which is an ARM (not thumb) function, which includes a bl to __gnu_uldivmod_helper. __gnu_uldivmod_helper is from bpabi.c and is in Thumb mode.

The linker helpfully adds a little trampoline to switch to Thumb mode  with the bx instruction (____gnu_uldivmod_helper_from_arm). However GCC returns from __gnu_uldivmod_helper with:

  pop     {r3, r4, r5, r6, r7, pc}

This means that no mode switch happens on the return from the Thumb mode function to the ARM mode aeabi_uldivmod function.

As well as the obvious aeabi_ldivmod, someone may want to double-check whether this sort of problem may apply to other functions (although I haven't found any others yet myself, but I'm not sure about how functions like those in unaligned-funcs.c would be called).

I am attaching a potential fix for the problem with aeabi_ldivmod and aeabi_uldivmod, so it would be good for that to be applied at least, including on the 4.7 branch.

2013-10-07  Jonathan Larmour  <jifl@eCosCentric.com>

	* config/arm/bpabi.S (aeabi_ldivmod, aeabi_uldivmod): Allow for
	non-interworking Thumb builds.

Thanks,

Jifl