Re: GAS for ARM: does not support BX instruction for ARMv3 and lower

[Nicholas Clifton]

Hi Sergey,

> I try to compile program for ARMv3 or ARMv2 CPU target using GCC (4.8.x).
> It produces assembler code with BX LR instructions (return from function)
> which GAS (2.23.2) cannot compile:
>     Error: selected processor does not support ARM mode `bx lr'

This is really a gcc bug.  ARM architectures before v4T do not support 
the BX instruction, and so gcc should not be generating it.


> It is work for ARMv4 mode and greater (ARMv4 has no BX <reg> instruction
> support, but GAS compiles it as MOV PC,<reg>).

You could try adding "--fix-v4bx" (or -Wa,--fix-v4bx) to your command 
line, but whilst this will allow the instruction to be assembled, it 
will not convert it into a MOV PC, LR instruction.


> As I know BX <reg> is strongly recommended to be used instead of MOV
> PC,<reg> by the ARM architecture manual (section A.4.1.1).

True - although I think that the ARM ARM also assumes that you will be 
using a newer architecture - v2 and v3 are very old now.

Cheers
  Nick