GAS compiling problem on ARM

[Richard Earnshaw]

> Hi,
> 
> I'm using binutils-2.13.92, configured with:
> ../binutils-2.13.92/configure 
> --prefix=/homedirectories/home6/mlil/proj_links/mingw/cross_arm 
> --target=arm-elf -v
> 
> When compiling, I get the following error messages which I do not 
> understand:
> 
> /homedirectories/home6/mlil/proj_links/mingw/cross_arm/lib/gcc-lib/arm-elf/3.3/../../../../arm-elf/bin/as 
> --gdwarf2 -EL -mcpu=arm7tdmi -mthumb-interwork -o inthdlr_gnu.o 
> /var/tmp//cc2yfq0p.s
> ../src/inthdlr_gnu.S: Assembler messages:
> ../src/inthdlr_gnu.S:249: Error: invalid immediate for address 
> calculation (value = 0xFFFFFFD0)
> ../src/inthdlr_gnu.S:249: Error: cannot represent THUMB_ADD relocation 
> in this object file format
> 
> The relevant code looks like:
> 
>      244         .thumb
>      245        
>      246         .globl  arm_lock
>      247         .globl  arm_lock32
>      248 arm_lock:      
>      249         adr     r0,arm_lock32
>      250         bx      r0
>      251         .align 2
>      252         .arm
>      253 arm_lock32:    
> 
> I compiling using GCC 3.3, which in turn invokes "as" as seen above.
> 
> What am I doing wrong here?

Because the symbol arm_lock32 is exported, gas is refusing to resolve the 
offset at assembly time.  Since it has no way of representing the 
relocation in the object file it is then generating an error.  You can 
work around the problem by defining an alternative local symbol as follows:

        .thumb

        .globl  arm_lock
        .globl  arm_lock32
arm_lock:      
        adr     r0,.Larm_lock32_internal
        bx      r0
        .align 2
        .arm
arm_lock32:
.Larm_lock32_internal:

Note, if all you want to do is switch to ARM state from thumb state (eg 
add a Thumb veneer to an ARM function, then the following trick will also 
work:

	.thumb
	.align 2
arm_lock:
	bx	pc	// PC is (. + 4), bit 0 == 0, '.' is word aligned.
	nop
	.arm
arm_lock32:
	...

Note that for this to work, the bx instruction must be word aligned (hence 
the ".align 2" before the Thumb code sequence.

R.