How to define a symbol with absolute address for AArch64?

[Sebastian Huber]

On 12.09.23 15:46, Xi Ruoyao wrote:
> On Tue, 2023-09-12 at 14:58 +0200, Sebastian Huber wrote:
>> On 12.09.23 13:21, Richard Earnshaw wrote:
>>>
>>> On 12/09/2023 11:02, Sebastian Huber wrote:
>>>> Hello,
>>>>
>>>> I would like to define a global symbol with an absolute address in an
>>>> assembly/C source file for the AArch64 target. This works for all
>>>> other architectures I tried so far, but not for AArch64:
>>>>
>>>> extern char abs_symbol[];
>>>> extern char abs_symbol_2[];
>>>>
>>>> __asm__(
>>>>     "\t.globl abs_symbol\n"
>>>>     "\t.set abs_symbol, 0x123\n"
>>>> );
>>>>
>>>> unsigned long f_abs_symbol(void)
>>>> {
>>>>     return (unsigned long)abs_symbol;
>>>> }
>>>>
>>>> unsigned long f_abs_symbol_2(void)
>>>> {
>>>>     return (unsigned long)abs_symbol_2;
>>>> }
>>>>
>>>> unsigned long _start(void)
>>>> {
>>>>     return f_abs_symbol() + f_abs_symbol_2();
>>>> }
>>>>
>>>> aarch64-rtems6-gcc abs.c -Wl,--gc-sections -Wl,--defsym=abs_symbol_2=291
>>> Have you tried -mcmodel=large?  With that I get:
>> I get the same result with -mcmodel=large. With -mcmodel=tiny I get:
>>
>> aarch64-rtems6-gcc abs.c -Wl,--gc-sections -Wl,--defsym=abs_symbol_2=291
>> -mcmodel=tiny
>> /tmp/ccKUnvyq.o: in function `f_abs_symbol_2':
>> abs.c:(.text+0x8): relocation truncated to fit: R_AARCH64_ADR_PREL_LO21
>> against symbol `abs_symbol_2' defined in*ABS*  section in a.out
>> collect2: error: ld returned 1 exit status
>>
>> Is this a tool bug?
> No, it's how code models are defined.  GCC documentation says clearly:
> 
>         -mcmodel=tiny
>             Generate code for  the  tiny  code  model.   The  program  and  its
>             statically  defined  symbols  must  be  within  1MB  of each other.
>             Programs can be statically or dynamically linked.
> 
> Here the text is located at 0x400000 but abs_symbol_2 is at 0x123, thus
> violating the definition of -mcmodel=tiny.

Yes, this makes sense.

> 
>> Is there some way to make this working with -mcmodel=small?
> No because -mcmodel=small only assumes the program and the symbols are
> within*a*  4GB range - for example it's allowed to be [47GB, 51GB).
> This is different from the default code model of RISC-V (-
> mcmodel=medlow) where the symbols must be in [-2GB, 2GB).
> 
> If you really think GCC should support this you can ask GCC for adding a
> new code model.  Anyway this is not a linker issue because the adrp-add
> pairs are generated by GCC.

Thanks for the explanation.

I guess what reliably works across code models is using the address of 
an existing symbol and then add a constant which fits into the code 
model. To retrieve the constant, we just have to subtract the address of 
the other symbol.

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