as constant calculation question

[Nick Clifton]

Hi Efim,

> does anyone know why as for ARM can use
> .word __load_section_end__
> .word __init_start__
> .word __load_section_end__ - 0x300000
> 
> but not:
> .word __load_section_end__ - __init_start__

Because the ARM ABI does not define a reloc (or rather pair of relocs) 
to handle the difference between two undefined symbols.

A single reloc can only reference at most one symbol.  Thus the first 
three cases can all be handled by the R_ARM_ABS32 reloc which will refer 
to the appropriate (undefined) symbol.  The last case however requires 
that *two* symbols be referenced.  This requires that two relocs be 
generated and the ARM ABI has not been set up to enable this feature.

Cheers
   Nick