Alexander Voropay wrote:
> <hellokishore@gmail.com> wrote:
>
> >>>However, if I compile the code with gcc3, it exits with the
> >>>error "Cannot branch to unknown symbol".
>
> >On the other hand, if I replace
> >
> >bal jump_to_label
> >
> >by
> >
> >la t9, jump_to_label
> >jalr t9
> >
> >I don't see any warning. What could be the reason ?
>
>
> 1) With "jal label " you'll have a MIPS_26 RELOC type in the ELF obj file:
>
> jal lablel
>
> 00000000 <.text>:
> 0: 0c000000 jal 0x0
> 4: 00000000 nop
>
> RELOCATION RECORDS FOR [.text]:
> OFFSET TYPE VALUE
> 00000000 R_MIPS_26 label
>
>
> 2) The "la r,label" is a syntetic inctruction wich expanded into two:
> "lui r,%hi label ; addui r,%lo label". It gives you two RELOCs:
>
> la t0, label
> jalr t0
>
> 00000000 <.text>:
> 0: 3c080000 lui t0,0x0
> 4: 25080000 addiu t0,t0,0
> 8: 0100f809 jalr t0
> c: 00000000 nop
>
> RELOCATION RECORDS FOR [.text]:
> OFFSET TYPE VALUE
> 00000000 R_MIPS_HI16 label
> 00000004 R_MIPS_LO16 label
>
> 3) AFAIK (correct me), there is no MIPS-specific RELOC type for
> the "branch" instruction format in the BFD, so "bal" to the *external*
> symbols is impossible.
There is R_MIPS_PC16 which was ill-defined (missing rightshift for the
immediate) in the old ELF spec and thus unused.
> May be, old gas generates something like RELOC_PCREL
> for "bal external" ?
Old gas/ld used R_MIPS_PC16 without reasonable range checking, IOW it
broke silently for branch spans greater than +-32k.
Implementing external branches for gas/ld with the correct R_MIPS_PC16
definition is somewhere on my TODO list but is unlikely to get priority
any time soon.
Thiemo
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