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Re: bal instruction in gcc 3.x

To: Alexander Voropay <alec@artcoms.ru>
Subject: Re: bal instruction in gcc 3.x
From: Thiemo Seufer <ths@networkno.de>
Date: Wed, 20 Jul 2005 14:54:43 +0200
Cc: Kishore K <hellokishore@gmail.com>, linux-mips@linux-mips.org
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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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