ELF instruction for section and branch.

Richard Sandiford rsandifo@redhat.com
Fri Dec 19 16:24:00 GMT 2003


"Gagneet Singh" <gagneet@acmet.com> writes:
> In your example,
> "
>>         .section .text1
>> foo:    b        bar
>> 
>>         .section .text2
>> bar:
> "
>
> If we take "b  bar" as a 32-bit instruction, then to get the relocation
> information, of the label "bar", we can calculate the 'bar' value using
> a 16-bit offset. 
>
> If so, then it would be a 16-bit relocation.

Yes, but the point of what I said here:

>> It sounds like you're asking about code such as:
>> 
>>         .section .text1
>> foo:    b        bar
>> 
>>         .section .text2
>> bar:
>> 
>> Is that right?  If so, then no, this isn't supported.  The 
>> assembler can't calculate the branch offset because the 
>> distance between foo and bar isn't known until link time.  
>> And unfortunately, due to an infamous problem with the MIPS 
>> ELF spec, there's no relocation that the assembler can use either.
>> 
>> (The R_MIPS_PC16 relocation was probably designed for 
>> branches, but the ELF spec says it gives a byte offset, 
>> whereas branch instructions need a word offset.)

is that the official ELF spec does not define a suitable relocation.

Richard



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