Home Original page

Multiple relocation in ELF

Ian Lance Taylor iant@google.com
Tue Jul 25 14:50:00 GMT 2006 
Paul Brook <paul@codesourcery.com> writes:

> > Note that you can always do a jump table using a PC relative
> > relocation against a single symbol, plus an addend.
> 
> Not always. You may want to put the table itself in .rodata (to reduce code 
> size for better constant pool placement), and have the values in the table be 
> relative to the branch instruction. Especially with hot/cold partitioning 
> there's no guarantee that both the jump and the target will be in the same 
> section. eg:

I agree that that case won't work, but I stand by my statement.  You
do have to make the values in the table be relative to the start of
the table.  That is a restriction, but it is unlikely to be a serious
one.  You have to put the address of the table into a register either
way, and then you have to load the branch offset either way.  The
question then is whether you want to jump to the sum of two registers
(which generally means an addition followed by a jump to register
indirect), or jump to a PC relative address plus a register.  Most
modern processors generally do not have an instruction which jumps to
a PC relative address plus a register (if you need a separate
addition, then you might as well use the other approach).

(I agree that in the specific case of such a processor, it would be
desirable to define a relocation pair which can compute the different
of two arbitrary symbols.  Such a relocation pair is conceptually no
different than, e.g., R_MIPS_HI16/R_MIPS_LO16.)

Ian
More information about the Binutils mailing list