Home Original page

Puzzle:Is addend ok when handling relocations in linking mips32-elf

Amker.Cheng amker.cheng@gmail.com
Tue Mar 10 02:21:00 GMT 2009 
Hi :
Thanks for your reply.

On Tue, Mar 10, 2009 at 7:26 AM, Alan Modra <amodra@bigpond.net.au> wrote:
> On Mon, Mar 09, 2009 at 07:11:45PM +0800, Amker.Cheng wrote:
>> here comes the problem, If i have a howto structure with nonzero bitpos,
>> I think addend will be wrong and should be right shift with howto->bitpos.
>
> Left shift actually.  reloc.c:bfd_perform_relocation shows what should
> be done.

Well, I think there is a little difference between
bfd_perform_relocation & _bfd_mips_elf_relocate_section,

given a reloc entry "relent" and a 4 bytes relocation target
instruction "insn" which
including in place "addend", They do do the same thing: calculate
relocation adjustment
from "relent" and in place "addend". in a slightly different way as
described below.

1 : for bfd_perform_relocation

It first calculates relocation from "relent" and then do:
relocation >>= (bfd_vma) howto->rightshift;
relocation <<= (bfd_vma) howto->bitpos;

at last adds the relocation with in place "addend", puts the result
back to "insn".

2 : for _bfd_mips_elf_relocate_section

It first retrieves in place "addend" from the "insn" using follow codes
addend = mips_elf_obtain_contents (...)
addend &= howto->src_mask;
addend <<= howto->rightshift;
/*I think addend need to be $right$ shift by howto->bitpos,
because it is calculated from in place "addend", which was
left shift by then assembler or relocatable linker before.
*/

then it uses the retrieved "addend" and the supplied "relent" to
calculate the result and
puts the result back into "insn" like:

result = "value of relent" + "addend"  /*in func mips_elf_calculate_relocation*/
/*need do "result<<howto->bitpos" if howto->bitpos nonzero */

insn &= howto->dst_mask;
insn |= (value & howto->dst_mask);     /*in func mips_elf_perform_relocation*/
-------------------------------cut here
Anyway, relocation types with non howto->bitpos are not supported currently,
so it's ok now.

Here is all my understanding, please point it out if wrong. any reply
will be appreciated.
Thanks.

Regards.
More information about the Binutils mailing list