Re: R_MIPS_PC16 relocation handling, a proposal

[Ian Lance Taylor <ian at zembu dot com>]

   Date: Tue, 12 Oct 1999 18:41:07 +0200 (MET DST)
   From: "Maciej W. Rozycki" <macro@ds2.pg.gda.pl>

    Guessing from MIPS ISA and ABI documents it seems there is no much use of
   R_MIPS_PC16 relocations.  On the other hand, the b* branch opcode family
   uses a shifted by two bits 16-bit PC-relative field that is not handled by
   any of MIPS relocations.  I think the authors of ABI might just missed the
   fact of the shift when documenting R_MIPS_PC16.

    Anyway, I propose using R_MIPS_PC16 for BFD_RELOC_16_PCREL_S2 relocations
   which would allow object modules to contain external references from b*
   opcodes.  As R_MIPS_PC16 is pretty unusable at the moment, nothing should
   break.  Otherwise, we might create a BFD-specific extension (just like
   e.g. R_386_PC8 for i386).

The ABI defines R_MIPS_PC16.  Our implementation should match the ABI.
We can't argue that the ABI got it wrong and implement it differently;
we will fail to interoperate with other implementations.

I don't know why the ABI defines R_MIPS_PC16 the way it does, but I
doubt the authors made such a simple mistake.  ELF ABIs traditionally
only define the relocations needed to dynamically link programs on a
Unix system.  The 16 bit branch can not be used between object files
in an ordinary Unix program, because there is no way to guarantee that
it will not overflow.

Ian