On Mon, 28 Sep 2015, Matthew Fortune wrote:
> > > + /* lapc <symbol> is an alias to addiupc reg, <symbol> - .
> > > + *
> > > + * We can't use addiupc because there is no label-label
> > > + * support for the addiupc reloc
> > > + */
> > > + __asm__("lapc %0, _start \n"
> > > + : "=r" (addr) : :);
> >
> > Just curious - if lapc is just an alias to addiupc, why does that work
> > but not addiupc? IIRC I did try addiupc previously but removed it
> > because it wasn't working, didn't know about lapc!
>
> This is just an unfortunate quirk of how the implementation is done in
> binutils. We don't recognise the special case that:
>
> addiupc <reg>, <sym> - .
>
> is the same as
>
> lapc <reg>, <sym>
>
> And therefore don't know that we can just use the MIPS_PC19_S2 reloc
> (name of that reloc may not be perfectly correct). It is a special
> case as the RHS of the expression in ADDIUPC above can be theoretically
> anything so we only support assembly time constants with addiupc.
>
> Apart from the need to document the LAPC alias somewhere I'm not sure
> we need do anything to improve addiupc itself particularly.
For the record -- this corresponds to how the LA macro and the
PC-relative ADDIU instruction are handled when assembling MIPS16 code.
And the place to document such peculiarities is obviously an assembly
language manual. A few have been written for the MIPS architecture
already and with recent updates to the instruction set perhaps it is time
for a revised edition or yet another book.
Maciej
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