RE: [PATCH 1/3] MIPS: Initial implementation of a VDSO

[Maciej W. Rozycki]

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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