%pc relative addressing of string literals/const data

[Joakim Tjernlund]

Alan Modra <amodra@gmail.com> wrote on 2010/10/11 14:58:45:
>
> On Sun, Oct 10, 2010 at 11:20:06AM +0200, Joakim Tjernlund wrote:
> > Now I have had a closer look at this and it looks much like -fpic
> > on ppc32, you still use the GOT/TOC to load the address where the data is.
>
> No, with ppc64 -mcmodel=medium you use the GOT/TOC pointer plus an
> offset to address local data.

I guess I have to look closer then.

>
> > I was looking for true %pc relative addressing of data. I guess this is really
> > hard on PowerPC?
>
> Yes, PowerPC lacks pc-relative instructions.
>
> > I am not sure this is all it takes to make -fpic to work with -mrelocatable,
> > any ideas?
>
> You might be lucky.  With -mrelocatable, .got2 only contains
> addresses.  No other constants.  So a simple run-time loader can
> relocate the entire .got2 section, plus those locations specified in
> .fixup.  You'll have to make sure gcc does the same for .got, and your
> run-time loader will need to be modified to handle .got (watch out for
> the .got header!).

yes, almost there. I have this reloc routine:

#define GOT(NAME)		NAME ## @got (r30)

	 * Adjust got pointers, no need to check for 0, this code
	 * already puts a few entries in the table.
	 */
	li	r0,__got_entries@sectoff@l
	la	r3,GOT(_GOT_TABLE_)
	lwz	r11,GOT(_GOT_TABLE_)
	mtctr	r0
	sub	r11,r3,r11
	addi	r3,r3,-4
1:	lwzu	r0,4(r3)
	cmpwi	r0,0
	beq-	2f
	add	r0,r0,r11
	stw	r0,0(r3)
2:	bdnz	1b

Linker script:

.reloc   :
  {
    _GOT_TABLE_ = .;
    *(.got)
    _GOT2_TABLE_ = .;
    *(.got2)
    _FIXUP_TABLE_ = .;
    *(.fixup)
  }

The above depends on that the first entry in the got to
which _GOT_TABLE_ points, has an entry to itself.
Using -fPIC I could create such an entry but I cannot find a way
to do it using -fpic. Any ideas?

       Jocke