On Tue, Oct 05, 2010 at 11:56:55AM -0700, Richard Henderson wrote:
> On 10/05/2010 06:54 AM, Joakim Tjernlund wrote:
> > Ian Lance Taylor <[email protected]> wrote on 2010/10/05 15:47:38:
> >> Joakim Tjernlund <[email protected]> writes:
> >>> While doing relocation work on u-boot I often whish for strings/const data
> >>> to be accessible through %pc relative address rather than and ABS address
> >>> or through GOT. Has this feature ever been considered by gcc?
> >>
> >> The feature can only be supported on processors which are reasonably
> >> able to support it. So, what target are you asking about?
> >
> > In my case PowerPC but I think most arch's would want this feature.
> > Is there arch's where this cannot be support at all or just within
> > some limits? I think within limits could work for small apps
> > like u-boot.
>
> PowerPC doesn't really have the relocations for pc-relative offsets
> within code -- primarily because it doesn't have pc-relative insns
> to match. Nor, unfortunately, does it have got-relative relocations,
> like some other targets. These are normally how we get around not
> having pc-relative relocations and avoiding tables in memory. C.f.
The new PowerPC medium code model for 64-bit now uses GOT-relative relocations
for local data: The compiler issues (r2 has the TOC/GOT pointer):
addis <reg>,r2,la...@toc@ha
addi <reg2>,<reg>,la...@toc@l
to load the address in <reg2>. If the data happens to be within 32K of the TOC
pointer, the linker will transform this to:
nop
addi <reg2>,r2,la...@toc@l
For non-local loads or the large code model, the compiler issues:
addis <reg>,r2,la...@got@ha
ld <reg2>,la...@got@l(<reg>)
If the address happens to be defined in the current program unit (main program
or shared librar), the linker can transform this to:
addis <reg>,r2,la...@toc@ha
addi <reg2>,<reg>,la...@toc@l
or:
nop
addi <reg2>,r2,la...@toc@l
or:
nop
ld <reg2>,la...@got@l(r2)
--
Michael Meissner, IBM
5 Technology Place Drive, M/S 2757, Westford, MA 01886-3141, USA
[email protected]
