Re: %pc relative addressing of string literals/const data

[Richard Henderson <rth at redhat dot com>]

On 10/05/2010 06:54 AM, Joakim Tjernlund wrote:
> Ian Lance Taylor <iant@google.com> wrote on 2010/10/05 15:47:38:
>> Joakim Tjernlund <joakim.tjernlund@transmode.se> 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.

  #pragma GCC visibility push(hidden)
  extern int x, y;
  int foo(void) { return x + y; }

Without pragma (-O2 -fpic):
i386:
        movl    y@GOT(%ecx), %eax
        movl    x@GOT(%ecx), %edx
        movl    (%eax), %eax
        addl    (%edx), %eax

alpha:
        ldq $1,y($29)           !literal
        ldl $0,0($1)
        ldq $1,x($29)           !literal
        ldl $1,0($1)
        addl $0,$1,$0

In both cases here, we have load the address from memory, from
the GOT table, then load the data (X or Y) from memory and 
perform the addition.


With pragma:
i386:
        movl    y@GOTOFF(%ecx), %eax
        addl    x@GOTOFF(%ecx), %eax

alpha (-fpic):
        ldl $1,x($29)           !gprel
        ldl $0,y($29)           !gprel
        addl $0,$1,$0

alpha (-fPIC):
        ldah $1,y($29)          !gprelhigh
        ldl $0,y($1)            !gprellow
        ldah $1,x($29)          !gprelhigh
        ldl $1,x($1)            !gprellow
        addl $0,$1,$0

In all cases here, we've replaced the load from the GOT table
with arithmetic.  In the case of i386 this gets folded into the
memory reference.  The alpha cases are essentially the same as
what ppc could generate if it had the appropriate relocations.

If you're sufficiently motivated, you could work with the ppc
maintainers to add the missing support.



r~