%pc relative addressing of string literals/const data

[Joakim Tjernlund]

Alan Modra <amodra@gmail.com> wrote on 2010/10/06 00:19:26:
>
> On Tue, Oct 05, 2010 at 11:40:11PM +0200, Joakim Tjernlund wrote:
> > yes, but this could be a new PIC mode that uses a new better
> > PIC mode for everything. Especially one that doesn't require each function
> > to calculate the GOT address in the function prologue(why is that so?)
>
> The ppc32 ABI is old, much like x86.  cf. x86 -O2 -fPIC (without
> hidden pragma).
>
> foo:
>    call   __i686.get_pc_thunk.cx
>    addl   $_GLOBAL_OFFSET_TABLE_, %ecx
>    pushl   %ebp
>    movl   %esp, %ebp
>    popl   %ebp
>    movl   y@GOT(%ecx), %eax
>    movl   x@GOT(%ecx), %edx
>    movl   (%eax), %eax
>    addl   (%edx), %eax
>    ret
> [snip]
> __i686.get_pc_thunk.cx:
>    movl   (%esp), %ecx
>    ret
>
> The new ppc64 -mcmodel=medium support does give you pic access to
> locals.
>
> -fPIC -O2 without hidden
> .LC0:
>    .tc x[TC],x   <-- compiler managed GOT entries
> .LC1:
>    .tc y[TC],y
> [snip]
> .L.foo:
>    addis 11,2,.LC0@toc@ha
>    addis 9,2,.LC1@toc@ha
>    ld 11,.LC0@toc@l(11)
>    ld 9,.LC1@toc@l(9)
>    lwz 3,0(11)
>    lwz 0,0(9)
>    add 3,3,0
>    extsw 3,3
>    blr
>
> -fPIC -O2 with hidden pragma
> .L.foo:
>    addis 11,2,x@toc@ha
>    addis 9,2,y@toc@ha
>    lwz 3,x@toc@l(11)  <-- TOC/GOT pointer relative
>    lwz 0,y@toc@l(9)
>    add 3,3,0
>    extsw 3,3
>    blr
>
> x@toc is equivalent to @GOTOFF on other processors.

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.

I was looking for true %pc relative addressing of data. I guess this is really
hard on PowerPC?

I have hacked -fpic support for -mrelocatable too:

>From d8ff0b3f0b44480542eab04d1659f4368b6b09cf Mon Sep 17 00:00:00 2001
From: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Date: Sun, 10 Oct 2010 10:34:50 +0200
Subject: [PATCH] powerpc: Support -fpic too with mrelocatable


Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
---
 sysv4.h |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/gcc/config/rs6000/sysv4.h b/gcc/config/rs6000/sysv4.h
index 8da8410..e4b8280 100644
--- a/gcc/config/rs6000/sysv4.h
+++ b/gcc/config/rs6000/sysv4.h
@@ -227,7 +227,8 @@ do {									\
     }									\
 									\
   else if (TARGET_RELOCATABLE)						\
-    flag_pic = 2;							\
+    if (!flag_pic)							\
+      flag_pic = 2;							\
 } while (0)

 #ifndef RS6000_BI_ARCH
--
1.7.2.2

I am not sure this is all it takes to make -fpic to work with -mrelocatable,
any ideas?

           Jocke