[U-Boot] [PATCH] mpc83xx: Add -fpic relocation support

[Joakim Tjernlund]

Wolfgang Denk <wd at denx.de> wrote on 2010/10/12 15:47:19:
> 
> Dear Joakim Tjernlund,
> 
> In message <OF4DCFBD28.58E81A84-ONC12577BA.0047150B-C12577BA.
> 0047D319 at transmode.se> you wrote:
> >
> > > What happens when one uses this patch in combination with a 
"standard"
> > > (i. e. older, unpatched) GCC?
> > 
> > Nothing, gcc will produce -fPIC relocs and the code will/should just 
work.
> 
> OK, so your change means effectively a no-op (except for the moderate
> size increase in start.S) to most of us?

Yes.

> 
> You mentioned -fpic was smaller and faster; do you have any numbers
> for that (especailly for the faster part) ?

No, but I can show a code fragment,

char *f()
{
        return "string";
}

-fPIC -mplt-bss:
.LC1:
        .long .LC0
        .section        ".text"
        .align 2
        .globl f
.LCL0:
        .long .LCTOC1-.LCF0
        .type   f, @function
f:
        stwu 1,-16(1)
        mflr 0
        bcl 20,31,.LCF0
.LCF0:
        stw 30,8(1)
        mflr 30
        stw 0,20(1)
        lwz 0,.LCL0-.LCF0(30)
        add 30,0,30
        lwz 0,20(1)
        lwz 3,.LC1-.LCTOC1(30)
        mtlr 0
        lwz 30,8(1)
        addi 1,1,16
        blr
.LC0:
        .string "string"
        .ident  "GCC: (Gentoo 4.4.4-r2 p1.2, pie-0.4.5) 4.4.4"
        .section        .note.GNU-stack,"", at progbits

-fpic -mbss-plt:
f:
        stwu 1,-16(1)
        mflr 12
        bl _GLOBAL_OFFSET_TABLE_ at local-4
        stw 30,8(1)
        mflr 30
        mtlr 12
        lwz 3,.LC0 at got(30)
        lwz 30,8(1)
        addi 1,1,16
        blr
.LC0:
        .string "string"
        .ident  "GCC: (Gentoo 4.4.4-r2 p1.2, pie-0.4.5) 4.4.4"
        .section        .note.GNU-stack,"", at progbits

> 
> > _GLOBAL_OFFSET_TABLE_ is a predefined symbol that the linker defines 
to
> > be in the middle of the -fpic GOT table. It marks the end of the GOT 
table
> > as far as we are concerned(u-boot does not generate so many relocs 
that 
> > the
> > linker needs to use the space above _GLOBAL_OFFSET_TABLE_)
> > 
> > There is no predefined symbol that marks the start of fpic relocs so
> > one is added(_GOT_TABLE_) by me to the linker script.
> 
> OK - can you please include these explanations into the commit
> message?  Thanks.

Will do.

       Jocke