[PATCH] riscv: Fix breakage caused by linker relaxation

[Rick Chen]

> > From: Sean Anderson [mailto:seanga2 at gmail.com]
> > Sent: Wednesday, December 18, 2019 10:36 AM
> > To: u-boot at lists.denx.de; Rick Jian-Zhi Chen(陳建志)
> > Subject: [PATCH] riscv: Fix breakage caused by linker relaxation
> >
> > Due to the two-instruction sequence needed to access arbitrary memory
> > locations, the RISC-V linker aggressively optimises memory accesses and jumps
> > at link-time. This is called "linker relaxation," and is discussed in this SiFive
> > article
> > <https://www.sifive.com/blog/all-aboard-part-3-linker-relaxation-in-riscv-tool
> > chain>.
> > One of the optimizations in place is to assume that the __global_pointer
> > symbol is placed in the gp register. To quote the article:
> >
> > "...The magic __global_pointer$ symbol is defined to point 0x800 bytes past
> > the start of the .sdata section. The 0x800 magic number allows signed 12-bit
> > offsets from __global_pointer$ to address symbols at the start of the .sdata
> > section. The linker assumes that if this symbol is defined, then the gp register
> > contains that value, which it can then use to relax accesses to global symbols
> > within that 12-bit range. The compiler treats the gp register as a constant so it
> > doesn't need to be saved or restored, which means it is generally only written
> > by _start, the ELF entry point."
> >
> > However, U-Boot instead keeps the global data pointer in gp. This causes
> > memory accesses and jumps optimized to use the gp pointer to fail. To fix this
> > problem, we undefine the __global_pointer symbol.
> >
> > Signed-off-by: Sean Anderson <seanga2 at gmail.com>
> > ---
> >  arch/riscv/cpu/u-boot.lds | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/arch/riscv/cpu/u-boot.lds b/arch/riscv/cpu/u-boot.lds index
> > 838a844399..c00d17c736 100644
> > --- a/arch/riscv/cpu/u-boot.lds
> > +++ b/arch/riscv/cpu/u-boot.lds
> > @@ -32,7 +32,6 @@ SECTIONS
> >
> >       . = ALIGN(4);
> >       .data : {
> > -             __global_pointer$ = . + 0x800;
> >               *(.data*)
> >       }
> >       . = ALIGN(4);

Reviewed-by: Rick Chen <rick at andestech.com>