[PATCH] riscv: Fix breakage caused by linker relaxation

[Bin Meng]

On Wed, Dec 18, 2019 at 8:09 PM Sean Anderson <seanga2 at gmail.com> wrote:
>
> 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-toolchain>.
> 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);
> --

Good catch!

Reviewed-by: Bin Meng <bmeng.cn at gmail.com>

But I wonder how U-Boot managed to work on RISC-V till today?

Regards,
Bin