On Wed, Dec 18, 2019 at 8:09 PM Sean Anderson <sean...@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 <sean...@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...@gmail.com> But I wonder how U-Boot managed to work on RISC-V till today? Regards, Bin
