Hi Andrew,
Andrew Dyer wrote:
Typically _gp is set in the linker script to 0x7ff0 above the start of the 'small' sections (all ELF sections with SHF_MIPS_GPREL flag), so that all small items can be accessed by a 16 bit signed offset off of the register allocated for _gp. The 16-bit signed offset corresponds to the native MIPS addressing mode. The .got section usually comes first in the link map and so .got + 0x7ff0 = _gp.
Somewhere in the linking process the linker calculates the signed offset necessary for each instruction accessing data in these sections based on the _gp symbol from the link scripts and puts that into the appropriate offset field in the instruction.
For us, I don't believe it matters where _gp is, as long as the relative offsets to all small items can be held in the 16 bit field and that _gp is aligned properly. Obviously we need to fixup the _gp register correctly during relocation. Since _gp is only a pointer, IMHO it makes sense to adhere to the standard map with a 0x7ff0 offset.
(BTW, ${CROSS_COMPILE}ld --verbose will dump the normal link script that gets used in an ELDK setup)
Good abstract. I hope that I don't miss anything. I'll submit patches later, please review them.
thanks a lot,
Shinya