On 10/16/07, Wolfgang Denk wd@denx.de wrote:
Dear Shinya Kuribayashi,
on IRC you asked:
In Current u-boot.lds for MIPS ports, _gp is basically equals to __got_start. Is this intentional, or not? If this is intentional for simplification, I'll take into account such original intention. If not, I'll take Vlad suggests because what he changes is basically right as PIC code.
I'm afraid I don't remember this any more. The code is as old as the very first MIPS port for the Infineon INCA-IP board, back in March 2003. It is easily possible that we didn't know any better by then, and everybody just copied the code without thinking since.
Speaking for myself, I'm not much of a MIPS expert and will rely on your judgment. All I can do is helping to test changes on the INCA-IP board.
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)