How ld decides offset for relocation type MIPS GPREL16?

[jigsaw]

Hi,

I'm writing a runtime linker for MIPS32 arch, and have problems with
GPREL16 relocation type.
For instance, the compiler/assembler outputs a relocatable object with
an instruction as below :

    ff838018        sd      v1,-32744(gp)

The symbol involved here belongs to .sbss section, and is set to
runtime address 0x12b119e8.
Given that _gp is 0x12b12220, and .sbss section starts at 0x12b11868.
( The values are read from target ELF object.)

After relocated by ld, the instruction becomes:

    ff83f7c8        sd      v1,-2104(gp)

My question is: How ld calculates the offset 0xf7c8?
My guess is that 0xf7c8 is calculated by _gp, .sbss address and
original offset 0x8018.
And I believe the related code is in routine
_bfd_mips_elf_gprel16_reloc of binutils/src/bfd/elf32-mips.c.

But I'm not able to figure out how this is calculated.
Could someone give a hand?

thanks &
regards,
-Qinglai