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Question about MIPS GOT / value computed into $gp.
- From: John <divilment at gmail dot com>
- To: binutils at sources dot redhat dot com
- Date: Tue, 30 Jan 2007 21:12:05 +0000
- Subject: Question about MIPS GOT / value computed into $gp.
Hi Folks,
I have a question about the value held in $gp pointing to the GOT in
MIPS generated code after static linking.
I'm not sure whether this is/was an issue with binutils (i'm stuck
with ld version 2.13.90.0.20 for a couple of reasons), or whether I've
just missed something from the MIPS/SysV abi. Apologies if this is the
wrong place to solicit an answer to my question.
My question is why the gp computed in the statically linked code is
0x10 off what i expected it to be.
For example, given the following source (in functions.c)
int sum(int a, int b)
{
return a + b;
}
[snip]
I get the following code after doing a gcc -c functions.c followed by an objdump
00000000 <sum>:
0: 3c1c0000 lui gp,0x0
0: R_MIPS_HI16 _gp_disp
4: 279c0000 addiu gp,gp,0
4: R_MIPS_LO16 _gp_disp
8: 0399e021 addu gp,gp,t9
[snip]
after static linking against uClibc (and an objdump -D), this becomes:
00400300 <sum>:
400300: 3c1c0005 lui gp,0x5
400304: 279c8660 addiu gp,gp,-31136
# distance between start of sum and the middle of the got now in gp
400308: 0399e021 addu gp,gp,t9
# virtual address of sum is in $t9, so gp now points to middle of got
[...]
00440970 <_GLOBAL_OFFSET_TABLE_>:
440970: 00000000 nop
440974: 80000000 lb zero,0(zero)
It evaluates $gp to 0x448960 in 'sum' rather than the middle of the
GOT, which would be 0x440970 + 2^15 (which is 0x448970)?
Have I missed something obvious?
Thanks :)
John.