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Re: Question about MIPS GOT / value computed into $gp.
- From: David Daney <ddaney at avtrex dot com>
- To: John <divilment at gmail dot com>
- Cc: binutils at sources dot redhat dot com
- Date: Tue, 30 Jan 2007 13:49:16 -0800
- Subject: Re: Question about MIPS GOT / value computed into $gp.
- References: <firstname.lastname@example.org>
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 184.108.40.206.20 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.
It is unfortunate that you are stuck. The current CVS HEAD fixes many
bugs in mips support.
My question is why the gp computed in the statically linked code is
0x10 off what i expected it to be.
Look in the linker scripts. _gp and (thus _gp_disp) are 0x7ff0 from the
start of the .got output section. I think the ABI documents address
this point. The locations just prior to the got point to the dynamic
linker entry point used to resolve PLT entries via the linker stubs.
Your missing 0x10 is needed as part of the linker stub protocol as
access to these locations is always done relative to $gp.
For example, given the following source (in functions.c)I don't know. Are you having some problem? Or are you just trying to
understand how it works?
int sum(int a, int b)
return a + b;
I get the following code after doing a gcc -c functions.c followed by an
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
after static linking against uClibc (and an objdump -D), this becomes:
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
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?