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Re: Bad MIPS address arithmetic
On 05/10/2010 02:17 PM, Paul Koning wrote:
I spotted this in binutils 2.18.
Given the source file:
foo: ld $v0,40000($sp)
jr $ra
The resulting code is:
lui v0, 1
addu v0, v0, sp
jr ra
ld v0, -25536(sp)
The problem is that this produces wrong addresses in machines with 64
bit registers, if the current sp is 0x7fff0000 or higher. If so, the
addu produces 0xffffffff8000nnnn in v0, and the ld then references
0xffffffff7fffnnnn which is not likely to be a valid address.
Well things get tricky up there near the top of USEG. That is why the
Linux kernel never places the stack in that region.
It seems that daddu rather than addu should be used here, for O64 (and
probably N32) ABIs.
This is what I get:
$ cat koning.s
foo: ld $v0,40000($sp)
jr $ra
$ mips64-linux-as --version
GNU assembler (GNU Binutils) 2.20
Copyright 2009 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or later.
This program has absolutely no warranty.
This assembler was configured for a target of `mips64-linux'.
$ mips64-linux-as -o koning.o koning.s
$ mips64-linux-objdump -d -r koning.o
koning.o: file format elf32-ntradbigmips
Disassembly of section .text:
00000000 <foo>:
0: 3c020001 lui v0,0x1
4: 005d1021 addu v0,v0,sp
8: 03e00008 jr ra
c: dc429c40 ld v0,-25536(v0)
$ mips64-linux-as -mabi=32 -o koning.o koning.s
$ mips64-linux-objdump -d -r koning.o
koning.o: file format elf32-tradbigmips
Disassembly of section .text:
00000000 <foo>:
0: 3c010001 lui at,0x1
4: 03a10821 addu at,sp,at
8: 8c229c40 lw v0,-25536(at)
c: 8c239c44 lw v1,-25532(at)
10: 03e00008 jr ra
14: 00000000 nop
...
$ mips64-linux-as -mabi=64 -o koning.o koning.s
$ mips64-linux-objdump -d -r koning.o
koning.o: file format elf64-tradbigmips
Disassembly of section .text:
0000000000000000 <foo>:
0: 3c020001 lui v0,0x1
4: 005d102d daddu v0,v0,sp
8: 03e00008 jr ra
c: dc429c40 ld v0,-25536(v0)
So I think it is working as it was intended. For the default n32 ABI,
the ADDU is the proper instruction to use for pointer arithmetic. Other
ABIs do different things. If you want code for n64, you should specify
that when invoking the assembler. You can also add '.set nomacro' if
you don't want it to do these weird things.
In MIPS assembly, what you think is an instruction often isn't, the ABI
and other things have a big influence.
David Daney