Re: Immediate branch offset

[Alexis BRENON <abrenon@wyplay.com>]

Le 09/06/2013 16:43, Maciej W. Rozycki a écrit :
> On Fri, 7 Jun 2013, Alexis BRENON wrote:
>
>> To create the JIT, I have to load some MIPS instruction directly in memory
>> without passing through a .asm file or else. So, I cannot set some labels. So
>> to make some branches, I try to load the equivalent instruction of :
>>      bne $t0, $t1, -8
>> to go back, just before the bne instruction, if $t0 and $t1 are equals. But
>> when it run, I've got an illegal instruction error.
>   Please note that BNE means Branch-if-Not-Equal, your quoted instruction
> will jump backwards if $t0 and $t1 are *not* equal.
>

Yes, sorry, it's just a typing mistake, since I tried with all branch 
instructions.

>> To debug, I write a small program in the MARS MIPS simulator with this
>> instruction. But when compiling, assembler says me that -8 is an operand of
>> incorrect type.
>   The instruction you quoted assembles for me successfully, what version of
> binutils do you use and what exact error message do you get?

I didn't try to assemble it, but only to run it in the MARS simulator. 
If I assemble it with GNU AS, it assembles successfully.

>   Please note however that this instruction is not what I understand you
> need -- it is treated as a branch to the absolute address -8 (0xfffffff8
> in the o32 ABI), rather than 8 bytes back (there's an off-by-four bug in
> GAS here too making it jump to -4 instead, and some other issues; I'll see
> if I can get them fixed sometime -- see the discussion around
> http://sourceware.org/ml/binutils/2012-09/msg00288.html if interested in
> the gory details).
>
>   If you want to jump to the instruction immediately preceding the branch
> and avoid a label (assuming the standard MIPS ISA), use:
>
> 	bne	$t0, $t1, . - 4
>
> -- "." is a special "the address of this instruction" designator (see the
> GAS manual for further information), so this produces the machine
> instruction you require (the jump is calculated as relative to the next
> instruction -- that is (. + 4) -- so the ultimate effective (i.e. shifted
> rather than as encoded in the instruction's 16-bit immediate operand
> field) offset is -8).
>
> $ cat foo.s
> 	bne	$t0, $t1, . - 4
> $ mips-linux-as -o foo.o foo.s
> $ mips-linux-objdump -dr foo.o
>
> foo.o:     file format elf32-tradbigmips
>
>
> Disassembly of section .text:
>
> 00000000 <.text>:
>     0:	1509fffe 	bne	t0,t1,0xfffffffc
>     4:	00000000 	nop
> 	...
> $ mips-linux-as --version
> GNU assembler (GNU Binutils) 2.23.2
> [...]
>
> Likewise with the current binutils trunk.
>
>   I hope this helps.
>
>    Maciej

I downloaded MIPS32 Architecture For Programmers Volume II: The MIPS32 
Instruction Set. If you read the BNE-BEQ-B... description it says :

     Purpose:
To compare GPRs then do a PC-relative conditional branch

	Description: if rs = rt then branch
An 18-bit signed offset (the 16-bit offset field shifted left 2 bits) is added to the address of the instruction
following the branch (not the branch itself), in the branch delay slot, to form a PC-relative effective target
address.

It says that it's a PC-relative jump, the offset is added to the PC 
value, and not a absolute jump as the J instruction. Nevertheless, I try 
to type this :

loop:
         addiu $v0, $v0, 1
         bne $v0, $t1, .-4


or this :

loop:
         addiu $v0, $v0, 1
         bne $v0, $t1, -8

In both cases, the objdump command says me :

    8:   24420001        addiu   v0,v0,1
    c:   1449fffe        bne     v0,t1,8 <loop>
   10:   00000000        nop

It seems to be equivalent.

Thanks for your answer.
Friendly,
Alexis BRENON