Re: MIPS assembly question

[David Daney <ddaney@avtrex.com>]

Winson Yung wrote:
> Hi there, I have some general mips inline assembly question regards to
> 32 bit atomic operation, here a section of its assembly
> implementation:
> 
>                 "       .set    mips3                                   \n"
>                 "1:     ll      %0, %2                  # __cmpxchg_u32 \n"
>                 "       bne     %0, %z3, 2f                             \n"
>                 "       .set    mips0                                   \n"
>                 "       move    $1, %z4                                 \n"
>                 "       .set    mips3                                   \n"
>                 "       sc      $1, %1                                  \n"
>                 "       beqzl   $1, 1b                                  \n"
> 
> Questions:
> 
> 1) what does 'z' mean in the line of 'bne %0, %z3, 2f'?

I think this 'z' comes from print_operand() in gcc/config/mips/mips.c in 
  GCC:

    'z'	if the operand is 0, use $0 instead of normal operand.

This is an optimization so that if you are comparing against the value 
of zero, you can use $0 instead of loading up another register with the 
value of zero first.

> 2) Is $1 suppose to be use as an constant 1, I don't understand the
> line 'sc  $1, %1'

$1 is register 1.  AKA $at.

So that line is Store Conditional Word from register 1 into the memory 
location indicated by operand 1.

> 
> Will appreciate if someone can point out to me a good tutorial on
> explaining these little things.
> 

If it is not in the GCC documentation, then you have to look at the GCC 
source code.  I don't know of any better way.

David Daney