Clarification Req: Regarding "memory" clobber for cache operation is required (?)

["Ashok.A" <ashok_kumar_ak@yahoo.com>]

Hello,

Need one clarification regarding the issue given
below. Could you please clarify it? 

To invalidate a cache line, do we really need to
use "memory" clobber in inline asm?

Example: (for MIPS-III architecture) 

   asm ("cache 17, 0(%0)"
        : /* no outputs */
        : "r"(ptr));

Where, 'ptr' (pointer to unsigned char type) points
to base address of cache to be invalidated.

For this case, do we need "memory" clobber in the
inline asm? Or, anyway to specifiy appropriate 
memory constraints?

Expecting for yours response ...

Thanks,
Ashok

P.S: I have checked the linux source. Looks neither
     "memory" clobber nor any memory constraint is
     specified in inline asm for similar operations
     in linux source.

     
--- Jim Wilson <wilson@specifixinc.com> wrote:
> Ashok.A wrote:
> > I need clarification regarding the "+m"
> constraints.
> > I heard that "+m" constraints cannot be used
> > in "inline asm". Is it true?
> 
> Current sources will give a warning for both of your
> examples.  Neither 
> read-write "+" or matching "1" constraints should be
> used with mems in 
> an asm.  These both require that the operands are
> the same, but the 
> compiler is not able to guarantee this for mems.  We
> can only make this 
> work for register constraints.
> 
> Try changing the lw to use %2 instead of %1, and
> then use "m" for the 
> second operand.  I think this is the way it is
> supposed to work nowadays.
> -- 
> Jim Wilson, GNU Tools Support,
> http://www.SpecifixInc.com
> 


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