A question about "memory" clobbers in asm

[Nathan Sidwell]

Jamie Lokier wrote:
> I'm having trouble understand when it's appropriate to use a "memory"
> clobber, when to use volatile, and when to use both.  The manual is
> unclear to me.  Although I see that someone has tried to clarify it
> since I last read it, it's still not obvious how "memory" is different
> from `volatile'.

my understanding is

* volatile asm says something to the effect 'this changes state that you
(the compiler) don't know about' -- such as writing to an IO port.

* memory clobber says 'this asm will change memory, so don't cache anything
across this asm'.

So,

   asm volatile ("pokeIOport");
   v = *ptr;

could hoist the load before the asm, but the asm will never be deleted
from the code sequence.  Whereas

   asm ("readsomething %0" :"=r" (v2) :: "memory");
   v = *ptr;

would do no such hoisting, but would delete the asm, if V2 was never
subsequently used.

nathan

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