Query regarding volatiles and store CCP.

[Ramana Radhakrishnan]

Hi,

While upgrading a port of mine to trunk for a testcase I noticed the
following . Its more of a question for a language lawyer I guess.

The test looks like this.

int spinlock[2];

void foo (void)
{
volatile int * spinlock0;

while (*spinlock0 == 0)
{
/* do nothing */
}
}

Store CCP folds away the assignment in the following way :


Folded statement: *spinlock0_1 = 0;
            into: spinlock[0] = 0;

Folded statement: *spinlock1_2 = 0;
            into: spinlock[1] = 0;

Folded statement: D.1498_3 = *spinlock0_1;
            into: D.1498_3 = spinlock[0];

main ()
{
  volatile int * spinlock1;
  volatile int * spinlock0;
  int D.1498;

<bb 2>:
  spinlock0_1 = &spinlock[0];
  spinlock1_2 = &spinlock[1];
  spinlock[0] = 0;
  spinlock[1] = 0;

<bb 3>:
  D.1498_3 = spinlock[0];
  if (D.1498_3 != 0)
    goto <bb 3>;
  else
    goto <bb 4>;

<bb 4>:
  return;

}

which later results in the loop getting optimized away. However
marking spinlock as volatile let the loop remain.


I originally thought this to be a problem . However after chatting
about it on IRC I wasn't sure if this was (un)defined behaviour.   I
looked through the standard but was unable to figure out from the
prose , whether it stated some place that the object pointed to also
should also have the same type qualifiers as the pointer being used to
access this.


Thanks in advance
Ramana




-- 
Ramana Radhakrishnan