🚀 go-pugleaf

wiecek@ee.ualberta.ca (B. Wiecek) wrote:

>As far as I remeber (see above) the standard requires that statements
>are executed in the same order they are written, thus if I write:

>	a = 1;		/* statement1 */
>	b = a + 1;	/* statement2 */
>	c = 3;		/* statement3 */
>	d = c + 1;	/* statement4 */

>the standard requires that these statements shall be executed in this order,
>right?

As long as a, b, c and d are all non-volatile, it can reorder as it
wishes, as long as there's no way of telling the difference.  If they're
all non-volatile, the compiler is free to produce something like:

	a=1;
	b=2;
	c=3;
	d=4;

and execute them all simultaneously on a 4+ processor machine.  Or, if
these are static, and haven't been previously used, it could treat it
all as initialized data, with no actual execution involved at all.

>However, today's suprscalar processors (eg. Alpha) and compilers for them
>may/will reorder the above to something like:

>	a = 1;		/* statement1 */
>	c = 3;		/* statement3 */
>	b = a + 1;	/* statement2 */
>	d = c + 1;	/* statement4 */

>Does the standard allow this?

It all depends on whether any or all of a, b, c or d is volatile or not.
With volatile variables, the value of each variable must be correct at
each sequence point, and it's required to actually read/write the memory
location to obtain/store the value.  E.g. even if the value of a is in a
register after setting a in statement 1, it's still required to read the
value from memory when it executes statement 2.  Likewise with the value
of c in statement 3 & 4.
    Later,
    Jerry.

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