Ambiguous C?

[Andrew Koenig]

In article <111 at ssp1.idca.tds.philips.nl>, roelof at idca.tds.PHILIPS.nl (R. Vuurboom) writes:

> The following piece of code got me into trouble.
> I needed to access a register which only allows long accesses.

 . . .

> Does C specify which (if any) interpretation is correct?

> #define	SCUCMD		0 
> #define	SCU_BDID	0x2000		/* bit 14 */

> 	if ( (*(int *)(SCUCMD)) & SCU_BDID )

There are two separate questions:

	1.  Does C define the meaning of this program fragement?

	2.  How do you get the compiler to do what you want?

The answer to (1) is `no' -- once you cast an integer into
a pointer, you're on your own.

In practice, what's probably happening is that the compiler
realizes that there's only one bit turned on in the value
of SCU_BDID, so it's optimizing the code by not fetching
the entire word to test.

To suppress the optimization, you can probably conceal the
value of SCU_BDID from the compiler by using it in a way that
the compiler thinks might change.  For instance:

	static int SCU_BDID = 0x2000;

Now the compiler probably won't be smart enough to realize that
SCU_BDID is really a constant, so it will have to generate code
that caters to any bit in the whole word being on.
-- 
				--Andrew Koenig
				  ark at europa.att.com