May too many register variables hurt?

[Doug Gwyn]

In article <972 at mwtech.UUCP> martin at mwtech.UUCP (Martin Weitzel) writes:
>	... only the first few of such declarations are effective ...

Clearly what was meant by this is that only the first few OF THOSE CURRENTLY
IN SCOPE are effective.  Out-of-scope declarations are simply irrelevant.

As you note, the programmer has a hard time indicating which variables
are most important to registerize, when a variety of compilation
environments must be accommodated.  What some developers have done is
to add a batch of macros in their system-configuration header:
	/* Prioritized "register" declarations: */
	#define	REG_0	register
	#define	REG_1	register	/* last available for 6809 */
	#define	REG_2	register	/* last available for PDP-11 */
	#define	REG_3	/* nothing */
	#define	REG_4	/* nothing */
and in the application use these in a mutually-exclusive manner:
	func()	{
		REG_2 int	i;
		REG_0 char	*p;
		...
			{
			REG_1 char	*q;
			REG_3 int	j;
			...
			}
		}
so that no more "register" storage-class specifiers are seen by the
compiler in any scope than are actually effective for the implementation.

I don't use this method myself, preferring to use "register" as a hint
rather than a requirement, but if you are hyper-concerned about this
level of optimization you might want to consider such an approach.