Re: stop using P_, __P in header files

[Ken Raeburn <raeburn@raeburn.org>]

On Jul 6, 2010, at 00:32, Stephen J. Turnbull wrote:
> In principle, "register" is still occasionally useful.  If you have a
> large number of variables in a loop which has two long branches, then
> which variables belong in registers depend on which branch is taken
> more often, something that the compiler cannot know.

If the compiler pays attention to them, yes, it could help.  Though for reasons other than register allocation it's probably more useful to tell the compiler which branches are taken most often, though unfortunately that's still compiler-specific (e.g., __builtin_expect, or feedback from profiling runs).

> Also, "large" depends on the number of registers available, of course.
> In the case of x86, "large" is pretty small, about 8-10 IIRC.

Fewer -- x86 has 8 integer registers including stack pointer and frame pointer, and some are clobbered in function calls.  Slap too many "register" declarations onto variables and the compiler is still going to have to guess which of them should actually be heeded.  But if you keep the number small, you can't give the compiler much advice on x86-64 or ppc or other platforms with many more registers available.

I will (grudgingly :-) admit there's sometimes a need to give the compiler some hints that it can't necessarily work out for itself.  But I think the use of "register" in Emacs goes well beyond that, as I pointed out regarding some functions in data.c.  And I suspect any significant fine-tuning with register declarations is likely to be specific to an architecture and/or compiler.

Ken