On Tue, 16 Jan 2007 17:04:27 -0500, Jeff Garzik wrote: >Alan wrote: >> Jeff - at some point we could eliminate a lot of the NULL checks like >> these by having the registration code fill in the defaults where >> appropriate ? > >libata policy up until this point has been to require all drivers fill >in the hook, either with the commonly-used default, or with their own >variant. Thus, no NULL checks for required hooks, and you get an oops >if you fail this requirement. I like that better than defaults buried >inside libata-core, where it is easier for programmers to forget them. > >But actually, it's an open question for the compiler guys whether this >case is preferred: > > if (ap->ops->hook) > ap->ops->hook(foo, bar); > else > commonly_used_default(foo, bar); > >or this: > > ap->ops->hook(foo, bar); > >With advanced branch prediction in modern CPUs, ISTR the first case may >be worth considering, even with the branch. Indirect function calls incur at least two main costs: - the 'call *' instruction itself suffers from poor branch prediction unless the HW is clever and the call patterns are highly biased -- most machines have worse branch predictors for indirect jumps than for direct jumps - the compiler must generate code for setting up parameters and spilling the caller's registers, and for reloading the caller's registers after the call If there is a highly common case, the code for that case is available, and the code is cheap to execute (doesn't require too many additional variables), then having an explicit test + inline code is preferred. If any of these conditions isn't met, then I wouldn't bother converting the call to the if/inline/call combo. There are compilers for OOP languages that routinely use rewrites like the one above in order to optimise method calls. /Mikael - To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
