Re: IRA patch: use ORDER_REGS_FOR_LOCAL_ALLOC

[Bernd Schmidt <bernds at codesourcery dot com>]

On 04/28/2010 12:52 AM, Jeff Law wrote:
> Well we no longer have a distinction between local & global allocation,
> so ISTM that ORDER_REGS_FOR_LOCAL_ALLOC has been overtaken by events.  
> If you're getting better allocations with the patch then I would think
> that you need to be defining REG_ALLOC_ORDER for the thumb differently.

That's the thing, there is only one way to define REG_ALLOC_ORDER since
it's an initializer.  If you need different ones depending on target
switches, as on ARM/Thumb, you need ORDER_REGS_FOR_LOCAL_ALLOC.  It's
just misnamed.  Even pre-IRA, it had an effect on both local and
global-alloc.

> WRT prologue/epilogue costs -- that's an ongoing discussion between Vlad
> and myself -- there's definitely arguments for and against including
> those costs and if they're going to be included whether or not the cost
> should be scaled or not, how to account for the fact that the cost to
> save/restore can be spread across multiple insns, etc etc.    I don't
> think we've reached any kind of definitive answer on this -- meaning
> it'll probably be punted until we can do some extensive benchmarking
> across several platforms.

What's there now is clearly wrong in some cases - e.g. on ARM, when
optimizing for size, once you save one register it doesn't cost anything
else to save more of them.

Should I add a new target macro that decides whether to ignore those costs?


Bernd