Re: [PATCH][AArch64] Separate shrink wrapping hooks implementation

[Kyrill Tkachov <kyrylo.tkachov@foss.arm.com>]

On 18/11/16 12:50, Segher Boessenkool wrote:
> On Fri, Nov 18, 2016 at 09:29:13AM +0000, Kyrill Tkachov wrote:
>>> So your COMPONENTS_FOR_BB returns both components in a pair whenever one
>>> of those is needed?  That should work afaics.
>> I mean I still want to have one component per register and since
>> emit_{prologue,epilogue}_components knows how to form pairs from the
>> components passed down to it I just need to restrict the number of
>> components in any particular basic block to an even number.
>> So say a function can wrap 5 registers: x22,x23,x24,x25,x26.
>> I want get_separate_components to return 5 components since in that hook
>> we don't know how these registers are distributed across each basic block.
>> components_for_bb has that information.
>> In components_for_bb I want to restrict the components for a basic block to
>> an even number, so if normally all 5 registers would be valid for wrapping
>> in that bb I'd only choose 4 so I could form 2 pairs. But selecting only 4
>> of the 5 registers, say only x22,x23,x24,x25 leads to x26 not being saved
>> or restored at all, even during the normal prologue and epilogue because
>> x26 was marked as a component in components_for_bb and therefore omitted
>> from
>> the prologue and epilogue.
>> So I'm thinking x26 should be removed from the wrappable components of
>> a basic block by disqualify_components. I'm trying that approach now.
> My suggestion was, in components_for_bb, whenever you mark x22 as needed
> you also mark x23 as needed, and whenever you mark x23 as needed you also
> mark x22.  I think this is a lot simpler?

But then we'd have cases where we're saving and restoring x23
even when it's not necessary.
In any case, I tried it out and it didn't fix the gobmk issue, though it did reduce the code
size increase somewhat.

With the patch already posted at [1] the net result is still positive on
both SPECINT and SPECFP.

I also ran the numbers on a Cortex-A57. The changes are less pronounced
with SPECINT being neutral (gobmk shows only a 0.8% regression) and SPECFP
having a small improvement, due to povray improving by 2.9%.

Thanks,
Kyrill

[1] https://gcc.gnu.org/ml/gcc-patches/2016-11/msg01352.html

>
> Segher