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

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

On 17/11/16 17:45, Segher Boessenkool wrote:
> On Thu, Nov 17, 2016 at 04:50:46PM +0000, Kyrill Tkachov wrote:
>>>> Is loading/storing a pair as cheap as loading/storing a single register?
>>>> In that case you could shrink-wrap per pair of registers instead.
>>> I suppose it can vary by microarchitecture. For the purposes of codegen
>>> I'd say
>>> it's more expensive than load/storing a single register (as there's more
>>> memory bandwidth required after all)
>>> but cheaper than two separate loads stores (alignment quirks
>>> notwithstanding).
>>> Interesting idea. That could help with code size too. I'll try it out.
>> I'm encountering some difficulties implementing this idea.
>> I want to still keep the per-register structures across the hooks but
>> basically restrict the number
>> of components in a basic block to an even number of FPRs and GPRs. I tried
>> doing this in COMPONENTS_FOR_BB
> 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.

Thanks,
Kyrill

>> but apparently this ended up not saving/restoring some of the registers at
>> all because the components that were
>> "filtered out" that way still made their way to the bitmap passed into
>> SET_HANDLED_COMPONENTS and so the normal
>> prologue/epilogue didn't end up saving and restoring them.
> I am not sure what this means?  "filtered out"?
>
>
> Segher