Re: [PATCH] AArch64: Improve address rematerialization costs

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Richard,

>> There isn't really a better way of doing this within the existing costing code.
>
> Yeah, I was wondering whether we could change something there.
> ADRP+LDR is logically more expensive than a single LDR, especially
> when optimising for size, so I think it's reasonable for the rtx_costs
> to say so.  But that doesn't/shouldn't mean that spilling is better
> (for either size or speed).
>
> So it feels like there's something missing in the way the costs are
> being applied.

Calculating accurate spill costs is hard. Spill optimization is done later, so
until then you can't know the actual cost of a spill decision already made.
Spills are also more expensive than you think due to store latency, more
dirty cachelines etc. There is little benefit in lifting an ADRP to the start of
a function but keep ADD/LDR close to references. Basically ADRP/MOV are
very cheap, so it's a waste to allocate these to long-lived registers.

Given that there are significant codesize and performance improvements,
it is clear that doing more rematerialization is better even in cases where it
takes 2 instructions to recompute the address. Binaries show a significant
reduction in stack-based loads and stores.

Cheers,
Wilco