Implement basic array prefetching hints in Hydrogen.

Implement basic array prefetching hints in Hydrogen.
Insert prefetch instructions for those array element accesses (reads) with the
prefetch hints in IA32 Lithium code generator.

With this CL Kraken reports 60% improvement on audio-dft on Atom, where huge
amount of cache misses were observed before.

Currently the write accesses are not prefetched, just in order to isolate
the individual impacts.

[yuqiang.xian%intel.com, 2013-01-10 06:41:20]

On 2012/12/04 08:39:13, yuqiang.xian%intel.com wrote:

Patch updated for latest bleeding_edge.
Can you please take a look? Thanks.

[Jakob Kummerow, 2013-01-11 17:48:16]

Thank you for contributing this patch! However, given the substantial size of
this change and the complexity it introduces, we concluded that it’s premature
to land it despite the benefit it provides. There’s a simpler way to achieve the
same result.

We are currently working on introducing a “single static information” (SSI)
representation to Crankshaft (see C. Scott Ananian’s paper from 1999), which
overlaps with what this patch is attempting to achieve, but in an
architecturally cleaner way. It opens up a whole range of future optimization
opportunities. Once a bit more of the SSI work is in place, it’s likely your
patch can be reworked to be much simpler.

We are aware that you are probably not too happy about a change you put hard
work into being rejected. To avoid this in the future, it would be great if you
could give us a heads-up when you have ideas that require larger changes to V8
so that we can figure out overlaps and shortcomings together before time is
invested in implementing those ideas.