[llvm-dev] linear-scan RA

[Preston Briggs via llvm-dev]

How precise is the interference checking (to my mind, a great weakness of
linear scan)?
Is there way to do coalescing (the great strength of coloring)?

I ask these questions because we (guys I work with) see loops
where there's a little register juggling that seems unnecessary.

Is there a paper that describes what y'all do?

Thanks,
Preston


On Mon, Sep 10, 2018 at 9:57 AM, Matthias Braun <mbraun at apple.com> wrote:

> I would not describe LLVMs register allocator as linear scan, it's closer
> to graph coloring than linear scan IMO (though doesn't really matcher
> either approach).
>
> RegAllocGreedy assigns the registers in an order based on the priority
> value computed in enqueu() we are not just scanning from top to bottom of
> the program. We also perform actual interference checks we just don't
> happen to build up an explicit interference graph up front.
>
> - Matthias
>
> On Sep 10, 2018, at 9:49 AM, Preston Briggs via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Why have we ended up using linear-scan register allocation
> by default (instead of, e.g., coloring)?
>
> Thanks,
> Preston
>
>
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