Re: Graph colouring and local register allocation

On Fri, 30 Nov 2007 16:17:40 +0100, Sid Touati <Sid.Touati@uvsq.fr>
wrote:


>Miles Bader a icrit :
>
>> So what do "modern" compilers use?
>
>The term "modern" has no sense. It's marketing. Maybe you mean "actual".
>
>Indeed, you should make a distinction what actual compilers do and
>what they should do. Most of the compilers use old register
>allocation techniques, designed for sequential processors. When ILP
>came, few of them used advanced techniques based on strong theoretical
>research results. What happened is that many ad hoc practical
>heuristics have been quickly designed to tackle the problem of
>register allocation with ILP. Some improvement has been measured, but
>on some simple non representative benchmarks. On real life codes, the
>old register allocation techniques are not as efficient as people
>think...


You didn't answer Miles's question.


Frankly, I don't see what impact ILP has on register allocation. The
fact that operations may be performed in parallel has no bearing
whatsoever on the number of live values in a block of code. Either
the processor has enough registers to handle them all concurrently or
it doesn't, and if there aren't enough registers to handle all the
live values, you have to prioritize. Existing instruction scheduling
and register allocation methods are perfectly adequate to do that.


If you know something different then how about providing some cites to
help the rest of us?


George

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