[PATCH 1/3] [PBQP] Tweak spill costs and coalescing benefits

[Arnaud A. de Grandmaison]

Hi Lang,

 

Could you please review the attached patch ? This is the first out of three.

 

This patch improves how the different costs (register, interference, spill

and coalescing) relates together. The assumption is now that:

- coalescing (or any other "side effect" of reg alloc) is negative, and

   instead of being derived from a spill cost, they use the block

   frequency info.

- spill costs are in the [MinSpillCost:+inf( range

- register or interference costs are in [0.0:MinSpillCost( or +inf

 

The current MinSpillCost is set to 10.0, which is a random value high

enough that the current constraint builders do not need to worry about

when settings costs. It would however be worth adding a normalization

step for register and interference costs as the last step in the

constraint builder chain to ensure they are not greater than SpillMinCost

(unless this has some sense for some architectures). This would work well

with the current builder pipeline, where all costs are tweaked relatively

to each other's, but could grow above MinSpillCost if the pipeline is

deep enough.

 

The current heuristic is tuned to depend rather on the number of uses of

a live interval rather than a density of uses, as used by the greedy

allocator. This heuristic provides a few percent improvement on a number

of benchmarks (eembc, spec, ...) and will definitely need to change once

spill placement is implemented: the current spill placement is really

inefficient, so making the cost proportional to the number of use is a

clear win.

 

This is the only patch in this PBQP improvement serie without a testcase

because this patch widely changes allocation, and it is hard to isolate
simple and

meaningful testcases for it. The 2 following patches, which are addressing
specific

areas of the allocator have a testcase.

 

Cheers,

--

Arnaud A. de Grandmaison

 
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