[PATCH 2/3] [PBQP] Callee saved regs should have a higher cost than scratch regs

[Arnaud A. de Grandmaison]

Hi Lang,

 

Could you please review this second patch in the PBQP improvement series ?

 

This patch teaches PBQP to handle callee saved registers: registers are not
all equal.

Some are not allocatable (infinite cost), some have to be preserved but can
be used,

and some others are just free to use.

 

Ensure there is a cost hierarchy reflecting this, so that the allocator will
favour scratch

registers over callee-saved registers. This does not model fully the reality
as the cost

for saving / restoring those CSR is paid only once, at their very first use,
and they then

become equivalent to scratch registers: with this patch, they will always
have a higher cost.

 

One way to improve this would be to handle that on a per allocation round
basis: if a CSR has

been used in one of the preceding rounds, then it becomes a scratch reg in
the sense that

it is as cheap to use as a scratch reg. This would be closer to reality.
What do you think ?

 

Cheers,

--

Arnaud A. de Grandmaison

 
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