[LLVMdev] PBQP & CalcSpillWeights

[Arnaud de Grandmaison]

Hi Lang,

> From memory your target is not public, so I won't be able to reproduce
> the crash myself. Is that correct?
Correct.
 
> If that's the case, I could add functionality to dump the PBQP graphs
> during allocation. I think they should give me enough information to
> debug the issue. Would you be able to share the PBQP graphs?
I can share the pbqp graph if you send me a patch with the added functionality 
you want. On my side, I already checked the node cost for this register, which 
is correctly set to inf for spill, as well as the edge costs, and they look 
ok.

I also attached my target's pbqp related file in case you want to double check 
what I did. This is llvm-3.0 based. It comprises 2 passes : the 
FemtoPBQPBuilder, plus a FemtoPairablepass, which undo some of the coalescer's 
work. The insertRegCopy may specifically need a double check, as I am not 100% 
sure to have updated correctly the LiveInterval information.

In terms of registers, the Femto target is simplistic : a single register 
class GR16, for data and pointers, all i16. It has 16 registers, R0 to R15; 
R15 is used as stack pointer, and R14 potentialy as framepointer. A pair is 
constituted from a register + its successor, i.e. (R0, R1), (R1,R2), (R2, R3), 
... are valid pairs. This is an instruction encoding constraint, as we only 
have 16bits wide instructions. Pairs involving R15 are never allowed, those 
with R14 may be allowed, depending on each function.

Most instructions have no pairing constraints, and do not appear here, being 
handled by the PBQPBuilder default base class. A few instructions have 1 or 2 
pairs of registers, and are all handled by the 2 passes above.

Thanks for your help,
-- 
Arnaud de Grandmaison
Senior CPU engineer
Business Unit Digital Tuner

Parrot S.A.
174, quai de Jemmapes
75010 Paris - France
Phone: +33 1 48 03 84 59
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