llvm dev - Dec 2012 - [LLVMdev] LLVM ERROR: ran out of registers during register allocation

On Dec 19, 2012, at 8:58 AM, Borja Ferrer <borja.ferav at gmail.com>
wrote:
> Hello Jakob,
> 
> I think I've found something interesting that may help you get a better
idea of what's going on.
> 
> While looking at the debug info I noticed that the coalescer was removing
lots of copies that could help the allocator make more cross class copies. As a
test, I disabled the join-liveintervals option in the coalescer which gave me
the surprise of  making the regalloc succeed. To show what I mean, I'm
attaching two text files that show the debug info generated when this option is
enabled and disabled for my target.
> 
> To push things a bit further, I wrote a dirty hack in
RegisterCoalescer::joinCopy() to return false when the dest regclass is too
constrained. This allowed me executing the coalescer without crashing the
regalloc. Obviously the generated code is not optimal at all, because there are
many useless copies around. I'm pretty sure this is not the right fix at
all, but it can give you a hint incase the problem is in the coalescer and not
in the regalloc.
We did something like this back when the register allocator couldn't split
live ranges.

The problem is that any heuristic you can come up with only makes the problem
less likely to happen. It doesn't actually fix it.

/jakob

>
> We did something like this back when the register allocator couldn't
split
> live ranges.
>
Yes, I remember the isWinToJoinCrossClass() function, removed here:
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp?r1=152016&r2=155551&diff_format=h
that prevented some coalescing to the cost of leaving many unnecessary
copies around for very constrained regclasses like the one I have.

>
> The problem is that any heuristic you can come up with only makes the
> problem less likely to happen. It doesn't actually fix it.
>
Indeed, that heuristic I wrote is a nasty hack and not the way of fixing it
correctly. Now that the regalloc has much more freedom on making changes in
the code I hope it is fixable. If you need any other info apart of the
dumps I've already attached please ask.

Thanks!
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Hello Jakob,

Did you get a chance to take a look into this, and if not, can you do it
when you get some spare time?

Thanks!

2012/12/19 Borja Ferrer <borja.ferav at gmail.com>
> We did something like this back when the register allocator couldn't
split
>> live ranges.
>>
>
> Yes, I remember the isWinToJoinCrossClass() function, removed here:
>
>
http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/CodeGen/RegisterCoalescer.cpp?r1=152016&r2=155551&diff_format=h
> that prevented some coalescing to the cost of leaving many unnecessary
> copies around for very constrained regclasses like the one I have.
>
>
>>
>> The problem is that any heuristic you can come up with only makes the
>> problem less likely to happen. It doesn't actually fix it.
>>
>
> Indeed, that heuristic I wrote is a nasty hack and not the way of fixing
> it correctly. Now that the regalloc has much more freedom on making changes
> in the code I hope it is fixable. If you need any other info apart of the
> dumps I've already attached please ask.
>
> Thanks!
>
>
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