[LLVMdev] PBQP & register pairing

Wed Jun 15 05:01:15 PDT 2011

Attached is a small patch to allow users of the PBQP allocator to optionally insert a custom pass. I believe it can be usefull to other users of the pbqp.
I used it to undo some of the coalescer work, and make sure that I have different virtual registers, inserting a copy if necessary, to build a pair.

I noticed an unexpected --- to me at least --- behaviour of the allocator.
I have some instructions using 2 pairs of registers, say "mpra R_x, R_x+1, R_y, R_y+1", and setting the pairing constraints R_x -> R_x+1 and R_y -> R_y+1 could silently produce wrong code like "mpra %R0, %R2, %R1, %R3". I add to explicitly add another constraint  to describe that y must differ from x, x-1 and x+1 to make the allocator build valid pairs, i.e "mpra %R0, %R1, %R2, %R3". Is there anything I missed ?

Now, 99% of my users' codebase is compiling. :)
The last issue I have is an assert during regalloc in LiveIntervalAnalysis : "attempt to spill already spilled interval!", and I do not know where to start looking. Any hint would be welcome.

Regards,
--
Arnaud de Grandmaison
________________________________
From: Lang Hames [mailto:lhames at gmail.com]
Sent: Tuesday, June 07, 2011 11:20 AM
To: Arnaud Allard de Grandmaison
Cc: Jakob Stoklund Olesen; Peter Lawrence; llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] PBQP & register pairing

Hi Arnaud,
That sounds great. I look forward to seeing a patch.

You may also look forward to big performance improvements in the PBQP allocator: I'm working on updates which will improve compile speeds and massively reduce memory use.

Regards,
Lang.

On Tue, Jun 7, 2011 at 7:02 PM, Arnaud Allard de Grandmaison <Arnaud.AllardDeGrandMaison at dibcom.com<mailto:Arnaud.AllardDeGrandMaison at dibcom.com>> wrote:

I also considered this approach, but did not want to dive in the constraint handling for now.

The PBQP path seemed easier at first sight --- and was easy to setup. And I always wanted to give a try to the pbqp :)

I will add the hook to the pbqp and propose a patch if this looks clean enough.

Thanks,
--
Arnaud de Grandmaison

-----Original Message-----
From: Jakob Stoklund Olesen [mailto:stoklund at 2pi.dk<mailto:stoklund at 2pi.dk>]
Sent: Monday, June 06, 2011 9:28 PM
To: Peter Lawrence
Cc: Arnaud Allard de Grandmaison; llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] PBQP & register pairing

On Jun 6, 2011, at 12:14 PM, Peter Lawrence wrote:

> Arnaud,
>                another way to look at it, if the description of your register sets includes "pairs",
> is that your assembly language syntax for MPQD is redundant, operand-2 is the second
> half of the register-pair in operand-0,  so an alternative is to let llvm think this is a two
> operand instruction (one of them being a pair) rather than a three operand instruction.
>
> even if you are not currently defining pairs in your register definitions, it might be less work
> to do that than to write and add an extra new pass.  Many many target machines have some
> notion of register pairs, so it should not be too hard to find examples of how to do this.

ARM's QQ and QQQQ register classes are examples.

It is currently a bit tedious to specify these constraints; cleaning that up is on my todo list.

/jakob

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