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Re: Live range Analysis based on tree representations

On Thu, 2015-09-03 at 15:22 +0000, Ajit Kumar Agarwal wrote:
> -----Original Message-----
> From: Aaron Sawdey [] 
> Sent: Wednesday, September 02, 2015 8:23 PM
> To: Ajit Kumar Agarwal
> Cc: Jeff Law;; Richard Biener;; Vinod Kathail; Shail Aditya Gupta; Vidhumouli Hunsigida; Nagaraju Mekala
> Subject: Re: Live range Analysis based on tree representations
> On Tue, 2015-09-01 at 17:56 +0000, Ajit Kumar Agarwal wrote:
> > All:
> > 
> > The Live ranges info on tree SSA representation is important step towards the SSA based code motion optimizations.
> > As the code motion optimization based on the SSA representation 
> > effects the register pressure and reasons for performance Bottleneck.
> > 
> > I am proposing the Live range Analysis based on the SSA 
> > representation. The Live range analysis traverses the dominator Tree. 
> > The SSA and phi variables are represented based on dominance frontier info and the SSA representation reflects The dominance info. Based on such dominance info Live range Overlapping Analysis can be derived.
> > 
> > Variable V intersects W if Vdef dominates the Wdef. The variable v 
> > intersects at point p if Vdef dominates P and Wdef Dominates the P. If 
> > Vdef dominates Wdef and Wdef dominates Udef , then the Vdef dominates 
> > Udef and thus Live range Of V intersect W and live range W intersect 
> > U, thus the live range V intersects the U. Such dominance info can be 
> > used to Represent the Overlapping Live range Analysis and the register pressure is derived from Overlapping Live ranges based On the dominator info inherited from the SSA representation. The SSA representation is derived based on dominance Frontier and the traversal of dominator tree based on SSA can derive the Overlapping Live ranges.
> > 
> > The above Overlapping Live range info can be used to derive the 
> > register pressure and the optimization based out of tree Representation can use the above overlapping live ranges to take register pressure into account.
> >>Ajit,
>   >>I did a prototype of this kind of analysis at one point last year to see if it could help improve inlining decisions in LTO. Basically I did >>exactly what you suggest and computed the number of overlapping SSA live ranges and used that as a proxy for register pressure. It >>did appear to be able to help in some circumstances but the real solution is to improve register allocation so it doesn't fall down when >>register pressure gets high.
> Aaron:
> Would you mind in explaining on what circumstances it helps and when it won't.  The Live ranges on SSA
> representation forms chordal Graphs that might have the different colorability requirements than the real
> register allocator. This may not give exact register pressure compared to  register allocator as register allocator 
> is  further down the optimization and the code generation pipeline  but forms the basis of optimization based 
> on SSA that effects the register pressure.
> >>The code is in a branch called lto-pressure.
> Thanks. I would like to see the code.

  The branch is here: svn://
The analysis is in ipa-inline.c; if you search for "pressure" you'll
find the code.

The live ranges in SSA are certainly different than what the register
allocator is going to see, but it's what we have to work with at the
point where the inlining decisions are made, which is why I was looking
at it. My hope was that it would be a reasonable proxy for downstream
register pressure. 

I went and did this after seeing a particular situation in bzip2 where a
bunch of inlining done by LTO sent register pressure up a lot and
resulted in a measureable increase in loads/stores due to extra spill
code. Functions that are called in only one place and not externally
visible will be inlined regardless of their size. There is one function
in bzip2 that has particularly complex control and inlining this into
another non-trivial function caused all the excess spill code.

Setting limits on "inline functions called only once" did work, i.e.
inhibited the particular inlining that I wanted to eliminate; it just
didn't help enough to justify the complexity of the extra analysis. So I
went a step further and put the caller and callee pressure in as a term
in the edge badness used to prioritize inlining of small functions. This
seemed to help in some cases and hurt others, probably because SSA
pressure doesn't accurately represent the kind of register pressure that
causes problems later.

Looking at the code a year+ later I see a bug already in how I
implemented the callee/caller pressure limit, but I tested it without
that enabled so that wasn't all of the problem.

I abandoned the work because it seemed to me the the community favored
fixing these issues in register allocation.


> Thanks & Regards
> Ajit
>   >>Aaron
> > 
> > Thanks & Regards
> > Ajit
> > 
> --
> Aaron Sawdey, Ph.D.
> 050-2/C113  (507) 253-7520 home: 507/263-0782 IBM Linux Technology Center - PPC Toolchain

Aaron Sawdey, Ph.D.
050-2/C113  (507) 253-7520 home: 507/263-0782
IBM Linux Technology Center - PPC Toolchain

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