[Vm-dev] Interpreter>>isContextHeader: optimization

[bryce at kampjes.demon.co.uk]

Eliot Miranda writes:
 >  On Sun, Feb 22, 2009 at 10:37 AM, <bryce at kampjes.demon.co.uk> wrote:
 > 
 > >
 > > Eliot Miranda writes:
 > >  >
 > >  > But what I really think is that this is too low a level to worry about.
 > >  >  Much more important to focus on
 > >  > - context to stack mapping
 > >  > - in-line cacheing via a JIT
 > >  > - exploiting multicore via Hydra
 > >  > and beyond (e.g. speculative inlining)
 > >  > than worrying about tiny micro-optimizations like this :)
 > >
 > > If you're planning on adding speculative, I assume Self style dynamic,
 > > inlining won't that reduce the value of context to stack mapping?
 > 
 > 
 > Not at all; in fact quite the reverse.  Context to stack mapping allows one
 > to retain contexts while having the VM execute efficient, stack-based code
 > (i.e. using hardware call instructions).  This in turn enables the entire
 > adaptive optimizer, including the stack analyser and the
 > bytecode-to-bytecode compiler/method inliner to be written in Smalltalk.
 >  The image level code can examine the run-time stack using contexts as their
 > interface without having to understand native stack formats or different
 > ISAs.  The optimizer is therefore completely portable with all machine
 > specificities confined to the underlying VM which is much simpler by virtue
 > of not containing a sophisticated optimizer (which one would have to squeeze
 > through Slang etc).

All you need is the optimiser to run early in compilation for it to be
portable. 

And we definately agree on trying to keep complex logic out of the
VM. Sound's like you're thinking of AoSTa.

 > So for me, context-to-stack mapping is fundamental to implementing
 > speculative inlining in Smalltalk.
 > 
 > 
 > My view with Exupery is context caches should be left until after
 > > dynamic inlining as their value will depend on how well dynamic
 > > inlining reduces the number of sends.
 > >
 > 
 > I know and I disagree.  Dynamic inlining depends on collecting good type
 > information, something that inline caches do well.  In-line caches are
 > efficiently implemented with native call instructions, either to method
 > entry-points or PIC jump tables.  Native call instructions mesh well with
 > stacks.  So context-to-stack mapping, for me, is a sensible enabling
 > optimization for speculative inlining because it meshes well with inline
 > caches.

PICs are a separate issue. Exupery has PICs, and has had them for
years now. PICs are just as easily implemented as jumps.

 > Further, context-to-stack mapping is such a huge win that it'll be of
 > benefit even if the VM is spending 90% of its time in inlined call-less
 > code.  We see a speedup of very nearly 2x (48% sticks in my head) for one
 > non-micro tree walking benchmark from the computer language shootout.  And
 > this is in a very slow VM.  In a faster VM context-to-stack mapping would be
 > even more valuable, because it would save an even greater percentage of
 > overall execution time.

I see only one sixth of the time going into context creation for the
send benchmark which is about as send heavy as you can get. That's
running native code at about twice Squeak's speed. Also there's still
plenty of inefficiency in Exupery's call return sequences.

 > Further still using call & return instructions as conventionally as possible
 > meshes extremely well with current processor implementations which, because
 > of the extensive use thereon of conventional stack-oriented language
 > implementations, have done a great job optimizing call/return.

Unconditional jumps for sends also benefit from hardware
optimisation. Returns turn into indirect jumps which are less
efficent, but getting better with Core 2.

Cheers
Bryce