[Vm-dev] Interpreter>>isContextHeader: optimization

Eliot Miranda eliot.miranda at gmail.com
Tue Feb 24 03:43:27 UTC 2009

On Mon, Feb 23, 2009 at 1:49 PM, <bryce at kampjes.demon.co.uk> wrote:

> Eliot Miranda writes:
>  >  On Sun, Feb 22, 2009 at 12:54 PM, <bryce at kampjes.demon.co.uk> wrote:
>  > > All you need is the optimiser to run early in compilation for it to be
>  > > portable.
>  >
>  >
>  > ...and for it to be untimely.  An adaptive optimizer by definition needs
> to
>  > be running intermittently all the time.  It optimizes what is happening
> now,
>  > not what happened at start-up.
> Exupery runs as a Smalltalk background thread, it already uses dynamic
> feed back to inline some primitives including #at: and #at:put.
>  > > 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.
>  >
>  >
>  > So you could get a 17% speedup if you could remove the context overhead.
>  >  That's quite a tidy gain.  I see a 26% increase in benchFib performance
>  > between base Squeak and the StackVM with no native code at all.
>  >
>  > What are the inefficiences in Exupery's call return sequences?
> Exupery uses a C call sequence so it's easy to enter from the
> interpreter,

that's a non sequitur.  The Cog VM has full interoperability between
interpreter and machine code without a C calling convention.  The
interpreter doesn't call machine code methods, instead it calls a trampoline
that jumps to the right point in a machine code method, allowing the system
to optimize the common case of machine-code to machine-code calls (through
inline caches).  The return address of a machine-code frame above an
interpreter frame is that of a routine that does a return to the interpreter
so that returns don't have to check for returning to the interpreter.

> that C call frame is torn down when exiting each
> compiled method then re-created when reentering native code. That's
> a complete waste when going from one native method to another.

So you go to all the effort of producing native code and then you wrap it in
so much gunk that you get minimal performance benefit from it.  I don't
understand.  What are your goals?  Experimenting with compilers or producing
an efficient Squeak VM?

Also the send/return sequence isn't yet that optimised, there's still
> plenty of inefficiencies due to lack of addressing modes etc and because
> it's fairly naive translation of the interpreters send code.
> 17% would be rather optimistic, some of the work required to set up a
> context will always be required. Temporaries will still need to be
> nilled out etc.

Again invalid assumptions.  Do without contexts (except as a frame access
abstraction).  Sufficient adaptive optimization can avoid temporary
initializations (e.g. by embedding information that records live ranges).

> Bryce
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