[Vm-dev] Finally Time to Make Good on "Everything is a message send"?

Bert Freudenberg bert at freudenbergs.de
Mon Sep 18 13:32:04 UTC 2017

On Sun, Sep 17, 2017 at 3:51 PM, Sean P. DeNigris <sean at clipperadams.com>

> On the "Squeak Beginners" ML, a new Smalltalker wrote:
> > I don't understand why Smalltalk doesn't allow me to have an or:
> > method that takes a block argument (except on Boolean).
> The answer apparently was that the compiler replaces the code with an
> optimization that is usually what one wants, and the actual message never
> gets sent:
> >    Ralph Johnson wrote
> >    Yes, when the compiler sees   exp or: [ ... ] then it assumes that
> > "exp" is
> >   a boolean-valued expression and generates code that fails if it isn't.
> I remember the pain of tripping over these little "everything is a message
> send to an object*" sins as a new Smalltalker. I wonder now, with the
> incredible speed of Cog, Spur, Sista, etc., if these devil's bargains from
> prior decades are still necessary. It would be psychologically satisfying
> (and nice for newbies) to remove the asterisk from the principle above.

​Yes, optimization of boolean-looking expressions is cheating that gets
caught once in a while.

But even in Cog it's still 400% faster:

[false or: [true]] bench

 '198,000,000 per second. 5.06 nanoseconds per run.'
 '211,000,000 per second. 4.75 nanoseconds per run.'
 '209,000,000 per second. 4.79 nanoseconds per run.'

... and again with #transformOr: disabled:

 '47,100,000 per second. 21.2 nanoseconds per run.'
 '47,500,000 per second. 21.1 nanoseconds per run.'
'48,000,000 per second. 20.8 nanoseconds per run.'
We don't want to give up that performance, but we would like to not get
caught cheating either. This would be very simple if instead of raising the
"must be boolean" error, we could simply retry the send as a regular
message send.

The major complication here is that in the byte code we don't know the
original method selector (e.g. ifTrue:ifFalse: etc), and we do not have the
original block closure arguments, because they never got constructed in the
first place.

There are various ways to tackle that problem with different speed/space
tradeoffs. If we don't care too much about the non-boolean receiver
performance (it's the slow path after all) then no modifications to the VM
would be needed.

You should be able to do it all in the #mustBeBoolean handler: perform the
original selector with the block arguments, then jump to after the
optimized block. Sounds like a fun, if not exactly trivial project :)

- Bert -
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