[Vm-dev] Making a Slower VM

[David T. Lewis]

On Sun, Feb 09, 2014 at 10:23:37AM -0800, tim Rowledge wrote:
> 
> On 09-02-2014, at 10:07 AM, David T. Lewis <lewis at mail.msen.com> wrote:
> > 
> > I think someone mentioned it earlier, but a very easy way to produce an
> > intentionally slow VM is to generate the sources from VMMaker with the
> > inlining step disabled. The slang inliner is extremely effective, and turning
> > it off produces impressively sluggish results.
> 
> Does that actually work these days? Last I remember was that turning
> inlining off wouldn?t produce a buildable interp.c file. If someone has
> had the patience to make it work then I?m impressed.
> 

You're right about one thing, it required a lot of patience ;-)

I did manage to get it working though, and the results are in VMMaker-dtl.342.

This turned out to be a useful exercise, as I flushed out a couple of type
declaration bugs along the way.

The major issue was that the refactoring of object memory and interpreter
into separate class hierarchies (which is a very good thing IMHO) requires
the use of accessor methods, and this leads to name conflicts in the generated
code if those accessor methods are not fully inlined.

I went with the approach of naming the accessors getFoo and setFoo: as well
as, for the case of array access, fooAt: and fooAt:put:. This is not very
pleasing from a readability point of view, but it is simple and it works.

If I compile a VM with inlining disabled and compiler optimization turned
off, the result is about 1/8th the speed of the same interpreter VM built
normally.

Dave