[squeak-dev] Re: [Vm-dev] Spur 64-bits. Ugh, this could be a slog...

Eliot Miranda eliot.miranda at gmail.com
Wed Nov 19 18:26:29 UTC 2014

On Tue, Nov 18, 2014 at 8:22 PM, David T. Lewis <lewis at mail.msen.com> wrote:

> On Tue, Nov 18, 2014 at 07:12:12PM -0800, Eliot Miranda wrote:
> >
> > Hi All,
> >
> >     I'm making good progress on 64-bit Spur in the Stack VM simulator.
> But
> > I've just noticed an image-level issue which could be indicative of lots
> of
> > 32-bit assumptions baked into the Squeal/Pharo/Newspeak systems.
> >
> > SmallInteger>>digitAt: n
> > "Answer the value of an indexable field in the receiver.
> > LargePositiveInteger uses bytes of base two number, and each is a 'digit'
> > base 256.  Fail if the argument (the index) is not an Integer or is out
> of
> > bounds."
> > n>4 ifTrue: [^ 0].
> > self < 0
> > ifTrue:
> > [self = SmallInteger minVal ifTrue:
> > ["Can't negate minVal -- treat specially"
> > ^ #(0 0 0 64) at: n].
> > ^ ((0-self) bitShift: (1-n)*8) bitAnd: 16rFF]
> > ifFalse: [^ (self bitShift: (1-n)*8) bitAnd: 16rFF]
> >
> > This assumes that SmallInteger is only ever 4 bytes, which is
> unacceptably
> > wasteful for my approach to 64-bits. In 64-bit Spur, SmallIntegers are
> > 61-bit 2's complement.
> >
> > I'm raising this example at this point to see if the community might find
> > similar issues and bring them to my attention.
> There will be quite a few issues like this scattered throughout the image.
> My suggestion would be to focus first on getting the 64-bit Spur image
> running with no changes to SmallInteger minVal and maxVal. Then, as a
> separate follow up step, expand the range of SmallInteger in the image
> and work out all the changes required to support it.

That's just a way of adding to the work.  Going whole hog means less work
after all.  It is also really important to me to get a full 64-bit system
working asap.  I'm not interested in a hybrid like the existing 64-bit VM.
What I am doing though is holding off on the 61-bit Float to SmallFloat
conversion.  I have to write new primitives for that and that can wait.

Another possible way to attack the problem is to start with a Squeak
> trunk image in 68002 image format (64-bit image), and work out the
> issues associated with extending SmallInteger minVal and maxVal to
> take advantage of the larger immediate integers range. This could be
> independent of the work you are doing now, so if someone else wanted
> to work on the "expand range of SmallInteger" aspect of the project,
> I expect that this can be done in the existing 68002 image, such that
> the resulting changes would be directly applicable to the format 68019
> (64-bit) Spur images.

The issues are rather straight-forward.  Further, the bootstrap from a
32-bit Spur to a 64-bit Spur image is quite simple.  Right now the
bootstrap only has 18 methods.  It has been by far the least complex
bootstrap I've done, and I've done Smalltalk-80 V2 16-bit to BrouHaHa
32-bit, VW 32-bit to 64-bit, VW 3 to VW 5, Squeak V3 to Spur 32 bit, Squeak
BlockContext to Squeak Closures.

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