[Vm-dev] VM Maker: VMMaker.oscog-eem.1426.mcz

[Eliot Miranda]

On Sat, Jul 18, 2015 at 8:10 PM, David T. Lewis <lewis at mail.msen.com> wrote:

>
> On Sat, Jul 18, 2015 at 06:09:22PM -0700, Eliot Miranda wrote:
> >
> > Hi David,
> >
> > On Sat, Jul 18, 2015 at 3:49 PM, David T. Lewis <lewis at mail.msen.com>
> wrote:
> >
> > >
> > > On Sat, Jul 18, 2015 at 08:55:27PM +0000, commits at source.squeak.org
> wrote:
> > > >
> > > > Eliot Miranda uploaded a new version of VMMaker to project VM Maker:
> > > > http://source.squeak.org/VMMaker/VMMaker.oscog-eem.1426.mcz
> > > >
> > > > ==================== Summary ====================
> > > >
> > > > Name: VMMaker.oscog-eem.1426
> > > > Author: eem
> > > > Time: 18 July 2015, 1:54:29.051 pm
> > > > UUID: 94ab92ba-c5c4-4953-8566-a4cd9c38df1f
> > > > Ancestors: VMMaker.oscog-eem.1425
> > > >
> > > > Fix at least one set of 64-bit issues caused by Slang.  In particular
> > > the integerObjectOf: code resulted in (objectMemory integerObjectOf:
> > > MillisecondClockMask) evaluating to the -1 object, instead of the
> > > 16r1FFFFFFF object, which was the cause of the initially nresponsive
> 64-bit
> > > image on the real VM (no problem in the simulator).
> > > >
> > >
> > > I can't test now to verify, but I wonder if this change is fixing the
> > > right problem.
> > >
> >
> > I'm pretty sure it is.  It only bites in a 64-bit implementation with >
> > 31-bit integers.  In the "standard" 64-bit image SmallIntegers are still
> > only 31-bits so the issue never occurs.  The issue is that the default
> type
> > of an integer constant in C is int.  So if one has to shift any constant
> > such that a non-zero bit will occupy bit 31 (0 relative), it must be cast
> > to a long type to avoid sign extension.
>
> I don't see it. The size of SmallInteger is enforced by the image, not by
> the object memory. There is nothing in the "standard" 64-bit image format
> that requires small integers to be restricted to the same limits imposed
> by a 32-bit object memory.
>
> The #integerObjectOf: method answers a sqInt, which may be either 32 or
> 64 bits. This should just work, and I don't see a need for conditional
> logic other than what might be needed to define the size of sqInt in the
> first place.
>
> >
> > Now of course I could generate all Integer constants with the L or UL
> > suffix, e.g.
> >
> > #define MillisecondClockMask 0x1FFFFFFFL
> >
> > instead of
> >
> > #define MillisecondClockMask 0x1FFFFFFF
> >
> > but that's a much more pervasive change than only generating the cast in
> > integerObjectOf when on 64-bits.  So I'm happy with the change that I've
> > made.  Experience can of course prove me wrong...
>
> I don't see the problem. Interpreter class>>initialize does this:
>         MillisecondClockMask := 16r1FFFFFFF.
>
> This works for all combinations or 32 and 64 bit image and host platform.
>
> There should be no need to generate all of the integer constants with
> L or UL suffix. It is perfectly acceptable to assign 16r1FFFFFFF to a
> 64 bit signed or unsigned variable, and no special handling should be
> required.
>
> I may be missing something, but I do not see anything about the Spur
> 64-bit image format that should require special handling for this.
>

Yes, I think you're missing something.  let me take you through it.  We're
talking about evaluating, in a 64-bit C, sizeof(long) == 8, sizeof(int) ==
4, the following two variants:

#define MillisecondClockMask 0x1FFFFFFF

(sqInt)((MillisecondClockMask << 3) + 1L)

(sqInt)(((sqInt)MillisecondClockMask << 3) + 1L)


So let's take the first.  The type of MillisecondClockMask is int.  The
type of MillisecondClockMask << 3 is int.  It's bit-pattern is 0xFFFFFFF8,
and hence its value is, incorrectly, -8.  So it evaluates to the
SmallInteger -1, not 16r1FFFFFFF as desired.

In the second, the type of MillisecondClockMask is int. The type of
(sqInt) MillisecondClockMask is long.  The type
of ((sqInt)MillisecondClockMask << 3).  It's bit pattern is also 0xFFFFFFF8
but its value is 0xFFFFFFF8, /not/ -8, and so it evaluates to the
SMallInteger 16r1FFFFFFF as desired.

Does that make sense?
-- 
_,,,^..^,,,_
best, Eliot
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