[Vm-dev] failing/errors Pharo Tests with CogVM

Thu Sep 16 21:49:25 UTC 2010

2010/9/16 Eliot Miranda <eliot.miranda at gmail.com>:
>
> Andreas,
>     I realise we need to be more precise.  Are you talking about NaNs specifically or NaN and Inf?  The Squeak VM happily answers Inf from its float primitives.  In fact the only guard against a NaN or Inf result being produced by the floating-point primitives is the guard against dividing by zero.  But e.g. in the interpreter (1.0e300 / 1.0e-300) isInfinite and there is no failure.  So specifically failing for aFloat / 0.0 seems a bit of a fig leaf to me.
> So what would your ideal semantics be?
> a) - fail whenever the result is Inf or NaN?
> b) - fail whenever the result is NaN and allow aFloat / 0.0 to answer Inf
> c) - fail whenever the result is NaN but fail aFloat / 0.0
> d) - the Interpreter status quo, fail only for aFloat / 0.0
> e) - never fail and answer Nan and Inf as specified in IEEE 754
> The situation with VW before IEEE was that it did a) and we changed it so that the mode switch selected either a) or e), with, IIRC, the current default being e).
> cheers,
> Eliot
>

Note that IEEE 754 authorize both:
- answering an exceptional value
- or raising an exception
It's all in programmer's hands... (well if they have a good library
from within C code, else in assembler).

Nicolas

> On Thu, Sep 16, 2010 at 1:25 PM, Andreas Raab <andreas.raab at gmx.de> wrote:
>>
>> On 9/16/2010 12:32 PM, Eliot Miranda wrote:
>>>
>>>     I need to check these carefully.  One thing that does differ in
>>> current Cog is that the machine-code arithmetic float primitives don't
>>> fail if they produce a NaN result; they simply answer a NaN result.  IMo
>>> what needs to be done is two-fold.
>>>
>>> a) we need a NaN mode flag in the VM, that persists across snapshots and
>>> e.g. is queryable/settable via vmParameterAt:put:, that puts the
>>> floating-pont primitive into a state where NaNs are answered instead of
>>> primitives failing.
>>
>> FWIW, I don't think we need that flag. Failing the primitive instead of producing something that is specifically declared not to be a number in an arithmetic computation is *always* the right thing to do. The problem with NaNs is that they propagate. So you start with an isolated NaN as the result of a division by an underflow number and may be able to catch it. But you don't because it's silent and then it propagates into a matrix because you're scaling the matrix by that number. Now you've got a matrix full of NaNs. As you push your geometry through that matrix everything becomes complete and utter NaN garbage. And of course, NaNs break reflexivity, symmetry and transitivity of comparisons.
>>
>> As a consequence we should never allow them to be introduced silently by the VM. If the error handling code for some arithmetic primitive decides that against all reasoning you'd like to produce an NaN as the result regardless, that's fine, you have been warned. But having the VM introduce NaNs silently is wrong, wrong, wrong.
>>
>> Cheers,
>>  - Andreas
>>
>>> b) the Cog code generator needs to respect this flag and arrange that
>>> when in the default mode (current behavior) the machine-code arithmetic
>>> float primitives also fail if they produce a NaN result.
>>>
>>> We can then decide at a later date whether to change the primitive
>>> behavior to answer NaNs or not.  This is also what we did in
>>> VisualWorks; there's an IEEE arithmetic mode and in recent releases VW's
>>> floating-point arithmetic will produce NaNs.
>>>
>>> Anyone interested in taking a look at this is very welcome.  Its
>>> probably a week long project at most.
>>>
>>> best,
>>> Eliot
>>>
>>> On Thu, Sep 16, 2010 at 12:19 PM, Nicolas Cellier
>>> <nicolas.cellier.aka.nice at gmail.com
>>> <mailto:nicolas.cellier.aka.nice at gmail.com>> wrote:
>>>
>>>
>>>    I mean the M7260-primitiveSmallIntegerCompareNan-Patch-nice.1.cs part,
>>>    the rest has already been applied in COG.
>>>
>>>    Nicolas
>>>
>>>    2010/9/16 Nicolas Cellier <nicolas.cellier.aka.nice at gmail.com
>>>    <mailto:nicolas.cellier.aka.nice at gmail.com>>:
>>>     > I see http://bugs.squeak.org/view.php?id=7260 was not integrated in
>>>     > COG, which was the cause of most of the Floating point failures
>>>    in old
>>>     > VM, but maybe it's now more complex than that ?
>>>     >
>>>     > Nicolas
>>>     >
>>>     > 2010/9/16 Mariano Martinez Peck <marianopeck at gmail.com
>>>    <mailto:marianopeck at gmail.com>>:
>>>     >>
>>>     >> Hi Eliot. I took a Pharo 1.1.1 image (which has included the
>>>    changes to run Cog) and I run all the tests with the build  r2219
>>>     >>
>>>     >> And these are the results:
>>>     >>
>>>     >> 9768 run, 9698 passes, 53 expected failures, 15 failures, 2
>>>    errors, 0 unexpected passes
>>>     >> Failures:
>>>     >> FloatTest>>#testRaisedTo
>>>     >> MCInitializationTest>>#testWorkingCopy
>>>     >> FloatTest>>#testReciprocal
>>>     >> ReleaseTest>>#testUndeclared
>>>     >> FloatTest>>#testDivide
>>>     >> MethodContextTest>>#testClosureRestart
>>>     >> FloatTest>>#testCloseTo
>>>     >> FloatTest>>#testHugeIntegerCloseTo
>>>     >> FloatTest>>#testInfinityCloseTo
>>>     >> WeakRegistryTest>>#testFinalization
>>>     >> PCCByLiteralsTest>>#testSwitchPrimCallOffOn
>>>     >> AllocationTest>>#testOneGigAllocation
>>>     >> FloatTest>>#testNaNCompare
>>>     >> FileStreamTest>>#testPositionPastEndIsAtEnd
>>>     >> NumberTest>>#testRaisedToIntegerWithFloats
>>>     >>
>>>     >> Errors:
>>>     >> MessageTallyTest>>#testSampling1
>>>     >> WeakSetInspectorTest>>#testSymbolTableM6812
>>>     >>
>>>     >>
>>>     >>
>>>     >> I think that most of these problems were fixed in latest
>>>    official SqueakVM. I guess they were integrated in VMMaker in
>>>    versions later than the one you used for Cog. Maybe you can
>>>    integrate them and create a new version?
>>>     >>
>>>     >> I am not a VM expert so please if you can help us with this
>>>    tests it would be cool.
>>>     >>
>>>     >> Thanks
>>>     >>
>>>     >> Mariano
>>>     >>
>>>     >>
>>>     >
>>>
>>>
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