[Vm-dev] Simulating the BalloonEnginePlugin, FloatArrayPlugin & Matrix2x3Plugin primitives

Eliot Miranda eliot.miranda at gmail.com
Mon Jan 6 17:30:17 UTC 2014

Hi All,

    I'm just revising plugin treatment in Spur and came across this old
snippet of mysterious code:

InterpreterSimulator>>loadNewPlugin: pluginString
| plugin simClass |
transcript cr; show:'Looking for module ', pluginString.
(#('FloatArrayPlugin' 'Matrix2x3Plugin')
includes: pluginString) ifTrue:
[transcript show: ' ... defeated'. ^ nil].

In the past I got as far as rewriting it to read...

InterpreterSimulator>>loadNewPlugin: pluginString
| plugin plugins simulatorClasses |
transcript cr; show: 'Looking for module ', pluginString.
"but *why*??"
(#('FloatArrayPlugin' 'Matrix2x3Plugin') includes: pluginString) ifTrue:
[transcript show: ' ... defeated'. ^nil].
plugins := InterpreterPlugin allSubclasses select: [:psc| psc moduleName
asString = pluginString asString].

In revising the code for Spur I removed the defeat code and found out more.
 It's essentially because the BalloonPlugin has difficulty simulating
accesses of 32-bit floats.  If you simply defeat the code and let things
run soon you get failures in FloatArrayPlugin & Matrix2x3Plugin primitives.
 These can be fixed by implementing the following in the FloatArrayPlugin &

cCoerce: value to: cType
^cType = 'float'
ifTrue: [value asIEEE32BitWord]
ifFalse: [value]

But soon you hit more difficult failures in the BalloonEnginePlugin, e.g.

BalloonEngineBase>>transformPointX: xValue y: yValue into: dstPoint
"Transform srcPoint into dstPoint by using the currently loaded matrix"
"Note: This should be rewritten so that inlining works (e.g., removing
the declarations and adding argument coercions at the appropriate points)"
| x y transform |
<inline: true>
<var: #dstPoint type:'int *'>
<var: #xValue type: 'double '>
<var: #yValue type: 'double '>
<var: #transform type:'float *'>
transform := self edgeTransform.
x := ((((transform at: 0) * xValue) +
((transform at: 1) * yValue) +
(transform at: 2)) * self aaLevelGet asFloat) asInteger.
y := ((((transform at: 3) * xValue) +
((transform at: 4) * yValue) +
(transform at: 5)) * self aaLevelGet asFloat) asInteger.
dstPoint at: 0 put: x.
dstPoint at: 1 put: y.

where x and y end up being the integer representation of 64-bit floats
while dstPoint accepts the integer representation of 32-bit floats.  At
least I think that's what's going on.

In any case I need to focus on Spur and can't spare the time to fix this.
 But I find it unsatisfactory.  It means the VM simulation isn't accurate.
 In the simulation the primitives fail and Smalltalk code is run.  In the
real VM the primitives work.  And that's deeply unsatisfying.

So if there's anyone itching for a VM challenge try and make
the BalloonEnginePlugin, FloatArrayPlugin & Matrix2x3Plugin primitives
simulate correctly, removing the defeat code above.  That would be a great
new year's gift.
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