[Vm-dev] StackInterp/Cog>>transferTo: - assertValidExecutionPointe:r:s differences

[Eliot Miranda]

Hi Ben,

    this time I'll try and answer your question rather than my brain fart...

> On Jun 9, 2016, at 6:46 PM, Ben Coman <btc at openinworld.com> wrote:
> 
> 
> Just curious about the difference in instructionPointer between these two...
> 
> StackInterpreter>>transferTo:
>     self assertValidExecutionPointe: instructionPointer + 1 r:
> framePointer s: stackPointer.
> 
> 
> CoIntepreter>>transferTo:from:
>    self assertValidExecutionPointe: instructionPointer r:
> framePointer s: stackPointer.

In the StackInterpreter, as in the context Interpreter the next bytecode is fetched by pre-incrementing the instructionPointer and then fetching the byte it points to.  Hence it may point to before the first bytecode and hence the + 1 above.  1 is subtracted from a context's pc (2 actually cuz pc is 1-relative and instructionPointer is 0-relative) to derive instructionPointer.

In the Cog VM instructionPointer may also be a machine code pc. So rather than add one, which only makes sense for interpreted methods, the CoInterpreter's assertValidExecutionPointer:s: allows for the - 1 if a frame us interpreted.  The StackInterpreter's method predates the CoInterpreter.

> 
> 
> cheers -ben

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