[Vm-dev] VM Maker: VMMaker.oscog-eem.267.mcz

[Eliot Miranda]

On Thu, Feb 21, 2013 at 11:24 AM, tim Rowledge <tim at rowledge.org> wrote:
>
>
> On 21-02-2013, at 11:10 AM, commits at source.squeak.org wrote:
>> Implement unknownBytecode.
>
> If you implement all unknown bytecodes then you won't need to...

First of all, in the Newspeak VM using the augmented SqueakV3 bytecode
set all the bytecodes are implemented.

Second of all, in the current newspeak VM we support two bytecode sets
and that second set has bytecodes to spare, and were the same approach
taken for Squeak there would be lots of bytecodes to spare for
experimentation.

Third, I can think of a couple of uses for unknownBytecode as
implemented above.  My current idea is to use it for a basic-block
coverage tool. Pepper a method with single-byte unknown bytecodes at
the start of each basic block, storing the original bytecodes in
properties. Now let the test suite run, and respond to unknownBytecode
by recording the coverage, restoring the old bytecode and continuing.
The same scheme can implement breakpoints.  But it depends crucially
on having a single bytecode that raises an exception.  For example, at
Cadence our current bytecode sequence is a clumsy three-bytecode one,
push false, long jump true 0, which gets rewritten into push self,
send breakpoint, pop.  Flipping a byte between a one-byte nop and a
one-byte unknown bytecode would be nicer.

>
>
> tim
> --
> tim Rowledge; tim at rowledge.org; http://www.rowledge.org/tim
> No one is listening until you make a mistake
>
>



-- 
best,
Eliot