Re: InstructionFactory, design issue, createDUP

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----- Original Message ----- 
From: <[email protected]>
To: <[email protected]>
Sent: Wednesday, November 19, 2008 1:09 PM
Subject: InstructionFactory, design issue, createDUP


> Hi,
>
> maybe I misunderstand the feature or the idea behind the 
> InstructionFactory but in the case I use it, the implementation of the 
> creation of some instructions e.g. DUPs it is really misleading. If you 
> create an invokeinstruction or an constant like ldc a new instruction 
> object is created. Pretty straight forward! But if you create a DUP 
> instruction you always will get the same object back. This is of course is 
> great for saving memory, but in the following example it leads to lots of 
> problems.
>
> Imagine you instrument a code sequence and add your own instructions one 
> after the other. You add invokestatic instructionsbut also the dups. At 
> the end you iterate again over the whole list to add some missing 
> instructions ( doing it in the first iteration is not possible or too much 
> work, due to the semantics of the algorithm). If you add new instructions 
> again it will not work properly if your insertion point is a DUP! The new 
> instruction will be inserted at the first match of the list.
> e.g.
> i1,... are instructions other than dup,
> dup_ are DUP instructions created with InstructionFactory.createDUP();
>
> you iterate from i1 to i6;
>
> init:       i1, i2, i3, dup_1, i4, i5, dup_2, i6
>
> 1.)  insert(i3, new_i)
> result:  i1, i2, new_i, i3, dup_1, i4, i5, dup_2, i6
>
> 2.) insert(dup_1, new_i2)
> result:  i1, i2, new_i, i3, new_i2,  dup_1, i4, i5, dup_2, i6
>
> 3.) insert(dup_2, new_i3)
> result:  i1, i2, new_i, i3, new_i2,  new_i3, dup_1, i4, i5, dup_2, i6
>
> The reason for this is that dup_1 dup_2 point to the same static DUP 
> instance (there is always one). Your call to insert()
> calls a method findInstruction() and this method only compares references 
> (==) and returns the first match.
> Even if you use copy() you get the same reference! To omit this problem 
> you have to create an instance without the factory by using a constructor.
>
> My point is, you can implement it this way BUT you have to mention this in 
> the API doc that this createDUP method does not behave like a createInvoke 
> method and always returns the same reference. I know usually the code is 
> the documentation but I think a lot of pain could be spared with a tiny 
> little sentence in the doc.
>
>
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