[DiSL-user] Handling Array store/load

[Alessio Gambi]

Hi Lubomir,

great answer ! I’ll try to implement that ASAP.

For the record I describe below the (hacky) solution that I adopted so far, which is based on storing partial states information from array accesses (getfield and getstaticfield): 

when I observe a getField or a getStatic which refer to an array, I keep the reference of the array inside a temporary variable, 
that I use later on. The time I observe the first ASTORE I raise the event that the array was modified and clear the temporary variable.
If I observe an ALOAD instead, I raise the event that the array element was read and clear the temporary variable.

— Alessio


Alessio Gambi, Ph.D.
gambi at st.cs.uni-saarland.de


> On 02 May 2017, at 22:47, Lubomír Bulej <lubomir.bulej at d3s.mff.cuni.cz> wrote:
> 
> Hi Alessio,
> 
> when you instrument the array store bytecodes, the information you need is not
> directly available, because the array access bytecodes (?ASTORE, ?ALOAD) work
> with the array reference on stack -- the reference could have been left there
> after a function all, or obtained using the GETFIELD or GETSTATIC bytecodes --
> the information on the class is only available those.
> 
> To do it properly, you would need to do a simple evaluation of the method's
> bytecode to trace how values got on the stack and which array references you
> use with the ?ASTORE and ?ALOAD bytecodes.
> 
> As a simpler solution, you could try to scan the method bytecode backwards
> from the point of the ?ASTORE or ?ALOAD bytecode and try to find the first
> GETFIELD or GETSTATIC bytecode that fetches an array (or a method invocation
> that returns the array), i.e., the source of the array reference, and then
> fetch the owner and the field name from that instruction.
> 
> This would be done in a custom static context, i.e., a class that implements
> the StaticContext interface.
> 
> I would suggest to look at the InstructionStaticContext implementation in the
> trunk version, where you will find the getIndex() method, which returns the
> index of an instruction's bytecode in a method. It only counts "real" bytecode
> instructions (not all ASM's AbstractInsnNode which are also used for labels).
> 
> The InstructionStaticContext extends the AbstractStaticContext class (which
> does not really do much at the moment, there are plans to make this interface
> nicer). Within the getIndex() method, the staticContextData.getRegionStart()
> gets you the start of the region the context is related to. The instruction
> nodes are linked, so you could try to walk back the list of instructions to
> find the field load that got the array reference on top of the stack and
> check the FieldAccessStatic context on how to retrieve the owner class and
> field name.
> 
> 
> Best regards,
> Lubomir
> 
> 
> 
> 
> On 28/04/17 12:15, Alessio Gambi wrote:
>> Hi DiSL-ers,
>> 
>> I am in the following situation:
>> 
>> I can capture the array store and load events, but I cannot understand how I can get the following information (if possible):
>> 
>> - Which field of which class the array store refers to?
>> 
>> Best
>> 
>> — Alessio
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