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Debugger Internals
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Last modified on 24/09/2013 12:01 a.m. by User.


Debugger Internals


There are some bugs with the Debugger in Pharo (and Squeak). This part of the system is quite complex and this page aims to document some of the important classes and interactions. As we learn more, hopefully we an fix it!


So the entries of interest for highlighting are
Then you see the DebuggerMethodMap>>forMethod:methodNode: takes both a CompiledMethod and its #methodNode. CompiledMethod>>methodNode invokes the Parser to get the AbstractSyntaxTree from method source, and if it ever fails ends up by trying to decompile the byteCodes.
This is the easy part. Now we to deal with #abstractPCForConcretePC: and #abstractSourceMap.
By reading CompiledMethod>>abstractPCForConcretePC: you should quickly understand that a concrete PC is a byte offset of current byteCode (the offsets displayed in the byteCode view) while the abstractPC is just the rank of current byteCode in the list of byteCodes instructions composing the CompiledMethod. This is just because "byteCodes" may spread on several bytes beside their name... This will use InstructionStream and InstructionClient which are just an iterator and a sort of visitor on byteCode instructions. So this is not really interesting.
The more interesting part is #abstractSourceMap There is a first step to obtain CompiledMethod>>rawSourceRangesAndMethodDo: This is the most important part. The rest is again a mapping from concretePC (instruction byte offset) to abstractPC (instruction rank). And some build of a dictionary mapping instruction rank (abstractPC) -> selected range.
Note that the last trick seems to use a regenerated CompiledMethod (theMethodToScan) rather than the original CompiledMethod. There is no assertion whether these two are equivalent or not. A priori, they should, unless the Compiler changed since last compilation or if its behaviour is affected by some Preferences... Would we introduce some customizable Compiler optimizations that this could become a problem (We would then add to map decompiled AST to source code AST, probably with guesses, unless the CompiledMethod would contain debugger friendly hints...) We will consider this is not a problem by now.
So let's now concentrate on rawSourceRangesAndMethodDo: The nice thing is that you now can just debug this
Also involves
A few more notes:
- Encoder>>rawSourceRanges answer a mapping AST Node -> sourceRange
Gnerally, I would expect sourceRange to be an Interval, but this has
to be confirmed.
Theses ranges are constructed at source code Parse time (see senders
of #noteSourceRange:forNode:) - The program counters are assigned to AST nodes at byte code #generate time - for inlined macros, like #to:do: in our case, this pc is after the initialize and test statements.
in CompiledMethod>>#rawSourceRangesAndMethodDo: I just evaluated this:
And what I see is that the original to:do: message (before macros inlining) has the right range
This node is the original #to:do: and has pc=42
There is also {a LeafNode}->a Text for '[ :index |
temp := index. collection add: temp ]'
which has a nil pc so it won't be taken into account in the source map.
But one of the messages produced by the inlining has this curious range: {index <= 5}->a Text for 'to: 5 do: [ :index |
temp := index. c'
This node has pc = 40, so it will be selected before the correct one above has a chance to be. {index <= 5} is the test statement produced by inlining, and this seems to be the incorrect highlighting we see. Thus we know we have to concentrate on macro inlining. This happens in MessageNode>>transformToDo: We'll see this.
In the interim, I played with eliminating the nodes having unitilialized pc Let us try to evaluate this snippet in debugger's inspector:
Oh god, a bug in the debugger: DoItIn: homeContext
^ ((homeContext namedTempAt: 2) encoder rawSourceRanges keys
select: [:e | e pc > 0]) collect: [:node |
| ival | ival := (node namedTempAt: 2) encoder rawSourceRanges at: node. node
-> (sourceText copyFrom: ival first to: ival last)]
(node namedTempAt: 2) does not mean a thing... A confusion occurred between the homeContext (DoItIn: method argument) and node (the block argument) Nevermind... forget about it.
Now let's just concentrate on MessageNode>>transformToDo: We see this code:
So the intention seems to select 'to: 5 do: '
But we see this:
So it seems intentional to select only the last instruction of the block for the debugger. We'd better not change this without prior asking Eliot. But obviously, this is not what is expected by the #transformToDo:
And by changing last line of this piece of code in #transformToDo:
I get a "correct" selection...
-- Nicolas --