[Vm-dev] Re: [Pharo-project] Block questions

Eliot Miranda eliot.miranda at gmail.com
Mon Apr 16 20:25:01 UTC 2012

On Sun, Apr 15, 2012 at 1:22 AM, Guillermo Polito <guillermopolito at gmail.com
> wrote:

> b := [ |a| a:=2 ].
> b asContext tempNames ->  #('a')
> b asContext tempNamed: 'a' -> Boom in MethodContext>>#tempAt: primitive
> tempAt: in Method context uses <primitive: 210>, which is failing.  Maybe
> there is a new primitive we are not aware of for Blocks...

No, 210 is what is needed.   The issue is my implementation of temporary
initialization in blocks, which simply uses pushNil to allocate and
initialize each temp. So if one inspects [|a|a:=2] and sends it self method
symbolic you get:

13 <8F 00 00 05> closureNumCopied: 0 numArgs: 0 bytes 17 to 21
17 <73> pushConstant: nil
18 <77> pushConstant: 2
19 <81 40> storeIntoTemp: 0
21 <7D> blockReturn
22 <7C> returnTop

And when you say self asContext pc you get 17, which is *before* the nil is
pushed.  So you'll want to modify tempNamed: to compare the index of the
temporary with the stack pointer and answer nil if the index is beyond the
stack pointer.

The same bug exists in Squeak, e.g. the below gave an error until I
fixed DebuggerMethodMap>>namedTempAt:in:

| b |
b := [ |a| a:=2 ].
{ b asContext tempNames. b method debuggerMap tempsAndValuesForContext: b
asContext }
#(#('b' 'a') 'b: [closure] in UndefinedObject>>DoIt
a: nil

Why did I use pushNil to allocate temps?  It was not a good choice; it
means for example that to determine how many temps a block has one has to
parse all the bytecodes and see where the stack pointer is at the end
(since initial pushNils are ambiguous; an initial pushNil might be
producing a parameter or result or initializing a temp).  But not using
pushNil would have meant a more complex bytecode for closure creation plus
another inst var in BlockClosure.  I made another similar misstep in not
storing the method in a BlockClosure, hence preventing simple
implementation of clean blocks (one needs to synthesize an outerContext
also).  We live and learn.

> On Sat, Apr 14, 2012 at 1:09 PM, Stéphane Ducasse <
> stephane.ducasse at inria.fr> wrote:
>> I'm taking some times to go back the block closure chapter and I was
>> wondering how I can get access to block temporaries
>> [ |a | a :=2.  a]
>> for example I tried
>> [ |a | a :=2.  a]
>> outerContext contextForLocalVariables tempAt: 1 and variations but none
>> worked.
>> Stef

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