The Trunk: Collections-dtl.268.mcz

> On Mon, Jan 04, 2010 at 05:15:28PM +0100, Andreas Raab wrote:
>> Hi Jannik -
>>
>> A couple of comments. First, If wordSize is a constant, put it into a
>> class var. That's the best practice pattern for dealing with constants
>> and I see no reason why one would opt to use an ivar for a value that
>> never changes. See for example the EndianCache in SmalltalkImage and
>> other use of constants throughout the system.
>>
>> As for caching, if I understand your description correctly, then system
>> tracer is storing the correct value for the traced image. When the image
>> starts, your cache code invalidates the known to be correct value which
>> is later lazily filled in with the same value again. So what observable
>> effect does your cache invalidation have?
>>
>> #startUp isn't the first method sent, not by a very long shot - in
>> particular when you mess with the execution machinery you have to be
>> aware that a method like #initialPC might be called before you ever get
>> to the point where you invalidate that cache (I'm not sure if this can
>> happen in this concrete case). In any case you should trace through the
>> startup sequence to find out just how much other code is executed before
>> getting there.
>
> John and/or Bert previously pointed out that there is no need to ever
> set the cached value from the image, so this would be done from a system
> tracer only (and yes this should have a comment).
>
>> My recommendations would be to make WordSize a class variable, leave the
>> lazy initialization in (it might be helpful during install etc) but add
>> a nice comment explaining that only system tracer ever modifies that
>> value when a 32/64 bit image is written. And leave out the pointless
>> cache invalidation :-)
>
> I think that the change set on Mantis 7430 does what you describe. This
> puts the class variable in SystemDictionary in order to retain the current
> "Smalltalk wordSize" idiom, and uses the original Dan Ingalls #initialPC
> implementation from the "dist3" 64-bit image.
>
> It's a small change, so I'll copy it here:
>
> 'From Squeak3.10.2 of 16 December 2009 [latest update: #8496] on 18 December 2009 at 6:08:11 pm'!
> "Change Set: Smalltalk-wordSize-dtl-M7430
> Date: 18 December 2009
> Author: David T. Lewis
>
> Cache Smalltalk wordSize in class var in SystemDictionary..
>
> Update CompiledMethod>>initialPC to use #wordSize. This is the method as implemented in the original 64-bit image (author di)."!
>
> IdentityDictionary subclass: #SystemDictionary
> instanceVariableNames: 'cachedClassNames'
> classVariableNames: 'LastImageName LastQuitLogPosition LowSpaceProcess LowSpaceSemaphore MemoryHogs ShutDownList SpecialSelectors StartUpList StartupStamp SystemChanges WordSize'
> poolDictionaries: ''
> category: 'System-Support'!
>
> !CompiledMethod methodsFor: 'accessing' stamp: 'di 6/29/2004 12:28'!
> initialPC
> "Answer the program counter for the receiver's first bytecode."
>
> ^ (self numLiterals + 1) * Smalltalk wordSize + 1
> ! !
>
>
> !SystemDictionary methodsFor: 'sources, change log' stamp: 'dtl 12/18/2009 00:32'!
> wordSize
> "Answer the size (in bytes) of an object pointer."
> "Smalltalk wordSize"
>
> ^ WordSize ifNil: [WordSize := [SmalltalkImage current vmParameterAt: 40] on: Error do: [4]]! !
>
>
>
>

> On Mon, Jan 04, 2010 at 07:09:01PM +0100, Laval Jannik wrote:
>> Hi,
>>
>> I understand your comments,
>> So:
>> - i will put word size a class var,
>> - this class var will be modify only by systemTracer
>>
>> Now I have a question:
>> Why using Smalltalk wordSize whereas the vm parameter is getting by SmalltalkImage current ?
>
> Hi Jannik,
>
> Different people may answer this question in different ways, because it
> is a matter of style and personal opinion. But it may help to know that
> earlier versions of Squeak had the vm parameter query in SystemDictionary
> ("Smalltalk") rather than in SmalltalkImage. This was moved as part of
> a larger effort to reorganize SystemDictionary, which is a rather large
> class that has accumulated many functions over the years.
>
> As matter of function, it makes no difference either way. The part that
> is subject to opinion is whether it makes more sense for a person to
> say "Smalltalk wordSize" to refer to the size of a word in the object
> memory, or whether "SmalltalkImage current wordSize" is more meaningful.
>
> FWIW, my own opinion is that neither one is right, but "Smalltalk wordSize"
> is easier to remember. But no matter what approach is taken, you will be able
> to find someone who does not agree, so you should form your own opinion ;-)
>
> HTH,
>
> Dave
>
>
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