On 12-May-06, at 10:41 AM, Michael Haupt wrote:

> John,
>
> On 5/12/06, John M McIntosh <johnmci at smalltalkconsulting.com> wrote:
>> In Squeak we use 2 bits of the 32 in 32bit Squeak, 1 bit
>> for a tag for the GC logic, and the other bit to signify if this
>> object is a SmallInteger.
>>
>> xxxxxxxx00 -> object reference (tag bit not set; divisible by four)
>> xxxxxxxx10 -> Unused (tag bit not set; but not a valid pointer)
>> xxxxxxxx01 -> SmallInteger (tag bit set)
>> xxxxxxxx11 -> SmallInteger (tag bit set)
>
> I was not aware of that GC flag bit, but I had actually been wondering
> why one bit was "wasted" when pointers/references are word-aligned...
> Can you point me to a place in the image where that is made clear?


Ah, well this is wrong, let me dig further, this came from some  
discussions we were having about 64 bit
squeak and I wonder if some of that creep into the conversation,  
since mmm 30  bits of data, and 1 bit for sign.

Lets see, back to code, versus my faulty memory, I apologies for  
confusing the issue.

isIntegerObject: objectPointer

	^ (objectPointer bitAnd: 1) > 0

Interestingly enough in Slang and header files we optimize the  
isIntegerObject: and it turns into

(oop & 1)

Also see
isNonIntegerObject: objectPointer

	^ (objectPointer bitAnd: 1) = 0

As for the GC tag bit 32bit Squeak uses a bit in the Object header
	MarkBit := 16r80000000.
to note if an object has been traced when running the marker logic.
immediate objects (aka integers) don't need tracing and are skipped.
The Smaltalk code for the mark/trace is easy to read, the slang  
generated C code is not, since the slang logic folds multiple routines
together to avoid calls outside of the logic, plus performs  
localizing usage of global variables, turning some into  local  
variables.


However as noted since words are aligned we technically have a spare  
state where we could have 30 bits of data with bit 2 set to 1,
and bit 1 set to 0

xxxxxxxx00  oop reference, divisible by 4.
xxxxxxxx10  invalid oop reference, but could be something other than  
an oops reference
xxxxxxxxx1  smallinteger


--
======================================================================== 
===
John M. McIntosh <johnmci at smalltalkconsulting.com> 1-800-477-2659
Corporate Smalltalk Consulting Ltd.  http://www.smalltalkconsulting.com
======================================================================== 
===