[Vm-dev] Spur Object 64 bit format

[Eliot Miranda]

On Thu, Nov 14, 2013 at 3:43 AM, Igor Stasenko <siguctua at gmail.com> wrote:

>
>
>
>
> On 12 November 2013 19:23, Eliot Miranda <eliot.miranda at gmail.com> wrote:
>
>>
>> Hi Igor,
>>
>>
>> On Tue, Nov 12, 2013 at 3:37 AM, Igor Stasenko <siguctua at gmail.com>wrote:
>>
>>>
>>> And let me remind you what i proposed back then:
>>>
>>> :)
>>> ===========
>>> About immediates zoo.
>>>
>>> Keep in mind, that the more immediates we have, the more complex
>>> implementation
>>> tends to be.
>>>
>>> I would just keep 2 data types:
>>>  - integers
>>>  - floats
>>>
>>> and third, special 'arbitrary' immediate , which seen by VM as a 60-bit
>>> value.
>>> The interpretation of this value depends on lookup in range-table,
>>> where developer specifying the correspondence between the value
>>> interval and class:
>>> [min .. max] -> class
>>>
>>
>> for this idea to go anywhere you'd have to show at least the pseudo-code
>> for the inline cache test in machine code methods.  These schemes seem
>> great in theory but in practice end up with a long, complex and slow
>> fetchClassOf: and/or inline cache test.  To remind you, you have to compete
>> with the following in Spur Cog:
>>
>> Limm:
>> andl $0x1, %eax
>> j Lcmp
>>  Lentry:
>> movl %edx, %eax
>> andl $0x3, %eax
>>  jnz Limm
>> movl 0(%edx), %eax
>> andl $0x3fffff, %eax
>>  Lcmp:
>>
>
> It is extra comparison for immediate case:
>
> cmp eax, LowRange
> jl miss
> cmp eax, HighRange
> jg miss
>

I don't understand.  Does that mean there are two cache entries, one for
HighRange one for LowRange?  Spell it out please.  Give the pseudo-code.
 Right now it is too vague for me to see how it is supposed to work.



>
> so, if value between low & high range, it is already known what class it
> is otherwise
> you got cache miss.
>
> And this scheme works even better if you pick highest bit for tagging,
> like that you can get rid of testing tag bit completely:
>  andl $0x1, %eax
> j Lcmp
> and start straight from comparing with high and low values.
> That of course, if we can use highest bit for tagging.
>
> The class lookup then will be binary search in table of ranges, which is
> O(log n).
>

So the inline cache becomes a range of entries?  That will cause a
significant code bloat.


>
>
>
>>
>> This occurs at the start of every method.  Its efficiency and size is
>> really important for performance and compactness of machine code.  I'm
>> really not trying to be negative; it is simply a fact of life with a
>> Smalltalk JIT.
>>
>> intervals, of course, cannot overlap.
>>> Determining a class of such immediate might be slower - O(log2(n)) at
>>> best (where n is size of range table), but from other side,
>>> how many different kinds of immediates you can fit into 60-bit value?
>>> Right, it is 2^60. Much more than proposed 8 isn't? :)
>>>
>>> And this extra cost can be mitigated completely by inline cache.
>>> - in case of regular reference, you must fetch the object's class and
>>> then compare it with one, stored in cache.
>>> - in case of immediate reference, you compare immediate value with min
>>> and max stored in cache fields.
>>> And if value is in range, you got a cache hit, and free to proceed.
>>> So, its just 1 extra comparison comparing to 'classic' inline cache.
>>>
>>> And, after thinking how inline cache is organized, now you can scratch
>>> the first my paragraph related to  immediates!
>>> We really don't need to discriminate between small
>>> integers/floats/rest!!
>>> They could also be nothing more than just one of a range(s) defined in
>>> our zoo of 'special' immediates!
>>>
>>> So, at the end we will have just two kinds of references:
>>>  - zero bit == 0 -- an object pointer
>>>  - zero bit == 1 -- an immediate
>>>
>>> Voila!.
>>>
>>> We can have real zoo of immediates, and simple implementation to support
>>> them.
>>> And not saying that range-table is provided by language-side, so we're
>>> free to rearrange them at any moment.
>>>
>>> And of course, it doesn't means that VM could not reserve some of the
>>> ranges for own 'contracted'
>>> immediates, like Characters, and even class reference for example.
>>> Think about it :)
>>>
>>>
>>>
>>> On 12 November 2013 11:56, Henrik Johansen <henrik.s.johansen at veloxit.no
>>> > wrote:
>>>
>>>>
>>>>
>>>> On 12 Nov 2013, at 11:55 , Henrik Johansen <
>>>> henrik.s.johansen at veloxit.no> wrote:
>>>>
>>>> nterest ~= time and: [skills]
>>>>
>>>>
>>>> OT: I swear, missing parenthesis will be the death of me!
>>>>
>>>>
>>>
>>>
>>> --
>>> Best regards,
>>> Igor Stasenko.
>>>
>>>
>>
>>
>> --
>> best,
>> Eliot
>>
>>
>
>
> --
> Best regards,
> Igor Stasenko.
>
>


-- 
best,
Eliot
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