[Vm-dev] Spur Object 64 bit format

Eliot Miranda eliot.miranda at gmail.com
Tue Nov 12 18:23:55 UTC 2013

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:

andl $0x1, %eax
j Lcmp
movl %edx, %eax
andl $0x3, %eax
jnz Limm
movl 0(%edx), %eax
andl $0x3fffff, %eax
cmpl %ecx, %eax
jnz Lmiss

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.

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