[Vm-dev] Spur Object 64 bit format

Igor Stasenko siguctua at gmail.com
Fri Nov 15 13:04:23 UTC 2013

On 14 November 2013 17:44, Eliot Miranda <eliot.miranda at gmail.com> wrote:

> 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.
Two entries where? If i understand, in COG you using generated code for
inline cache, by jumping onto PIC entry for checking if it hits or misses.
So, you just generate different code for immediate case(s), to check that
receiver oop value is tagged and within certain range (cache hit) , or not
(cache miss).

A pseudo-code for bit-pattern matching is this:

"presuming least significant bit is tag"
(oop bitAnd: 2r101) == 2r101 ifTrue: [ hit ] ifFalse: [ miss ]

And code for range matching is this:

"presuming least significant bit is tag"
(oop odd and: [ oop between: lowValue and: highValue]) ifTrue: [ hit ]
ifFalse: [ miss ]

"or presuming most significant bit is tag"
(oop between: lowValue and: highValue) ifTrue: [ hit ] ifFalse: [ miss ]

>> 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.

i'm not sure i understood, what that extra inline cache you talking about?
presuming that each PIC is generated code, you just generate different code
for entry which specialized to detect
that given oop is of certain immediate class.

But if its not, then yes, you need to hold these two values somewhere to be
able to compare them with input oop.
But even in that case, i fail to see how single extra comparison could
cause so much bloat.

And it always good to compare, how much bloat will cause bit pattern
matching code.

What is clearly beneficial for immediate ranges that you can redefine them
at any moment by introducing
new kinds of immediates , without need to even touch VM: language can
easily control that.
And the potential number of immediate classes is much bigger , if you
encode them in ranges, because:

2^31 (smallint) + 2^32 (short float) + 2^24 (character unicode)  =

which leaves us:

2^63 - 6459228160 =  9223372030395547648
space for other kinds of immediates, which we can introduce later
!!without!! need to ever bother VM again.

Best regards,
Igor Stasenko.
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