[Vm-dev] Cog Primitive Performance

[Eliot Miranda]

Hi Andres,

On Tue, Apr 18, 2017 at 1:52 AM, Andres Valloud <
avalloud at smalltalk.comcastbiz.net> wrote:

>
> Yes, the SqR initials are mine.  That's too bad about the comments, at the
> very least there should be something like this:
>
> "This code performs a multiplication by 1664525 mod 2^28 without
> overflowing into large integers."


Thanks.  I'll update the primitives appropriately.  Could you supply a
reference both for the technique and the choice of  1664525 in particular?
I'd like to include that info in the comments too.


On 4/18/17 1:48 , Levente Uzonyi wrote:
>
>> The methods in SmallInteger and LargePositiveInteger have the initials
>> SqR (that's yours IIRC), but they have no comment.
>> The variant in String is originally from 2001 and has never had that
>> comment.
>>
>> Levente
>>
>> On Mon, 17 Apr 2017, Andres Valloud wrote:
>>
>>
>>> On 4/17/17 16:45 , Eliot Miranda wrote:
>>>
>>>> FYI, hash multiply is
>>>> hashMultiply
>>>> | low |
>>>> low := self bitAnd: 16383.
>>>> ^(16r260D * low + ((16r260D * (self bitShift: -14) + (16r0065 * low)
>>>> bitAnd: 16383) * 16384))
>>>> bitAnd: 16r0FFFFFFF
>>>>
>>>> and when written as a primitive is
>>>> hashMultiply
>>>> | low |
>>>> low := self bitAnd: 16383.
>>>> ^(16r260D * low + (16r260D * (self bitShift: -14) + (16r0065 * low) *
>>>> 16384))
>>>> bitAnd: 16r0FFFFFFF
>>>> because we don't care about the multiply by 16384 overflowing; when
>>>> written as a primitive it won't overflow into a LargePositiveInteger.
>>>>
>>>
>>> Hopefully this doesn't mean the primitive got implemented by actually
>>> doing these operations verbatim.  As you have probably seen, that
>>> convoluted arithmetic is done this way in Smalltalk only to simulate a
>>> 32x32 multiplication bit-anded down to 28 bits without overflowing
>>> into large integers (the original code from August 2000 had my initials).
>>>
>>> At a sufficiently low level such as C, all that complexity is just an
>>> unsigned multiplication by 1664525.  The image code should still have
>>> a comment to that effect, did it get lost?
>>>
>>> Andres.
>>>
>>>
>>


-- 
_,,,^..^,,,_
best, Eliot
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