[Vm-dev] Cog Primitive Performance

[David T. Lewis]

On Tue, Apr 18, 2017 at 10:27:41AM +0200, Nicolas Cellier wrote:
>  
> 2017-04-18 4:02 GMT+02:00 Andres Valloud <avalloud at smalltalk.comcastbiz.net>
> :
> 
> >
> > 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.
> >
> 
> More: if it's a 64 bit image, then we have 60 bit long unsigned small int
> since 1664525 highBit = 21, and self is a hash result not exceeding 30
> bits, we can implement hashMultiply as
>     ^self * 1664525 bitAnd: 16r0FFFFFFF
> 
> Maybe the JIT can be given a second chance too.

AFAIK, primitiveStringHash has always been translated directly from 
ByteArray class>>hashBytes:startingWith: and the hashMultiply logic that we are
discussing here currently is repeated in four different methods in Squeak trunk
(String class>>stringHash:initialHash:, TypeString class>>stringHash:initialHash:,
SmallInteger hashMultiply, and ByteArray class hashBytes:startingWith:).

At this point, and particularly given the differences in various VMs, it would
make sense to reimplement primitiveStringHash directly in MiscPrimitivesPlugin
with whatever optimizations are needed (maybe differently based on size of a
small integers), rather than translating it from a default implementation in
the image.

But I think we may be missing Eliot's main point. There is overhead associated
with accessing the stack (at least on a stack interpreter) and in returning
values to image, and this will be the case regardless of what calculations are
being done in the primitive itself. So at least for some VMs we may be reaching
the point where that overhead becomes a significant part of the performance profile.

Whether that overhead would justify creating a new primitive callout mechanism
may be another question, but it is certainly worth understanding where the
overhead occurs, and how that performance profile changes as the Cog jit and
Sista progress.

Dave