Re: LW compiler optimizations

And yet, if I follow that advice in the simple DOTIMES loop, I get even worse timings than before. 

So the boxing advice only seems to apply to the innards of a closure being mapped across the arrays. Otherwise, avoid this advice.



> On Jan 13, 2025, at 07:41, David McClain (as dbm at refined-audiometrics dot com) <[email protected]> wrote:
> 
> So I see that the inner closure is calling Compiler Double-float Box on the computation. The allocations must be reflecting that across 5633 calls, which translates to 48 bytes per call.
> 
> So to see if I can get rid of boxing, I set up the SUM as a one-element array, specialized to double-float elements. And sure enough, that more than doubled the speed and cut the allocations down to a little over half the original number.
> 
> Oddly, when I reran my bench test, at the start, this time the mapping routine took twice as long as in my earlier bench tests. (very puzzling behavior) 
> 
> So now with the 1 element array SUM, I’m back to 124 μs/invocation on 5633 elements.
> 
> But the fact remains, if you want to avoid boxing, use a specialized array destination for the computed result.
> 
>> On Jan 13, 2025, at 07:13, David McClain <[email protected]> wrote:
>> 
>> I divided down the allocations to per-invocation costs. 
>> 
>> The worst offender in this regard is the implementation that MAP’s a closure across the pairs of elements in the dot-product. It takes a whopping 270 kB per buffer dot-product. 
>> 
>> And yet it is so simple. The closure should only be formed just once per invocation.
>> 
>> (defun fir-dotpr-map (fir vec &key (start2 0))
>>   ;; 128 μs/invocation for 5633 element vectors
>>   ;; 270,502 bytes/invocation
>>   #F
>>   (declare ((vector double-float *) fir vec)
>>            (fixnum start2))
>>   (let ((sum   0d0)
>>         (v     (make-array (length fir)
>>                            :element-type 'double-float
>>                            :displaced-to vec
>>                            :displaced-index-offset start2)))
>>     (declare (double-float sum)
>>              ((vector double-float *) v)
>>              (dynamic-extent v))
>>     (map nil (lambda (a b)
>>                (declare (double-float a b))
>>                (incf sum (* a b)))
>>          fir v)
>>     sum))
>> 
>