`first` and `rest` need to check if the input is a list (a linear-time
but memoized test), whereas `car` and `cdr` check only if the inputs
are `pair?`s, and match-define uses unsafe operations, doing the pair
check only once.
I see from raco demod that match-define does not generate very
beautiful code, which leads me to this version (which is what I think
match-define should have expanded into). It seems to be about 4x
faster on my computer.
#lang racket
(require racket/unsafe/ops)
(define (as)
(build-list 1000000 (λ (n) (random 100))))
(define (bs)
(build-list 1000000 (λ (n) (random 100))))
(define (z as bs [acc 0])
(cond
[(or (null? as) (null? bs))
acc]
[else
(match-define (cons this-a more-as)
as)
(match-define (cons this-b more-bs)
bs)
(z more-as more-bs (+ acc (* this-a this-b)))]))
(let ([as (as)]
[bs (bs)])
(collect-garbage)
(collect-garbage)
(collect-garbage)
(time (for ([i (in-range 10)])
(z as bs))))
(define (z2 as bs [acc 0])
(cond
[(or (null? as) (null? bs))
acc]
[else
(define this-a (car as))
(define more-as (unsafe-cdr as))
(define this-b (car bs))
(define more-bs (unsafe-cdr bs))
(z2 more-as more-bs (+ acc (* this-a this-b)))]))
(let ([as (as)]
[bs (bs)])
(collect-garbage)
(collect-garbage)
(collect-garbage)
(time (for ([i (in-range 10)])
(z2 as bs))))
Robby
On Thu, Jul 27, 2017 at 8:27 PM, Philip McGrath
<phi...@philipmcgrath.com> wrote:
> `match-define` seems to be the real source of the speedup for me:
>
> #lang racket
>
> (define (as)
> (build-list 1000000 (λ (n) (random 100))))
> (define (bs)
> (build-list 1000000 (λ (n) (random 100))))
>
> (define (f as bs [acc 0])
> (if (or (null? as) (null? bs))
> acc
> (f (rest as) (rest bs) (+ acc (* (first as) (first bs))))))
>
> (let ([as (as)]
> [bs (bs)])
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
> (time (f as bs)))
>
>
>
> (define (g as bs)
> (let g ([as as]
> [bs bs]
> [acc 0])
> (cond
> [(or (null? as) (null? bs))
> acc]
> [else
> (match-define (cons this-a more-as)
> as)
> (match-define (cons this-b more-bs)
> bs)
> (g more-as more-bs (+ acc (* this-a this-b)))])))
>
>
> (let ([as (as)]
> [bs (bs)])
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
> (time (g as bs)))
>
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
>
> (define (z as bs [acc 0])
> (cond
> [(or (null? as) (null? bs))
> acc]
> [else
> (match-define (cons this-a more-as)
> as)
> (match-define (cons this-b more-bs)
> bs)
> (z more-as more-bs (+ acc (* this-a this-b)))]))
>
> (let ([as (as)]
> [bs (bs)])
> (collect-garbage)
> (collect-garbage)
> (collect-garbage)
> (time (z as bs)))
>
> ; timed at the command line
> ;cpu time: 242 real time: 241 gc time: 0
> ;2452993452
> ;cpu time: 21 real time: 21 gc time: 0
> ;2450414634
> ;cpu time: 22 real time: 22 gc time: 0
> ;2450010495
>
> -Philip
>
> On Thu, Jul 27, 2017 at 8:22 PM, Matthias Felleisen <matth...@ccs.neu.edu>
> wrote:
>>
>>
>> On Jul 27, 2017, at 9:18 PM, Matthew Flatt <mfl...@cs.utah.edu> wrote:
>>
>> I don't think optional arguments are all that slow.
>>
>>
>>
>> This isn’t just Matthew’s opinion. Vincent’s feature-specific profiler
>> could not detect a significant impact of optional or keyword arguments. (See
>> Compiler Construction paper, London 2014 I think)
>>
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