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[PATCH] Make vector-map and vector-for-each in (rnrs base) fast

From: Linus Björnstam
Subject: [PATCH] Make vector-map and vector-for-each in (rnrs base) fast
Date: Thu, 18 Feb 2021 08:34:23 +0100
User-agent: Cyrus-JMAP/3.5.0-alpha0-141-gf094924a34-fm-20210210.001-gf094924a

Hi there!

I was spelunking through the guile source tree and found (rnrs base). The 
vector-map and vector-for-each in there are horribly inefficient. They are 
doing (list->vector (apply map PROC (map vector->list vectors))), which means 
it spends quite some time checking for circular references.

This fixes that. The speedup is surprisingly small, considering we pass through 
the elements 2 fewer times and don't chase pointers through memory trying to 
find cycles. Anywhere from 30 to 300% depending on how the stars are aligned on 
things like (vector-map + vec vec)

One potential speedup we could do is using eq? to compare numbers, but I don't 
know how well fixnums in guile overlap size_t, regardless of how realistic such 
a limitation would be. If I change the behaviour of vector-map to go 
back-to-front (order is unspecified in r6rs) we can easily do (eq? -1 index) as 
a stop condition to avoid any eventual overhead of type checking with =. (If 
those are not elided, which I suspect might be the case. ). I did not look at 
that, since I have too little computer time these days.

As an added bonus, this speeds up quicksort.scm in ecraven's benchmarks by a 
  Linus Björnstam

Attachment: 0001-Write-a-proper-vector-map-and-vector-for-each-for-rn.patch
Description: Text Data

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