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From: Robin Vowels <robin.vowels@gmail.com>
Newsgroups: comp.lang.ada
Subject: Re: Efficient Sequential Access to Arrays
Date: Thu, 26 Jul 2012 07:47:14 -0700 (PDT)
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Date: 2012-07-26T07:47:14-07:00
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On Jul 25, 5:09=A0pm, Niklas Holsti <niklas.hol...@tidorum.invalid>
wrote:
> On 12-07-24 19:00 , rrr...@gmail.com wrote:

> > For the PL/I optimising compiler for the CDC Cyber (c. 1980),
> > multiplication was eliminated for matrix elements
> > by using a table of offsets into each row.
> > Thus, double subscripting reduced to simple addition.
>
> That sounds like a specific choice of code generation for arrays, rather
> than a general optimization.

It was a general optimisation for matrices,
with significant benefits for intensive use of subscripting.

Optimisation comes from a number of optimisation strategies,
not just one.

> Keean could simulate this code by defining the array as a
> one-dimensional array of one-dimensional arrays (rows), which would
> replace the row-length multiplication by an additional indexing.
>
> >> (We are of course (I believe) assuming that index range checks are
> >> turned off.)
>
> > Index range checks should be left on.
>
> Sure, in general. But here the OP (Keean) is trying to beat the
> performance of C++ that uses pointers without index checks. Enabling
> index checks in the Ada code would probably (in this case, where the
> algorithm has scatterered indexing of the array) slow it by an amount
> that swamps the effect of the multiplication optimization.

There's really no point in executing code without such checks.
You may never know that there is a bug in the code.