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From: "Tim Behrendsen" <tim@a-sis.com>
Subject: Re: Ada to C/C++ translator needed
Date: 1996/09/30
Message-ID: <01bbaef3$73605680$87ee6fce@timpent.a-sis.com>#1/1
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references: <32499FA0.4B5E@magic.fr> <52e5t5$m28@btmpjg.god.bel.alcatel.be>
 <52feul$os2@goanna.cs.rmit.edu.au> <01bbad6e$67743f20$32ee6fcf@timhome2>
 <52ltk5$qlf@news1.halcyon.com> <01bbae25$67c669a0$32ee6fcf@timhome2>
 <dewar.844055713@schonberg> <01bbae8f$dffbd440$32ee6fcf@timhome2>
 <mheaney-ya023180003009960805430001@news.ni.net>
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organization: A-SIS
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newsgroups: comp.lang.c,comp.lang.ada
Date: 1996-09-30T00:00:00+00:00
List-Id: <comp.lang.ada>


Matthew Heaney <mheaney@ni.net> wrote in article
<mheaney-ya023180003009960805430001@news.ni.net>...
> In article <01bbae8f$dffbd440$32ee6fcf@timhome2>, "Tim Behrendsen"
> <tim@airshields.com> wrote:
> 
> >I know that Ada does run-time checks of array bounds that C
> >doesn't do; that could be one source of loss of performance.
> >Does Ada do any other run-time checks that C doesn't do?
> 
> Not necessarily.  Using a subtype actually *turns off* type checking during
> iteration through an array.  
> [snip example]

> Here you are asserting that I lies in the index subtype of O.  The checking
> of that assertion need only take place once, on entry to the loop.
> 
> It's a common misunderstanding that a subtype puts checks in, but very
> often just the opposite is true.  The idea is the give the compiler as much
> information as you can when declaring the index that dereferences an array
> object.  For example:
>[snip example]

So it is possible to give the compiler enough hints to eliminate
the check in looping cases.  In fact, I would expect the compiler
could be smart enough to detect (excuse C syntax)

for (i = 0; i < n; ++i) {
    if (A[i] > j)
        ...
}

It knows the working range of i is 0..n, so it could do the
bounds check once (A[n]) at the beginning of the loop.

All that having been said, I wonder how much bounds checking
is actually kept in a real good-sized project where you are
passing general values into subroutines that get used as
array indexes (where the compiler couldn't necessarily validate
at compile time)?

To tell you the truth, I doubt it's a big deal, unless the
latter subroutine call is being used in a hard loop, and you're
doing character-by-character processing or something. Then the
bounding checks could start adding up.

-- Tim Behrendsen (tim@a-sis.com)