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From: Bart <bc@freeuk.com>
Newsgroups: comp.compilers
Subject: Re: Bounds checking, Optimization techniques and undefined behavior
Date: Wed, 8 May 2019 14:37:31 +0100
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On 07/05/2019 10:04, David Brown wrote:
> On 06/05/2019 10:15, Hans-Peter Diettrich wrote:

>> I learned to love the Pascal indexing instead of pointers, because a
>> loop like
>>    for i := 1 to 10 do sum := sum + A[i];
>> can be optimized safely by the Pascal/Delphi compiler into pointer and
>> auto increment, so that no speed penalty exists vs. explicit pointer
>> usage. But in a C for loop the index variable can be changed in code, so
>> that even above code would execute slower with bounds checks.
>>
>
> It will not be slower in C - because the compiler knows that "i" is
> never changed in the loop.  (I'd like some way to have that enforced in
> C, but I don't know of any good method.)

Making loop variables declared in a for-header be implicitly 'const'
could be done:

    for (int i=0; i<N; ++i)

This can be the same as 'const i=0'.

The problem is that in such a loop (of the kind I frequently malign in
c.l.c and you always disagree), the code to increment this loop variable
is in user-code. So it is subject to the same rules as all other code,
and i cannot be changed.

In a more typical for-loop, such increments are done behind the scenes,
while still keeping the loop index read-only in user-code.