[fpc-pascal] Illegal counter variable?

Tue Sep 10 14:14:08 CEST 2019

I’m trying to use it in a way that always worked in Turbo Pascal for DOS, I have no idea how to program in C,  I find Pascal much more clear and easy to follow and C just confuses me, so I never spent much time other than a few stray projects here and there on C…  I’ve always used pascal since the 80s, and in fact my units that change the for loop variable that run in {$Mode TP} are the original code from turbo pascal written in the 80s.. over the decades I’ve modified stuff all around it, but the actual code with for loops with changing control variables is the exact same code I was running on old 4.7MHz Dos machines with turbo pascal for DOS…  so for nearly 40 years I have never even known there was some rule against changing the control variable in for loops,  it’s always worked and I’ve always used it.. and it still works if I use {$Mode TP}  it may not work if I try to use Delphi or something else, and there may be some pascal version even older than turbo pascal for DOS that didn’t act this way, but from the entire scope of my programming experience, this is something that has just always worked and I never had a reason not to use it.

James

From: fpc-pascal <fpc-pascal-bounces at lists.freepascal.org> On Behalf Of Markus Greim
Sent: Monday, September 9, 2019 1:32 PM
To: FPC-Pascal users discussions <fpc-pascal at lists.freepascal.org>
Subject: Re: [fpc-pascal] Illegal counter variable?

James, 

not every body is using a GHz machine. I am , for example, programming a 80186 in an embedded system with very limited speed an RAM. 

But I understand thats not a general argument. 

But look at MISRA C. Its a big set of rules for "real" save C programming, more or less now the standard in the automobile industry. 

Also here are strict rules for counters in for loops. 

See: 

https://www.misra.org.uk/forum/viewtopic.php?t=272

So what they are doing is writing Pascal programs in C... ;-) And they need huge and expensive and complex  tools to check the C code for 

MISRA compatibility!

So we should NOT give up the safety of Pascal for the sloppiness of C! 

Kind Regards

Markus

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On September 9, 2019, 7:19 PM GMT+2 james at productionautomation.net <mailto:james at productionautomation.net>  wrote:

Yes, today such limitations do seem too restrictive, I wonder if the reasons for the restrictions have become obsolete. You would have to have a really slow computer with very limited resources to optimize loops to the point of reducing functionality like this, and the tendency with modern pc's is to have a larger library of much more powerful and more flexible tools, even if they are more complex and take more memory and resources. For example Case statements used to only work with characters or integers, but the modern version now also works with strings, very much added functionality for sure, but also would use more resources.... but we would all rather have the capability because even a raspberry pi is blazing fast and has ram to burn compared to our old 8086s It seems silly to me that I can't so what I used to do with Turbo Pascal for DOS where I was limited to programs of a few hundred K, due to optimizations that just can't make much of a difference at all on modern computers. I am happy that FPC has TP compatibility mode though! It's just I get to a point where I eventually can't cross units since I can't have a circular unit reference. So I have to choose for any given unit.. do I want my old for loops and change the control variables, or do I want cool new case statements... etc.. it would be nice to have {$Mode everything_the_way_I_want_it}

Since the compiler knows I was trying to change the variable inside the for loop, could it not just compile in not quite as efficient code (TP MODE for loop) when it detects this, and use the more efficient optimized code when it detects that it is able to use it?

James

-----Original Message-----
From: fpc-pascal <fpc-pascal-bounces at lists.freepascal.org <mailto:fpc-pascal-bounces at lists.freepascal.org> > On Behalf Of Bernd Oppolzer
Sent: Monday, September 9, 2019 10:46 AM
To: FPC-Pascal users discussions <fpc-pascal at lists.freepascal.org <mailto:fpc-pascal at lists.freepascal.org> >
Subject: Re: [fpc-pascal] Illegal counter variable?

Am 09.09.2019 um 16:11 schrieb James Richters:

> I just don't see why having the limitation, there is no technical 
> reason that the for loop couldn't change that I can see.. especially since it works in TP mode.

The original reason why some Pascal implementations had this limitation:

for performance or optimization reasons, the loop control variable was transferred to a register at the beginning of the loop, and changing the variable (at its storage location) inside the loop simply had no effect, because the variable was not fetched from there again during loop execution.
Worse: maybe, to make read accesses to the loop control variable valid inside the loop, they are prepared by storing the control register value into the loop control variable, thus turning changes to the loop control variable useless.

Forbidding (write) accesses to the loop control variable allows for many aggressive optimization strategies around loops.

Maybe today such limitations seem too restrictive.

Kind regards

Bernd

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