Re: [jallib] Compiler doesn't honour "volatile"

'Oliver Seitz' via jallib Fri, 27 Nov 2020 12:17:42 -0800

 Hi Rob!
So we *do* have REPEAT/UNTIL, I thought to miss it just today :-D
Thanks for the hint. Earlier today I only found an older version of the 
documentation which contained just that problematic example: increasing a word 
variable inside an interrupt, but that example is gone (or shortened) in the 
current version.
Yes, stopping the interrupt before reading the variable would be a simpe 
solution, but I wanted to time my pulses as accurate as possible. I can't 
disable just the timer update interrupt, as another interrupt copies the timer 
value. The hardware timer keeps running, and so the timer values may become 
inconsistent for the interrupt-on-change. Ok, I already handled that 
inconsistency for it can occur very rarely even without stopping the interrupt, 
but I did not want to stop it. Well, and I did not think of just stopping that 
very interrupt, I only thought about stopping all interrupts. 
As the place where the actual problem was right now is a very unimportant one, 
I have two very simple solutions:
If the low-byte value is larger than 253, just skip that part of the program 
this time.
Or, what I did now:
var volatile word check_valueREPEAT  check_value=changing_value  
result=changing_vaue-whateverUNTIL check_value==changing_value
It would be sufficient to only check the high-byte.
Thanks again for answering even my stupid questions ;-)
Greets,Kiste
 
    Am Freitag, 27. November 2020, 18:54:08 MEZ hat Rob CJ <[email protected]> 
Folgendes geschrieben:  
 
 Hi Kiste,
Some answers to the things you mention.
About volatile variables. This is what the JAL documentation says about is.
"The VOLATILE keyword guarantees that a variable that is either used or 
assigned will not be optimized away, and the variable will be only read (or 
written) once when evaluating an expression.Normally, if a variable is assigned 
a value that is never used, the assignment is removed and the variable is not 
allocated any space. If the assignment is an expression, the expression *will* 
be fully evaluated. If a variable is used, but never assigned, all instances of 
the variable will be replaced with the constant 0 (of the appropriate type) and 
the variable will not be allocated any space."

Volatile is especially needed if a perhiperal would update a variable since the
compiler does not know that and so it would optimize it away since it does not
'see' that the variable is used by the program
About documentation. The JAL documentation is in the 'compiler' directory that
is present in the install or the bee-package.
You are right that the documentation does not talk about mapping bytes onto
words. I could add that as an example since it is something which is used quite
often.
Then my thoughts about the issue you mention. I see that you are updating a
word value (not a byte value) and since the PIC is byte oriented a word
consists of two bytes. Updating a word value requires several instructions and
I can imagine that if an interrupt occurs exactly in between these instructions
that things go wrong. What you would need is a semaphore but since we do not
have that, a way around is that you before printing your word you disable that
specific interrupt that is also updating that same word. If you do not want to
delay the interrupt too much because of the print function you could also do
the following:-) Disable the interrupt-) Copy the word value to a local word
variable-) Enable the interrupt
Kind regards,
Rob

Van: 'Oliver Seitz' via jallib <[email protected]>
Verzonden: vrijdag 27 november 2020 12:28
Aan: [email protected] <[email protected]>
Onderwerp: Re: [jallib] Compiler doesn't honour "volatile" Hi!
Yes, thanks, memory is fine, the data logger is up and running, storing 2048
values at 24 bit resolution, great! :-)
I have a program, but it uses a watch crystal and counts external pulses. And,
for saving time... I've now seen the error three times within ten days.
Thinking again about the issue, I'm not really sure if "volatile" is the right
term to describe the problem. "volatile" specifies, that for every read/write
in the source code, exactly one read/write in the machine code is done.
Perhaps it is up to the program author to ensure the correct reading outside of
the interrupt if a variable is modified inside the interrupt and vice versa.
Then it would be a subject of documentation. Which, by the way, I didn't read
:-D
But, while disgressing already, a bit to make JAL popular and usable...Where is
the official documentation? I lately had to place byte variables over dword
variables and couldn't find the syntax in any document, only placing bits. I
kind of tried luckily with var byte name1 at name2+byte_offset.

(Ok, found the website and the language reference. But no byte offsets :-) )
Greets,Kiste
Am Freitag, 27. November 2020, 11:39:06 MEZ hat Rob CJ <[email protected]>
Folgendes geschrieben:

Hi Kiste,
Hmm, my next puzzle for the weekend. Can you send a complete sample program
that I can use? It will save me time.
BTW. Did you already test the new build compiler that solves the memory issue?
Thanks.
Kind regards,
Rob
Van: 'Oliver Seitz' via jallib <[email protected]>
Verzonden: vrijdag 27 november 2020 07:44
Aan: Jallib <[email protected]>
Onderwerp: [jallib] Compiler doesn't honour "volatile" Sorry... This isn't in
fact the proper list for compiler issues, is it?
This one is probably more severe. I'll start with the example. This is the data
I've received:
?2807 2808 1?2807 2809 2?2807 2810 2?2807 2811 4?2807
2812 4?2807 2813 6?2807 2814 7?2807 2815 8?2807 2816 264
It's generated from this part of code, running on an PIC18F27K42. extra_time is
increased in an interrupt every 16 seconds. It is declared as var volatile
word, so the compiler should make sure that the bytes of the variable are in a
consistent state:

diff=word(extra_time-last_pulse_time_uw)

if print_debug then
print_debug=false
serial="?"
print_word_dec(serial,last_pulse_time_uw)
serial=" "
print_word_dec(serial,extra_time)
serial=" "
print_dword_dec(serial,diff)
print_crlf(serial)
end if

Let me repeat some of the results above in hex:

AFE-AF7 =7AFF-AF7 =8B00-AF7 =108
So, what has happened? The last line is clearly wrong. The subtraction was done
just as extra_time was in the process of increasing. The low byte was processed
before, the high byte was processed after the interrupt fired. So the actual
subtraction done was BFF-AF7, with the result of 108.
I've had a look at the asm file. Both bytes of the variable are just handled
one after the other, both in the main program and in the interrupt service
routine. The "volatile" declaration does not seem to do anything.
Greets,Kiste

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