Long signed integer multiplication issue?

[Leonardo Hirokazu De Souza Hamada]

Hi,

Currently I am messing around a JAL adapted PID code from Arduino library (FastPID to be precise). I'm using a pic 18f4620.

So, it came to my attention that a multiplication between two SDWORD value does not give a correct value, when one of then is negative.

-----Code section

delay_1s(1)
print_string(serial_hw_data, "Results:\r\n")

var sdword E = -2
var sdword c, p
p = 50
c = p * E
print_sdword_dec(serial_hw_data, c)

print_crlf(serial_hw_data)

-- This code gives correct results
E = -2
p = 50
if E < 0 then 
   E = -E
   c = p * E
   c = -c
end if
print_sdword_dec(serial_hw_data, c)

for 32 loop
    print_crlf(serial_hw_data)
end loop

---End code

----Terminal output:

Results:
3276700
-100

[Rob Hamerling]

Try with the  -fastmath  compiler option (see compiler docs for its effects).

On 16/08/2021 20.25, 'Leonardo Hirokazu De Souza Hamada' via jallist wrote:

Hi,

Currently I am messing around a JAL adapted PID code from Arduino library (FastPID to be precise). I'm using a pic 18f4620.

So, it came to my attention that a multiplication between two SDWORD value does not give a correct value, when one of then is negative.

-----Code section

delay_1s(1)
print_string(serial_hw_data, "Results:\r\n")

var sdword E = -2
var sdword c, p
p = 50
c = p * E
print_sdword_dec(serial_hw_data, c)

Try with the  -fastmath  compiler option (see compiler docs for effects).

print_crlf(serial_hw_data)

-- This code gives correct results
E = -2
p = 50
if E < 0 then 
   E = -E
   c = p * E
   c = -c
end if
print_sdword_dec(serial_hw_data, c)

for 32 loop
    print_crlf(serial_hw_data)
end loop

---End code

----Terminal output:

Results:
3276700
-100

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--
Rob Hamerling, Vianen, NL

[Leonardo Hirokazu De Souza Hamada]

So, just to complement, I did a diff against the two generated .asm files, one with -fastmath option and the other one with -no-fastmath option.

The only thing effectively changed was the name of multiplication subroutine, see:

$ diff 18f4620_integer_debug_stripped.asm_fm 18f4620_integer_debug_stripped.asm_no-fm
3c3
< ; command line:  C:\\JallibWorkplace\\compiler\\jalv2_64.exe d:\Users\Hamada\Google Drive\Controle-Forno-Eletrico\VALIDATION\18f4620_integer_debug_stripped.jal -s C://JallibWorkplace//lib -fastmath
---
> ; command line:  C:\\JallibWorkplace\\compiler\\jalv2_64.exe d:\Users\Hamada\Google Drive\Controle-Forno-Eletrico\VALIDATION\18f4620_integer_debug_stripped.jal -s C://JallibWorkplace//lib -no-fastmath
57c57
< l__pic_multiply_4_4
---
> l__pic_multiply
112c112
<                                call     l__pic_multiply_4_4
---
>                                call     l__pic_multiply
154c154
<                                call     l__pic_multiply_4_4
---
>                                call     l__pic_multiply

Greetings,

Leonardo

[Rob CJ]

Hi Leonardo,

I did the following tests (next time also mention the PIC):

-) Ran the program on a PIC18F26K42. Incorrect results with and without -fastmath

-) Ran the program on a PIC16F1825. Correct results.

So the problem seems to be for the PIC18F. I did not test this for a PIC12F but at least this narrows the problem down a bit.

I registered the issue under.

Error is calculation using sdword type · Issue #23 · jallib/jalv2compiler (github.com)

Error is calculation using sdword type · Issue #23 · jallib/jalv2compiler Given the following program. The results should in both cases be -100 but the first result shows 3276700 and the second one the correct -100. This seems to be the case for PIC18F types, not PIC16F ... github.com

I am also working on some other stuff so it might take some time before I can have a look at this issue and the other issue you reported (I am not a very good multi-tasker).

Kind regards,

Rob

---

Van: 'Leonardo Hirokazu De Souza Hamada' via jallist <jal...@googlegroups.com>
Verzonden: woensdag 18 augustus 2021 16:29
Aan: jallist <jal...@googlegroups.com>
Onderwerp: Re: [jallist] Long signed integer multiplication issue?

 

To view this discussion on the web visit https://groups.google.com/d/msgid/jallist/c457aa9d-293f-4381-a142-c53d7e5ea274n%40googlegroups.com.

[Rob Hamerling]

Hi Leonardo,

Found out why I have different asm files between with and without -fastmath!
I copied your program, but left out the second part with the good result of the multiply (reason: "why debug good code?"). 
So  in my program there is only a single multiply. And the doc text of -fastpath explains that in that case this single multiply is generated 'inline'. Therefore I get different asm files for cases with and without -fastpath.
I did a check: when I add the second multiply I get idential asm files (apart from the name of the procedure), exactly like with you.
I just didn't realize this code difference would have this effect, sorry for the confusion.

Remains the 'puzzle' why the not-inline routine gives a wrong result.

Regards, Rob.

 

On 18/08/2021 16.29, 'Leonardo Hirokazu De Souza Hamada' via jallist wrote:

So, just to complement, I did a diff against the two generated .asm files, one with -fastmath option and the other one with -no-fastmath option.

The only thing effectively changed was the name of multiplication subroutine, see:

$ diff 18f4620_integer_debug_stripped.asm_fm 18f4620_integer_debug_stripped.asm_no-fm
3c3
< ; command line:  C:\\JallibWorkplace\\co

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[Rob Hamerling]

Hi Leonardo,

Section 3.2 of the compiler docs (jalv2.pdf) contains a warning concerning multiplications: casting might be required!
Following the recommendation and modifying the multiplication into:

    c = sbyte*8(p) * E

gives me a warning that the value of c may be truncated (because c is smaller than sbyte*8), but in this case there is no truncation and the value of c is correct!

The compiler might be a little more user-friendly for these occasions, but at least it is documented!

Regards, Rob.

On 16/08/2021 20.25, 'Leonardo Hirokazu De Souza Hamada' via jallist wrote:

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[Leonardo Hirokazu De Souza Hamada]

Hi Rob,

Thanks for the tips and attention given to this.

Best regards,

Leonardo