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Problems with optimization

 
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slavka012



Joined: 10 Feb 2012
Posts: 28

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Problems with optimization
PostPosted: Tue Apr 09, 2013 9:25 pm     Reply with quote

Running CCS compiler

CCS PCM C Compiler, Version 4.129, 59140
Executing: "C:\Program Files\PICC\Ccsc.exe" +FM "dental-main.c" +DF +LY +T +A +M -Z +Y=7 +EA #__16F1946=TRUE

----------------------------------------------------
.c:
Code:

M_INLINE void Encrypt(unsigned char source[], unsigned char number) {

   // this is supposed to be a const, but error in CCS compiler
   // does not allow it hence we get a warning
    unsigned char *pass_ptr = Passphrase + PassphraseIndex;
    unsigned int8 TmpIndex = PassphraseIndex;
    int i = 0;
    for (i = 0; i < number; i++ ) {
        source[i] = source[i] ^ (Passphrase[TmpIndex]);
        pass_ptr++;
        TmpIndex++;
        if (TmpIndex >= sizeof(Passphrase)) {
            TmpIndex = 0;
            pass_ptr = Passphrase;
        };
    };

 }
M_SEPARATE void AdvancePassphrase() {
    PassphraseIndex = (PassphraseIndex + PassphraseInc);
    PassphraseIndex = PassphraseIndex % sizeof(Passphrase);
}



.lst file:
024 PassphraseIndex

.asm:
Code:
34:                M_SEPARATE void AdvancePassphrase() {
35:                    PassphraseIndex = (PassphraseIndex + PassphraseInc);
  0707    300D     MOVLW 0xd
  0708    07A4     ADDWF 0x24, F
36:                    PassphraseIndex = PassphraseIndex % sizeof(Passphrase);
  0709    13A4     BCF 0x24, 0x7
37:                }
  070A    0008     RETURN

The problem here is that compiler does not set the BSR inside the AdvancePassphrase function.

It is sort of set before the call to AdvancePassphrase:
Code:
  1135    0020     MOVLB 0
140:                   AdvancePassphrase();
  1189    3180     MOVLP 0
  118A    2707     CALL AdvancePassphrase


However the "movlb 0" is never executed. I set the PC to the preceding function:
Code:
 
    Encrypt(SerialBuff+1, REQ_ADC_RESPONSE_SIZE);
    AdvancePassphrase();


And execution never goes to movlb. The function is executed with BSR=0x5, and does not really update PassphraseIndex.

That's pretty horrible. I spend whole day figuring this one out - the code was working before. Today I set optimization to a higher level because the code did not fit into the memory anymore.


Last edited by slavka012 on Tue Apr 09, 2013 9:55 pm; edited 1 time in total
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Tue Apr 09, 2013 9:40 pm     Reply with quote

Quote:
+Y=7

This means you are running #opt 7. The default opt level is 9.

I never change the #opt level. To do so, means you're running an
untested version of the compiler. 99.9% of the people on this forum
are using the default level, I would bet. So any bugs we find are
for the #opt 9 default level. No one really knows what the compiler
is doing for other #opt levels. I don't think they are tested by anyone.
Don't use them. And don't ask why #opt is included, lol.
slavka012



Joined: 10 Feb 2012
Posts: 28

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PostPosted: Tue Apr 09, 2013 10:00 pm     Reply with quote

Hm... You might be right. I checked at #opt level 9, it works correctly. It works at level 6 and below as well.

I can't recall why I lowered it in the first place.
Ttelmah



Joined: 11 Mar 2010
Posts: 19540

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PostPosted: Wed Apr 10, 2013 12:48 am     Reply with quote

I think 'black art' at opt=7. Smile
I think PCM programmer is 'spot on'. 99.9% of code will be at default, with the occasional 'test' at opt=0, if you think you have an optimisation problem.
Some years ago, there were some memory management problems which were fixed by dropping a couple of levels.

Best Wishes
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