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Optomisation levels

 
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Morgano
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Optomisation levels
PostPosted: Mon Aug 13, 2007 4:50 am     Reply with quote
Hi

We use the compiler PCWH 4.049 on a PIC18F66J15.

Is there any information on what is optimised in different optimisation levels?

Can we select prority for what to optimise for? (speed, codesize, RAM size)

It seems setting optimisation levels 10 or 11 give same result as level 0, without warnings? This is a PIC18. Maybe it is because the compiler still can not utilise the enhanced instruction set?

I am reading the generated assembler code, this is a part of what is generated in optimisation level 0, 10 and 11:

Code:
....................         printf(Maxputc, "Chain_IsToBeTrusted: %u,
potShadow: %u, keyShadow: %u, PHDTemp: %u\n\r", Chain_IsToBeTrusted,
potShadow, keysShadow, PHDTemp);
004400:  MOVLB  1
004402:  CLRF   x8C
004404:  MOVLB  0
004406:  MOVLB  1
004408:  MOVF   x8C,W


Note the really weird bank switching, the last two MOVLB are just plain unnecessayr! I would not call this unoptimised code, i would call it bloatisized code...

Using optimisation level 9 there seem not to be unnecessary bank swithing.

The header of the list file:
Code:
CCS PCH C Compiler, Version 4.049, 40082               13-aug-07 11:29

               Filename: main.lst

               ROM used: 26294 bytes (1%)
                         Largest free fragment is 65536
               RAM used: 394 (10%) at main() level
                         488 (13%) worst case
               Stack:    9 worst case (6 in main + 3 for interrupts)


Command line:
Code:
+DF +LN +T -A +M +Z +Y11 +EA I="C:\Program\PICC\Drivers;
C:\Program\PICC\Devices; c:\mplab\project_libs"
RossJ



Joined: 25 Aug 2004
Posts: 66

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PostPosted: Mon Aug 27, 2007 11:31 pm     Reply with quote
Hello Morano,

I don't know if there is a list of optimisations, but I have found a definite improvement in ROM usage with optimisation levels above 9.

One thing I have found which might be tricking you is that the +Y command line option only accepts one digit (i.e. +Y9 is level 9, whereas +Y10 is level 1!). You can avoid this problem by using the #opt preprocessor directive.

To be sure which level is actually being used, check the 'Opt Level:' at the end of the .SYM file.

/Ross.
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