Editing BoostC Optimizations








== some notes from the forum still to be formatted ==

The following post yeilded several real and unreal optimizations 
http://forum.sourceboost.com/index.php?showtopic=2433&pid=9574&mode=threaded&start=#entry9574


==== Incrementing ====

subtracting/deincrementing is faster than adding/incrementing

Valid?  believed not to be true

==== Avoid Division ====

not using division saves ~35 bytes

Valid?  division is slow, but we think not by poweres of 2, which are shifts ( if divisor know at compile time )

==== Left vs Right Shift ====

Left Shifting is faster than Right

==== Function call in ISR =====

Calling a function in an ISR takes longer than outside an ISR
Valid?  believed not to be true

==== Function called Only Once =====

-Never use a function for a single operation, you waste time calling and returning

Valid?  believed not to be true

==== Shift for Division =====

Use bit shifting instead of division for speed savings and possible ram savings

(shifting one bit right is equal to dividing by two, etc)

Valid?  believed not to be true, optimizer is smart enough to do this


==== Arrays and Pointers ====

Both arrays and pointers generate a lot of code. Use plain variables instead where possible.

I just saved a lot of bytes by reducing the number of index access on array vars.

a stupid and useless example:

   char var[10];
   
   for( i= 0; i<10;i++){
      if ((var[i]==this) || (var[i]==that)) var[i]=somefunction(var[i]);
      var[i]+=2;
   }


if you need to have access on a particular index several times i'm using an extra dummy var.

   char var[10], dummy;
   
   for( i= 0; i<10;i++){
      dummy= var[i];
      if ((dummy==this) || (dummy==that)) dummy=somefunction(dummy)  {
         var[i]= dummy+=2;
      }
   }

something similar happens when you are working with struct variables. 	




==== ROM vs RAM ====
-Write constants into ROM not RAM 

Valid?  In that you save RAM at the expense of ROM, normally you have more ROM than RAM.

==== Return Values ====

Do not return values from functions that you wont use/do not need.

Valid?  We think so, seems like it must be.


==== ROM vs RAM ====


-Reuse common code or write a common function and call it will save huge amounts of space
( i suspect agressive opt in BoostC may negate this last but i've yet to test)

Valid?  We think so, but even if not it good coding pratice.



i had forgotten about this one but the good old ()?:; (tri-unary?) function conditionally yields 
slightly smaller/faster code on every compiler i've ever used versus using if()else();
The gain varies from 1-3 lines of asm code, which, if in a high use test will amount
to a conciderable savings.

The conditional aspect of this is that the fail function must do something it cannot be
left empty or there is no value in using it and will actually result in longer code.

CODE
(test)?(result=1):(result=0);

VS
CODE
if(test)
{
   result=1;
}
else
{
   result=0;
}



i had forgotten about this one but the good old ()?:; (tri-unary?) function conditionally yields slightly smaller/faster code...

I tried that one out and there's no gain on SourceBoost C (for PIC16) on any optimization level.

For code like that, I would expect this:
CODE
result = (test ? 1 : 0)
to do better, but it doesn't, either... 	

	  
 

		QUOTE (edt @ Dec 29 2006, 01:23 PM)
QUOTE (emte @ Dec 27 2006, 05:07 PM)
i had forgotten about this one but the good old ()?:; (tri-unary?) function conditionally yields slightly smaller/faster code...

I tried that one out and there's no gain on SourceBoost C (for PIC16) on any optimization level.

For code like that, I would expect this:
CODE
result = (test ? 1 : 0)
to do better, but it doesn't, either...





Odd, altho my tests were simply:

CODE
    (testBit)?(_ONE = 1):(_ONE = 0);
0039  08A1      MOVF main_1_testBit, F
003A  1903      BTFSC STATUS,Z
003B  283E      GOTO    label268438744
003C  1486      BSF gbl__ONE,1
003D  283F      GOTO    label268438746
003E        label268438744
003E  1086      BCF gbl__ONE,1
003F        label268438746

        if(testBit)
003F  08A1      MOVF main_1_testBit, F
0040  1903      BTFSC STATUS,Z
0041  2844      GOTO    label268438747
0044        label268438747

        {
            _ONE = 1;
0042  1486      BSF gbl__ONE,1

        }
        else
0043  2837      GOTO    label268438739

        {
            _ONE = 0;
0044  1086      BCF gbl__ONE,1

        }


with the asm disassembly, but it looks like i am wrong about the new version of
the compiler. i am glad to see that these are now the same as they are identical
chunks of code.

If i recall correctly, the reason they were different is that the if-else used to use two
bit tests to determine logic, which is how you would do it if you had more ifelse tests.

Now i am back to using ?:; just for a cleaner look to code i guess. Thanks for testing
that with the new version, it did not even occur to me that it may have changed 


----------------

Hmm it looks like the if-else might actually be one line shorter now ... altho it is
just a lable line. 


result= test&1;


Yes that would possibly work in this case as it is an extremly simple example.
But on the otherhand if you were writting code for an indicator light, say to
notify of an AD conversion, you could not use the return value as your test bit.
ie. if test = 4 that mask would fail but (test)?(result=1):(result=0); would work
since all non-zero/positives would set result.

i find this handy if you have more than one AD module on your device instead 
of using the flags as you can now build an AD FIFO cyclical stack or non-blocking 
time sensitive functions. (all that slow eeprom write stuff while your doing another 
conversion)
... hmm maybe i should add that example to handy functions ...



==== Still More we are working on ====


Software stack is slower than hardware ( but deeper )

Valid? we think so.

Inline function does not really use call and return so is faster, but if called multiple time may take more space ( but for short funtions the call return... can take more space than the function itself.  Would be nice if someone did measurements.

Valid? we think so.


Local values vs global variables.