Pointer Autodecrement

Merlin

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Subject:

Pointer Autodecrement

· Posted: 26.05.2011 - 18:23 · #1

The opertors ^++ and ++ are useful in speed sensitive applications. It would be good to have the decrement counterpart ^-- and -- too.

Regards

Merlin.

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Subject:

Re: Pointer Autodecrement

· Posted: 26.05.2011 - 18:37 · #2

Hello Merlin,

nice to have. But there is a small problem.
The auto-increment of pointer accesses is build-in with some AVR opcodes. It uses post-increment.
The auto-decrement feature is somewhat more difficult to handle because it uses pre-decrement.
If this is implemented with AVRco then the user must do some precautions:

Code

var
  ar      : array[0..10] of byte;
  p       : pointer;
  bb     : byte;
...
  p:= @ar + size(ar); // +1 ???
  bb:= p^--;

rolf

Merlin

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Age: 24
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Subject:

Re: Pointer Autodecrement

· Posted: 27.05.2011 - 12:46 · #3

Hi Rolf.

A good point. The intention (at pascal level) was post-decrement, not pre-decrement. So this is what I would want to do (and nearly can). (I know it is buggy - it is just to give an idea...)

Code

var
  ar : array[1..10] of byte;
  pa, pb : pointer to byte;
  br : array[1..9] of byte;
  carry : boolean;

...
  pa := @ar[10];
  pb := @br[9];
  carry := FALSE;
  for i := 1 to 9
    if carry then
      dec( pa^);
   endif;
   carry := pb^ > pa^;
   pa^-- := pa^ - pb^--;  // not pa^ := pa^-- - pb^--; !!!
  endfor;

But yes, at assembler level efficiency will be reduced because of pre-decrement, so maybe the produced code would be no more efficient than I already have...

Code

var
  ar : array[1..10] of byte;
  pa, pb : pointer to byte;
  br : array[1..9] of byte;
  carry : boolean;

...
  pa := @ar[10];
  pb := @br[9];
  carry := FALSE;
  for i := 1 to 9
    if carry then
      dec( pa^);
    endif;
    carry := pb^ > pa^;
    dec( pa^, pb^ );
    dec( pa );
    dec( pb );
  endfor;

Thanks for insight.

Regards

Merlin.

Pointer Autodecrement

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