Re: [Pharo-project] Two complement smallInteger question

Dec. 23, 2011


      I read somewhere that in C short int range value are 	-32767 to +32767

http://www.ericgiguere.com/articles/ansi-c-summary.html

And I was wondering why it does not follow the  $-1 * 2^{N-1}\ to\ 2^{N-1}-1$ range encoding


Stef


On Dec 23, 2011, at 11:35 AM, StÃ©phane Ducasse wrote:
...
Hi
I'm doing a pass on the two complement chapter and small integer implementation and I wanted to check the SmallInteger>>primitiveAdd see below:
stackIntegerValue extracts from a smalltalk integer the 31 encoding number bit. I imagine that this is correct.
sqInt
stackIntegerValue(sqInt offset) {
DECL_MAYBE_SQ_GLOBAL_STRUCT
   sqInt integerPointer;
  integerPointer = longAt(GIV(stackPointer) + (offset * BytesPerWord));
  /* begin checkedIntegerValueOf: */
  if ((integerPointer & 1)) {
      return (integerPointer >> 1);
  }
  else {
      /* begin primitiveFail */
      if (GIV(primFailCode) == 0) {
      	GIV(primFailCode) = 1;
      }
      return 0;
  }
}
Now I have three questions:
  - is two complement used to manage small integers at VM level?
  I saw digitAdd on Integer and it deals with large integer.
- integerResult = (stackIntegerValue(1)) + (stackIntegerValue(0)); 
  performs the real add. I imagine that it comes from plain C. Now does anybody has an idea
  where I could find the definition of such + procedure?
  In ANSI C
- from the books, performing 54 - 5 is equivalent to adding the two complement 
  i.e. 54 + 5' (two complement) 
  this work well for 54 - 5 correct results and we can follow it looking at binary operations:
(2r110110 - 2r101) bitString	
      --> '0000000000000000000000000110001'
  (2r110110 bitString) 	
      --> '0000000000000000000000000110110'
  2r101 bitString
      --> '0000000000000000000000000000101'		
  2r101 negated bitString
      --> '1111111111111111111111111111011'
Now doing 5 - 15 does not (or I miss something ilke always subtracting the smaller one and taking the two complement of the smallerâ¦)
  so does anybody has the answer on how to perform 5 - 15 ?
static void
primitiveAdd(void) {
DECL_MAYBE_SQ_GLOBAL_STRUCT
 sqInt integerResult;
 char *sp;
/* begin pop2AndPushIntegerIfOK: */
  integerResult = (stackIntegerValue(1)) + (stackIntegerValue(0));
  if (GIV(primFailCode) == 0) {
      if ((integerResult ^ (integerResult << 1)) >= 0) {
      	/* begin pop:thenPush: */
      	longAtput(sp = GIV(stackPointer) + ((2 - 1) * BytesPerWord), ((integerResult << 1) | 1));
      	GIV(stackPointer) = sp;
      }
      else {
      	/* begin success: */
      	if (!(0)) {
      		if (GIV(primFailCode) == 0) {
/* Don't overwrite an error code that has already been set. */
GIV(primFailCode) = 1;
      		}
      	}
      }
  }
}
Stef