[Pharo-project] Two complement smallInteger question

Dec. 23, 2011

      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

[Pharo-project] Two complement smallInteger question

StÃ©phane Ducasse