[Pharo-project] Two complement smallInteger question

[Stéphane Ducasse]

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 smaller…)
        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