Why doesn't 1 + 1 use BINARY_ADD?

Question

I do this:

>>> dis.dis(lambda: 1 + 1)
0 LOAD_CONST        2 (2)
3 RETURN_VALUE

I was expecting a BINARY_ADD opcode to perform the addition. How was the sum computed?

Answer 1

This is the work of Python's peephole optimizer. It evaluates simple operations with only constants during the compile time itself and stores the result as a constant in the generated bytecode.

Quoting from the Python 2.7.9 Source code,

            /* Fold binary ops on constants.
               LOAD_CONST c1 LOAD_CONST c2 BINOP -->  LOAD_CONST binop(c1,c2) */
        case BINARY_POWER:
        case BINARY_MULTIPLY:
        case BINARY_TRUE_DIVIDE:
        case BINARY_FLOOR_DIVIDE:
        case BINARY_MODULO:
        case BINARY_ADD:
        case BINARY_SUBTRACT:
        case BINARY_SUBSCR:
        case BINARY_LSHIFT:
        case BINARY_RSHIFT:
        case BINARY_AND:
        case BINARY_XOR:
        case BINARY_OR:
            if (lastlc >= 2 &&
                ISBASICBLOCK(blocks, i-6, 7) &&
                fold_binops_on_constants(&codestr[i-6], consts)) {
                i -= 2;
                assert(codestr[i] == LOAD_CONST);
                cumlc = 1;
            }
            break;

Basically, it looks for instructions like this

LOAD_CONST c1
LOAD_CONST c2
BINARY_OPERATION

and evaluates that and replaces those instructions with the result and a LOAD_CONST instruction. Quoting the comment in the fold_binops_on_constants function,

/* Replace LOAD_CONST c1. LOAD_CONST c2 BINOP
   with    LOAD_CONST binop(c1,c2)
   The consts table must still be in list form so that the
   new constant can be appended.
   Called with codestr pointing to the first LOAD_CONST.
   Abandons the transformation if the folding fails (i.e.  1+'a').
   If the new constant is a sequence, only folds when the size
   is below a threshold value.  That keeps pyc files from
   becoming large in the presence of code like:  (None,)*1000.
*/

The actual evaluation of this particular code happens in this block,

    case BINARY_ADD:
        newconst = PyNumber_Add(v, w);
        break;