mirror of https://github.com/python/cpython
bpo-44376 - reduce pow() overhead for small exponents (GH-26662)
Greatly reduce pow() overhead for small exponents.
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Exact integer exponentiation (like ``i**2`` or ``pow(i, 2)``) with a small exponent is much faster, due to reducing overhead in such cases.
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@ -4239,18 +4239,58 @@ long_pow(PyObject *v, PyObject *w, PyObject *x)
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REDUCE(result); \
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} while(0)
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if (Py_SIZE(b) <= FIVEARY_CUTOFF) {
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/* Left-to-right binary exponentiation (HAC Algorithm 14.79) */
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/* http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf */
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for (i = Py_SIZE(b) - 1; i >= 0; --i) {
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digit bi = b->ob_digit[i];
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for (j = (digit)1 << (PyLong_SHIFT-1); j != 0; j >>= 1) {
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MULT(z, z, z);
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if (bi & j)
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i = Py_SIZE(b);
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digit bi = i ? b->ob_digit[i-1] : 0;
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digit bit;
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if (i <= 1 && bi <= 3) {
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/* aim for minimal overhead */
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if (bi >= 2) {
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MULT(a, a, z);
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if (bi == 3) {
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MULT(z, a, z);
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}
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}
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else if (bi == 1) {
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/* Multiplying by 1 serves two purposes: if `a` is of an int
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* subclass, makes the result an int (e.g., pow(False, 1) returns
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* 0 instead of False), and potentially reduces `a` by the modulus.
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*/
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MULT(a, z, z);
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}
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/* else bi is 0, and z==1 is correct */
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}
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else if (i <= FIVEARY_CUTOFF) {
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/* Left-to-right binary exponentiation (HAC Algorithm 14.79) */
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/* http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf */
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/* Find the first significant exponent bit. Search right to left
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* because we're primarily trying to cut overhead for small powers.
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*/
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assert(bi); /* else there is no significant bit */
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Py_INCREF(a);
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Py_DECREF(z);
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z = a;
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for (bit = 2; ; bit <<= 1) {
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if (bit > bi) { /* found the first bit */
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assert((bi & bit) == 0);
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bit >>= 1;
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assert(bi & bit);
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break;
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}
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}
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for (--i, bit >>= 1;;) {
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for (; bit != 0; bit >>= 1) {
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MULT(z, z, z);
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if (bi & bit) {
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MULT(z, a, z);
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}
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}
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if (--i < 0) {
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break;
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}
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bi = b->ob_digit[i];
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bit = (digit)1 << (PyLong_SHIFT-1);
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}
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}
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else {
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/* Left-to-right 5-ary exponentiation (HAC Algorithm 14.82) */
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