diff --git a/Misc/NEWS.d/next/Core and Builtins/2021-06-11-17-37-15.bpo-44376.zhM1UW.rst b/Misc/NEWS.d/next/Core and Builtins/2021-06-11-17-37-15.bpo-44376.zhM1UW.rst new file mode 100644 index 00000000000000..f854d56b3c8419 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2021-06-11-17-37-15.bpo-44376.zhM1UW.rst @@ -0,0 +1 @@ +Exact integer exponentiation (like ``i**2`` or ``pow(i, 2)``) with a small exponent is much faster, due to reducing overhead in such cases. \ No newline at end of file diff --git a/Objects/longobject.c b/Objects/longobject.c index e1c1191e648dae..5e29e9a7257093 100644 --- a/Objects/longobject.c +++ b/Objects/longobject.c @@ -4239,17 +4239,57 @@ long_pow(PyObject *v, PyObject *w, PyObject *x) REDUCE(result); \ } while(0) - if (Py_SIZE(b) <= FIVEARY_CUTOFF) { + i = Py_SIZE(b); + digit bi = i ? b->ob_digit[i-1] : 0; + digit bit; + if (i <= 1 && bi <= 3) { + /* aim for minimal overhead */ + if (bi >= 2) { + MULT(a, a, z); + if (bi == 3) { + MULT(z, a, z); + } + } + else if (bi == 1) { + /* Multiplying by 1 serves two purposes: if `a` is of an int + * subclass, makes the result an int (e.g., pow(False, 1) returns + * 0 instead of False), and potentially reduces `a` by the modulus. + */ + MULT(a, z, z); + } + /* else bi is 0, and z==1 is correct */ + } + else if (i <= FIVEARY_CUTOFF) { /* Left-to-right binary exponentiation (HAC Algorithm 14.79) */ /* http://www.cacr.math.uwaterloo.ca/hac/about/chap14.pdf */ - for (i = Py_SIZE(b) - 1; i >= 0; --i) { - digit bi = b->ob_digit[i]; - for (j = (digit)1 << (PyLong_SHIFT-1); j != 0; j >>= 1) { + /* Find the first significant exponent bit. Search right to left + * because we're primarily trying to cut overhead for small powers. + */ + assert(bi); /* else there is no significant bit */ + Py_INCREF(a); + Py_DECREF(z); + z = a; + for (bit = 2; ; bit <<= 1) { + if (bit > bi) { /* found the first bit */ + assert((bi & bit) == 0); + bit >>= 1; + assert(bi & bit); + break; + } + } + for (--i, bit >>= 1;;) { + for (; bit != 0; bit >>= 1) { MULT(z, z, z); - if (bi & j) + if (bi & bit) { MULT(z, a, z); + } + } + if (--i < 0) { + break; } + bi = b->ob_digit[i]; + bit = (digit)1 << (PyLong_SHIFT-1); } } else {