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bpo-46020: Optimize long_pow for the common case (GH-30555) 

This cuts a bit of overhead by not initializing the table of small
odd powers unless it's needed for a large exponent.
This commit is contained in:
Tim Peters 2022-01-12 12:55:02 -06:00 committed by GitHub
parent e2a9c8ef09
commit fc05e6bfce
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1 changed files with 13 additions and 6 deletions
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19
Objects/longobject.c
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@ -4215,8 +4215,13 @@ long_pow(PyObject *v, PyObject *w, PyObject *x)
/* k-ary values. If the exponent is large enough, table is
* precomputed so that table[i] == a**(2*i+1) % c for i in
* range(EXP_TABLE_LEN).
* Note: this is uninitialzed stack trash: don't pay to set it to known
* values unless it's needed. Instead ensure that num_table_entries is
* set to the number of entries actually filled whenever a branch to the
* Error or Done labels is possible.
*/
PyLongObject *table[EXP_TABLE_LEN] = {0};
PyLongObject *table[EXP_TABLE_LEN];
Py_ssize_t num_table_entries = 0;
/* a, b, c = v, w, x */
CHECK_BINOP(v, w);
@ -4408,10 +4413,14 @@ long_pow(PyObject *v, PyObject *w, PyObject *x)
*/
Py_INCREF(a);
table[0] = a;
num_table_entries = 1;
MULT(a, a, a2);
/* table[i] == a**(2*i + 1) % c */
for (i = 1; i < EXP_TABLE_LEN; ++i)
for (i = 1; i < EXP_TABLE_LEN; ++i) {
table[i] = NULL; /* must set to known value for MULT */
MULT(table[i-1], a2, table[i]);
++num_table_entries; /* incremented iff MULT succeeded */
}
Py_CLEAR(a2);
/* Repeatedly extract the next (no more than) EXP_WINDOW_SIZE bits
@ -4472,10 +4481,8 @@ long_pow(PyObject *v, PyObject *w, PyObject *x)
Py_CLEAR(z);
/* fall through */
Done:
if (Py_SIZE(b) > HUGE_EXP_CUTOFF / PyLong_SHIFT) {
for (i = 0; i < EXP_TABLE_LEN; ++i)
Py_XDECREF(table[i]);
}
for (i = 0; i < num_table_entries; ++i)
Py_DECREF(table[i]);
Py_DECREF(a);
Py_DECREF(b);
Py_XDECREF(c);