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bpo-46504: faster code for trial quotient in x_divrem() #30856

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Merged
merged 2 commits into from
Jan 25, 2022
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bpo-46504: faster code for trial quotient in x_divrem() #30856

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a4eea39
bpo-46504: faster code for trial quotient in x_divrem()
tim-one Jan 24, 2022
6e5e4d2
Fix typo in comment.
tim-one Jan 24, 2022
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9 changes: 8 additions & 1 deletion Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -2767,8 +2767,15 @@ x_divrem(PyLongObject *v1, PyLongObject *w1, PyLongObject **prem)
vtop = vk[size_w];
assert(vtop <= wm1);
vv = ((twodigits)vtop << PyLong_SHIFT) | vk[size_w-1];
/* The code used to compute the remainder via
* r = (digit)(vv - (twodigits)wm1 * q);
* and compilers generally generated code to do the * and -.
* But modern processors generally compute q and r with a single
* instruction, and modern optimizing compilers exploit that if we
* _don't_ try to optimize it.
*/
q = (digit)(vv / wm1);
r = (digit)(vv - (twodigits)wm1 * q); /* r = vv % wm1 */
r = (digit)(vv % wm1);
while ((twodigits)wm2 * q > (((twodigits)r << PyLong_SHIFT)
| vk[size_w-2])) {
--q;
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