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bpo-46055: Streamline inner loop for right shifts #30243

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merged 6 commits into from
Dec 27, 2021
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bpo-46055: Streamline inner loop for right shifts #30243

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056495d
bpo-46055: Streamline inner loop for right shifts
mdickinson Dec 23, 2021
d0f1958
Move declarations to top of function; rename high to lobits
mdickinson Dec 23, 2021
f92f3f8
Now even faster!
mdickinson Dec 23, 2021
76e9a48
Remove unnecessary parentheses
mdickinson Dec 23, 2021
76dc476
Cosmetic changes to align the inner loop with that of long_lshift1
mdickinson Dec 23, 2021
08bb0bb
Merge remote-tracking branch 'upstream/main' into faster-rshift
mdickinson Dec 27, 2021
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15 changes: 8 additions & 7 deletions Objects/longobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4491,7 +4491,7 @@ long_rshift1(PyLongObject *a, Py_ssize_t wordshift, digit remshift)
{
PyLongObject *z = NULL;
Py_ssize_t newsize, hishift, i, j;
digit lomask, himask;
twodigits accum;

if (Py_SIZE(a) < 0) {
/* Right shifting negative numbers is harder */
Expand All @@ -4511,16 +4511,17 @@ long_rshift1(PyLongObject *a, Py_ssize_t wordshift, digit remshift)
if (newsize <= 0)
return PyLong_FromLong(0);
hishift = PyLong_SHIFT - remshift;
lomask = ((digit)1 << hishift) - 1;
himask = PyLong_MASK ^ lomask;
z = _PyLong_New(newsize);
if (z == NULL)
return NULL;
for (i = 0, j = wordshift; i < newsize; i++, j++) {
z->ob_digit[i] = (a->ob_digit[j] >> remshift) & lomask;
if (i+1 < newsize)
z->ob_digit[i] |= (a->ob_digit[j+1] << hishift) & himask;
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@mdickinson mdickinson Dec 23, 2021 •
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The new code also fixes a really subtle portability issue in this code. Here the result of a->ob_digit[j+1] * 2**hishift may not be representable in the target type. Normally that wouldn't matter, because a->ob_digit[j+1] has type digit, which is an unsigned type, so the C standards tell us that any out-of-range value wraps in the normal way. But integer promotions could result in the left-hand operand to the shift actually being of type int (a signed type), and then an out-of-range shift result gives undefined behaviour according to the standard (C99 §6.5.7p4). We don't run into this in practice because under any likely combination of integer type bit widths (e.g., 16-bit digit, 32-bit int), if digit is small enough to be promoted to int, then int is likely big enough to hold the shift result. But the C standard does allow potentially problematic bit widths (e.g., digit could be 16 bits and int 24 bits).

Not a real issue, since it's unlikely we'd ever meet this in practice, but it's nice not to have to worry about it. With the new code, the result of the shift is guaranteed to be representable in the target type (that type being either twodigits, or something larger in the case that there are integer promotions going on).

j = wordshift;
accum = a->ob_digit[j++] >> remshift;
for (i = 0; j < Py_SIZE(a); i++, j++) {
accum |= (twodigits)a->ob_digit[j] << hishift;
z->ob_digit[i] = (digit)(accum & PyLong_MASK);
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Would not be better to operate on a single digit? (digit)accum & PyLong_MASK

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Maybe. I find this code clearer as a statement of intent: we're only changing the value once, not twice (all other things being equal, I like my casts not to change values).

I'll run some timings and look at the generated code. If the cast-first variant is faster, I'll change this.

accum >>= PyLong_SHIFT;
}
z->ob_digit[i] = (digit)accum;
z = maybe_small_long(long_normalize(z));
}
return (PyObject *)z;
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