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A (very) slight speed improvement for iterating over bytes (#21705) 

My mentee @xvxvxvxvxv noticed that iterating over array.array is
slightly faster than iterating over bytes.  Looking at the source I
observed that arrayiter_next() calls `getitem(ao, it->index++)` wheras
striter_next() uses the idiom (paraphrased)

    item = PyLong_FromLong(seq->ob_sval[it->it_index]);
    if (item != NULL)
        ++it->it_next;
    return item;

I'm not 100% sure but I think that the second version has fewer
opportunity for the CPU to overlap the `index++` operation with the
rest of the code (which in both cases involves a call).  So here I am
optimistically incrementing the index -- if the PyLong_FromLong() call
fails, this will leave the iterator pointing at the next byte, but
honestly I doubt that anyone would seriously consider resuming use of
the iterator after that kind of failure (it would have to be a
MemoryError).  And the author of arrayiter_next() made the same
consideration (or never ever gave it a thought :-).

With this, a loop like

    for _ in b: pass

is now slightly *faster* than the same thing over an equivalent array,
rather than slightly *slower* (in both cases a few percent).
This commit is contained in:
Guido van Rossum 2020-08-03 09:04:13 -07:00 committed by GitHub
parent c36dbac588
commit 488512bf49
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1 changed files with 2 additions and 6 deletions
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8
Objects/bytesobject.c
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@ -3139,7 +3139,6 @@ static PyObject *
striter_next(striterobject *it) striter_next(striterobject *it)
{ {
PyBytesObject *seq; PyBytesObject *seq;
PyObject *item;
assert(it != NULL); assert(it != NULL);
seq = it->it_seq; seq = it->it_seq;
@ -3148,11 +3147,8 @@ striter_next(striterobject *it)
assert(PyBytes_Check(seq)); assert(PyBytes_Check(seq));
if (it->it_index < PyBytes_GET_SIZE(seq)) { if (it->it_index < PyBytes_GET_SIZE(seq)) {
item = PyLong_FromLong( return PyLong_FromLong(
(unsigned char)seq->ob_sval[it->it_index]); (unsigned char)seq->ob_sval[it->it_index++]);
if (item != NULL)
++it->it_index;
return item;
} }
it->it_seq = NULL; it->it_seq = NULL;