From 488512bf4953d856fcb049a05060a450af52fcdc Mon Sep 17 00:00:00 2001 From: Guido van Rossum Date: Mon, 3 Aug 2020 09:04:13 -0700 Subject: [PATCH] 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). --- Objects/bytesobject.c | 8 ++------ 1 file changed, 2 insertions(+), 6 deletions(-) diff --git a/Objects/bytesobject.c b/Objects/bytesobject.c index 3a922d32b16..836a736037b 100644 --- a/Objects/bytesobject.c +++ b/Objects/bytesobject.c @@ -3139,7 +3139,6 @@ static PyObject * striter_next(striterobject *it) { PyBytesObject *seq; - PyObject *item; assert(it != NULL); seq = it->it_seq; @@ -3148,11 +3147,8 @@ striter_next(striterobject *it) assert(PyBytes_Check(seq)); if (it->it_index < PyBytes_GET_SIZE(seq)) { - item = PyLong_FromLong( - (unsigned char)seq->ob_sval[it->it_index]); - if (item != NULL) - ++it->it_index; - return item; + return PyLong_FromLong( + (unsigned char)seq->ob_sval[it->it_index++]); } it->it_seq = NULL;