[Python-Dev] Comparison speed (original) (raw)

Tim Peters tim.one@home.com
Sun, 20 May 2001 05:31:25 -0400

[M0artin v. Loewis]

... If I set tprichcompare of strings to 0, I get past this code, and do

c = (*f)(v, w); if (PyErrOccurred())

Note that the usual way to write this is

     if (c < 0 && PyErr_Occurred())

More work for my artificial "ab" < "cd" case but a net win in real life (when c >= 0, it's an internal error if PyErr_Occurred() were to return true; alas, when c < 0 there's no way in the cmp protocol to use c's value alone to distinguish between "less than" and "error").

return NULL; return convert3waytoobject(op, c);

Here, I get 3 function calls: f is stringcompare, then PyErrOccurred, finally convert3waytoobject, which converts {-1,0,1} x Op -> {PyTrue, PyFalse}.

Unfortunately, it's 4 calls, as PyErr_Occurred() isn't a leaf.

Indeed, when I inline convert3waytoobject, I get the same speed in both cases (with the remaining differences attributed to measurement and gcc doing register usage differently in both functions).

OK, understood, and thanks for following up!

I'd still be in favour of giving strings a richcompare, since it allows to optimize what I think is the single most frequent case: PyEQ on strings.

In the absence of significant sorting, I agreed Py_EQ is most frequent.

With a control flow like

if (a->obsize != b->obsize) goto False; if (a->obsize == 0) goto True; if (a->obsval[0] != b->obsval[0]) goto False; if(memcmp(a->obsval, b->obsval, a->obsize)) goto False; else goto True; we can reduce the number of function calls

Suggest collapsing the third into the first:

    if (a->ob_size != b->ob_size
            || a->ob_sval[0] != b->ob_sval[0])
                goto False;

There's no danger of over-indexing when ob_size==0, because it doesn't include the trailing null byte Python always sticks at the end of string objects; and the first-byte check is much more likely to pay off than the zero-length check (comparison to a null string? gotta be rare as clear conclusions ), and better to test for the more common case first.