[Python-Dev] PATCH: Fast globals/builtins lookups for 2.6

[Neil Toronto]

Guido van Rossum wrote:
> Hm.
> 
> On my Linux box, in the trunk:
> 
> Before the patch:
> Pystone(1.1) time for 50000 passes = 1.16
> This machine benchmarks at 43103.4 pystones/second
> 
> After the patch:
> Pystone(1.1) time for 50000 passes = 1.14
> This machine benchmarks at 43859.6 pystones/second
> 
> That's only about 1.75% faster. But pystone is a lousy benchmark.

I'm not aware of any benchmark that isn't. :)

Can you humor me and change the PY_LONG_LONG to Py_ssize_t in both 
PyDictObject and PyFastGlobalsObject and see if that helps? It does on 
one of my test machines.

Speaking of which, here's a question for everybody. I was wondering 
whether 64 bits is necessary. It takes an hour of concerted effort - 
nothing but "module.d = 1; del module.d" for an hour straight - to 
overflow a 32-bit version number. Is anybody going to actually get close 
to doing that in a global namespace?

I don't think a malicious user could exploit it. The most they could do 
is segfault by doing exactly 2**32 entry-invalidating operations and 
then one get or set. They've got better things to do if they're running 
code on your machine.

FWIW - and I wouldn't bother with this if I weren't mucking about with 
dict internals - with a 32-bit version number, I've verified that gcc 
emits only one extra instruction in dict functions that increment it. 
It's two for a 64-bit number. The version test in LOAD_GLOBAL does take 
a bit more time with 64 bits, though.

Neil