[PATCH] Speed up manifest parsing by 4x

[Matt Mackall]

On Wed, 2008-03-26 at 11:18 +0100, Adrian Buehlmann wrote:
> Some questions/comments below by an unwary poor C++ Windows programmer
> who would like to learn from the *NIX gurus.
> 
> On 26.03.2008 00:45, Bryan O'Sullivan wrote:
> > manifest: improve parsing performance by 4x via a C extension
> 
> great!
> 
> > +static PyObject *parse_manifest(PyObject *self, PyObject *args)
> > +{
> > +	PyObject *binascii, *unhexlify;
> > +	PyObject *mfdict, *fdict;
> > +	char *str, *cur, *start, *zero;
> 
> I admit I've never written a single line of C code for UNIX, but
> I've learned the hard way that random pointer values found on
> stack, shooting into random memory places, can be damn hard to track down.
> So, I initialize stack pointers to 0 when defining them.
> Just in case they are used in error before initialized. A zero
> pointer deref is of magnitudes easier to debug.
> The speedup gained when not initializing is rarely worth the potential
> trouble.

Depends on the function. Ideally functions are made simple enough that
such problems are obvious and/or structually impossible.

>  The alternative is to initialize when defining, which has no speed
> penalty.

Speaking as someone who spends a lot of time doing optimization work in
the Linux kernel: definitely not true. Initializers are code that gets
run at every function invocation. It takes both cycles and, perhaps more
importantly, cache footprint. Taking an extra cacheline hit is very
expensive.

-- 
Mathematics is the supreme nostalgia of our time.