Re: [xslt] likely/unlikely hinting for libxml/libxslt

[Stefan Behnel <stefan_ml behnel de>]

Stefan Kost, 10.02.2010 09:43:
> in the quest of figuring our what canb be done to make gtk-doc less slow
> (where the only slow part is the docbook xslt processing) I was running
> it under oprofile and studying the report. Below is the profile
> (probably nothing new in there). Attached is also the callgraph as an
> image (not sure if that will get through to the list).

Note that KCacheGrind claims to have support for oprofile. Should make such
a graph a lot more accessible.


> Some questions:
> 
> * I'd suggest to make use of __builtin_expect() in libxml. glib does:
> 
> #if defined(__GNUC__) && (__GNUC__ > 2) && defined(__OPTIMIZE__)
> #define G_LIKELY(expr) (__builtin_expect ((expr), 1))
> #define G_UNLIKELY(expr) (__builtin_expect ((expr), 0))
> #else
> #define G_LIKELY(expr) (expr)
> #define G_UNLIKELY(expr) (expr)
> #endif

We actually do this in Cython:

#ifdef __GNUC__
/* Test for GCC > 2.95 */
#if __GNUC__ > 2 || \
              (__GNUC__ == 2 && (__GNUC_MINOR__ > 95))
#define likely(x)   __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 0)
#else /* __GNUC__ > 2 ... */
#define likely(x)   (x)
#define unlikely(x) (x)
#endif /* __GNUC__ > 2 ... */
#else /* __GNUC__ */
#define likely(x)   (x)
#define unlikely(x) (x)
#endif /* __GNUC__ */


> Could that be added to libxml as e.g. XML_{UN,}LIKELY?

+1

> It could be used e.g. in xpathInternals.h, e.g. for
> 
> #define CHECK_ERROR                            \
>     if (XML_UNLIKELY (ctxt->error != XPATH_EXPRESSION_OK)) return
> 
> With the attached (hackish patch) I got the % for xmlXPathCompOpEval
> from 22.69% down to 17.70%.
> (this was just on, but one long iteration).

Sounds like a case to me.


> If there is no one against such an approach I can make a proper patch
> and submit it to bugzilla.

Please do. I can't guarantee that it will go in, but once it's there, I
can't really see a reason why it shouldn't.

Stefan