notably improve performance of string escape function
String escaping is a surprisingly performance intense operation.
I spent many hours in the profiler, and the root problem seems
to be that there is no easy way to detect the character classes
that need to be escaped, where the root cause is that these
characters are spread all over the ascii table. I tried
several approaches, including call tables, re-structuring
the case condition, different types of if conditions and
reordering the if conditions. What worked out best is this:
The regular case is that a character must not be escaped. So
we want to process that as fast as possible. In order to
detect this as quickly as possible, we have a lookup table
that tells us if a char needs escaping ("needsEscape", below).
This table has a spot for each ascii code. Note that it uses
chars, because anything larger causes worse cache operation
and anything smaller requires bit indexing and masking
operations, which are also comparatively costly. So plain
chars work best. What we then do is a single lookup into the
table to detect if we need to escape a character. If we need,
we go into the depth of actual escape detection. But if we
do NOT need to escape, we just quickly advance the index
and are done with that char. Note that it may look like the
extra table lookup costs performance, but right the contrary
is the case. We get amore than 30% performance increase due
to it (compared to the latest version of the code that did not
do the lookups.
Rainer Gerhards
10 years ago