193804 - eliminate use of function pointers in nsRuleNode

Status: RESOLVED DUPLICATE of bug 210550

(Core :: CSS Parsing and Computation, defect)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

There are two places in nsRuleNode where we use function pointers but could
probably use a switch statement or something similar.  We originally started
using function pointers because it was shown to be faster.  However, a switch
(perhaps in order by style struct ID, if the old one wasn't? -- which we'd
automatically get by using nsStyleStructList.h) or some sort of hand-coded
binary search might be faster.  See bug 87229 for the previous work.

Comment 1

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

(sfraser suggested that the access to the global data, where the function
pointers are, is particularly bad on Mach-O.)

Comment 2

[Simon Fraser [no longer active]]

Actually, my pageload tests showed that changing it from using a static table to
using a switch had negligible impact on performance. A more isolated test would
be needed to show up differences more accurately.

There may be other areas in which frequent accesses of global data hurt
performance on Mach-O: for instance, comparisons against the static layout
atoms. Sampling a run of the pageload tests shows __cxa_get_globals as the most
common function at the top of the stack (after functions that can context switch).

Comment 3

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

Hey, I filed the same bug twice.

*** This bug has been marked as a duplicate of 210550 ***