Vim 7 performance notes

[Alexei Alexandrov]

Hi Bram et al.,

I'm doing some performance investigations of Vim code trying to understand 
whether there are any possibilities to improve it.
Currently I've made the following observations (all investigations are done on 
Windows):

Redundant work is done during regexp operations in syntax highlighting. On some 
files it is very noticable. The stack of the hotspot is ... > syn_current_attr 
> syn_regexec > vim_regexec_multi > vim_regexec_both > regtry > regmatch > 
ga_grow > alloc_clear > memset. So alloc_clear spends quite a few clockticks in 
lalloc() and memset().
The reason for this is pessimistically big grow size for regset growing array:

    ga_init2(&regstack, 1, 10000);

This is not very good: many regexp operations don't go deep - non-match is 
detected very quickly. But even one element on the stack will lead to 
allocating at least 10000 bytes (which should be fast with good CRT memory 
allocator) and (worse) initializing these 10000 bytes with zeros (won't be that 
fast).

One possible solution would be to keep regstack alive across calls to 
vim_regexec_both, but I'm not sure if it's can be done safely. What I did was 
replacing the grow size with smaller number and making the grow size for 
growing arrays dynamic with increase of 25%:

--- regexp.c (revision 136)
+++ regexp.c (working copy)
@@ -3350,7 +3350,7 @@
     /* Init the regstack empty.  Use an item size of 1 byte, since we push
      * different things onto it.  Use a large grow size to avoid reallocating
      * it too often. */
-    ga_init2(&regstack, 1, 10000);
+    ga_init2(&regstack, 1, 64);

     /* Init the backpos table empty. */
     ga_init2(&backpos, sizeof(backpos_T), 10);

--- misc2.c (revision 136)
+++ misc2.c (working copy)
@@ -1905,6 +1905,7 @@

     {
  if (n < gap->ga_growsize)
      n = gap->ga_growsize;
+        gap->ga_growsize += (gap->ga_growsize >> 2);
  len = gap->ga_itemsize * (gap->ga_len + n);
  pp = alloc_clear((unsigned)len);
  if (pp == NULL)

With this change I can see serious performance improvements, but I'm not sure 
if they are safe.
Bram, does it look making any sense?

-- 
Alexei Alexandrov