[PATCH] backend: compare word-at-a-time in bcTruelen

The current bcTruelen function uses a simple reverse array scan to
find the legth of a space-padded string. On some workloads (it shows
in sysbench), this can result in a lot of time spent in this function.

This change introduces a word-at-a-time comparison in bcTruelen, aiming
to reduce the number of compares where possible. Word-size compares
are performed on aligned sections of the string.

Results in a small performance increase; around 1-2% on my POWER6 test
box.

Signed-off-by: Jeremy Kerr <jk(at)ozlabs(dot)org>

---
Resend: context diff this time

---
src/backend/utils/adt/varchar.c | 24 +++++++++++++++++++++---
1 file changed, 21 insertions(+), 3 deletions(-)

*** a/src/backend/utils/adt/varchar.c
--- b/src/backend/utils/adt/varchar.c
***************
*** 624,639 **** varchartypmodout(PG_FUNCTION_ARGS)
static int
bcTruelen(BpChar *arg)
{
char *s = VARDATA_ANY(arg);
int i;
- int len;

! len = VARSIZE_ANY_EXHDR(arg);
! for (i = len - 1; i >= 0; i--)
{
if (s[i] != ' ')
break;
}
return i + 1;
}

--- 624,657 ----
static int
bcTruelen(BpChar *arg)
{
+ const uint32_t spaces = 0x20202020;
+ const int wordsize = sizeof(spaces);
char *s = VARDATA_ANY(arg);
int i;

! i = VARSIZE_ANY_EXHDR(arg) - 1;
!
! /* compare down to an aligned boundary */
! for (; i > 0 && !PointerIsAligned(s + i - (wordsize - 1), uint32_t); i--)
{
if (s[i] != ' ')
+ return i + 1;
+ }
+
+ /* now that we're aligned, compare word at a time */
+ for (; i >= wordsize - 1; i -= wordsize)
+ {
+ if (*(uint32_t *)(s + i - (wordsize - 1)) != spaces)
break;
}
+
+ /* check within the last non-matching word */
+ for (; i >= 0; i--)
+ {
+ if (s[i] != ' ')
+ break;
+ }
+
return i + 1;
}