[PATCH v3] Avoid manual shift-and-test logic in AllocSetFreeIndex

[Jeremy Kerr <jk(at)ozlabs(dot)org>]

Move the shift-and-test login into a separate fls() function, which
can use __builtin_clz() if it's available.

This requires a new check for __builtin_clz in the configure script.

Results in a ~2% performance increase on PowerPC.

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

---
v3: respin as context diff

---
 configure.in                  |   13 +++++++++++++
 src/backend/utils/mmgr/aset.c |   34 +++++++++++++++++++++++++++-------
 2 files changed, 40 insertions(+), 7 deletions(-)

*** a/configure.in
--- b/configure.in
***************
*** 1361,1366 **** case $host_os in
--- 1361,1379 ----
  		AC_FUNC_FSEEKO;;
  esac
  
+ # GCC builtins
+ #
+ # We need AC_TRY_LINK here, as the prototype generated by AC_CHECK_FUNC
+ # will cause gcc to try to reference a non-builtin symbol.
+ 
+ AC_MSG_CHECKING([for __builtin_clz])
+ AC_TRY_LINK([],
+ 	    [__builtin_clz(0);],
+ 	    [AC_DEFINE(HAVE_BUILTIN_CLZ, 1,
+ 				[Define to 1 if you have __builtin_clz().])
+ 			AC_MSG_RESULT(yes)],
+ 		[AC_MSG_RESULT(no)])
+ 
  
  #
  # Pthreads
*** a/src/backend/utils/mmgr/aset.c
--- b/src/backend/utils/mmgr/aset.c
***************
*** 255,260 **** static MemoryContextMethods AllocSetMethods = {
--- 255,285 ----
  #define AllocAllocInfo(_cxt, _chunk)
  #endif
  
+ /*
+  * fls: find last set bit.
+  *
+  * Returns the 1-based index of the most-significant bit in x. The MSB
+  * is bit number 32, the LSB is bit number 1. If x is zero, the result is
+  * undefined.
+  */
+ static inline int
+ fls(unsigned int x)
+ {
+ #ifdef HAVE_BUILTIN_CLZ
+ 	return 32 - __builtin_clz(x);
+ #else
+ 	int ls = 0;
+ 
+ 	while (x != 0)
+ 	{
+ 		ls++;
+ 		x >>= 1;
+ 	}
+ 
+ 	return ls;
+ #endif
+ }
+ 
  /* ----------
   * AllocSetFreeIndex -
   *
***************
*** 268,281 **** AllocSetFreeIndex(Size size)
  {
  	int			idx = 0;
  
! 	if (size > 0)
  	{
! 		size = (size - 1) >> ALLOC_MINBITS;
! 		while (size != 0)
! 		{
! 			idx++;
! 			size >>= 1;
! 		}
  		Assert(idx < ALLOCSET_NUM_FREELISTS);
  	}
  
--- 293,301 ----
  {
  	int			idx = 0;
  
! 	if (size > (1 << ALLOC_MINBITS))
  	{
! 		idx = fls((size - 1) >> ALLOC_MINBITS);
  		Assert(idx < ALLOCSET_NUM_FREELISTS);
  	}