Marginal performance hack: remove the loop that used to be needed to

look through a freelist for a chunk of adequate size.  For a long time
now, all elements of a given freelist have been exactly the same
allocated size, so we don't need a loop.  Since the loop never iterated
more than once, you'd think this wouldn't matter much, but it makes a
noticeable savings in a simple test --- perhaps because the compiler
isn't optimizing on a mistaken assumption that the loop would repeat.
AllocSetAlloc is called often enough that saving even a couple of
instructions is worthwhile.

diff --git a/src/backend/utils/mmgr/aset.c b/src/backend/utils/mmgr/aset.c
index c6608751a88e9f09610dc7183aac7e668e7f770f..71b6da1e493f0318244317871d57ce74c166539a 100644 (file)
--- a/src/backend/utils/mmgr/aset.c
+++ b/src/backend/utils/mmgr/aset.c
@@ -11,7 +11,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *   $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.71 2007/01/05 22:19:47 momjian Exp $
+ *   $PostgreSQL: pgsql/src/backend/utils/mmgr/aset.c,v 1.72 2007/04/30 00:12:08 tgl Exp $
  *
  * NOTE:
  * This is a new (Feb. 05, 1999) implementation of the allocation set
@@ -516,7 +516,6 @@ AllocSetAlloc(MemoryContext context, Size size)
    AllocSet    set = (AllocSet) context;
    AllocBlock  block;
    AllocChunk  chunk;
-   AllocChunk  priorfree;
    int         fidx;
    Size        chunk_size;
    Size        blksize;
@@ -578,26 +577,16 @@ AllocSetAlloc(MemoryContext context, Size size)
    /*
     * Request is small enough to be treated as a chunk.  Look in the
     * corresponding free list to see if there is a free chunk we could reuse.
-    */
-   fidx = AllocSetFreeIndex(size);
-   priorfree = NULL;
-   for (chunk = set->freelist[fidx]; chunk; chunk = (AllocChunk) chunk->aset)
-   {
-       if (chunk->size >= size)
-           break;
-       priorfree = chunk;
-   }
-
-   /*
     * If one is found, remove it from the free list, make it again a member
     * of the alloc set and return its data address.
     */
+   fidx = AllocSetFreeIndex(size);
+   chunk = set->freelist[fidx];
    if (chunk != NULL)
    {
-       if (priorfree == NULL)
-           set->freelist[fidx] = (AllocChunk) chunk->aset;
-       else
-           priorfree->aset = chunk->aset;
+       Assert(chunk->size >= size);
+
+       set->freelist[fidx] = (AllocChunk) chunk->aset;
 
        chunk->aset = (void *) set;
 
@@ -618,7 +607,7 @@ AllocSetAlloc(MemoryContext context, Size size)
    /*
     * Choose the actual chunk size to allocate.
     */
-   chunk_size = 1 << (fidx + ALLOC_MINBITS);
+   chunk_size = (1 << ALLOC_MINBITS) << fidx;
    Assert(chunk_size >= size);
 
    /*