lib: Avoid the hash-lookup division if possible

For Dwarf_Abbrev codes, the most common case is that they're packed at
the low end, saving uleb128 encoding size.  Since the hash table is
always resized to be no more than 90% full, those codes are always less
than the table size, and dividing for the remainder is unnecessary.

Dwarf_Dies are frequently created anew, and need to find abbrev each
time, so even that one division becomes a noticeable hotspot.  This
patch adds a branch to avoid it, which is very predictable for the CPU.

Signed-off-by: Josh Stone <jistone@redhat.com>

diff --git a/lib/ChangeLog b/lib/ChangeLog
index 6ce3168..9b8899e 100644 (file)
--- a/lib/ChangeLog
+++ b/lib/ChangeLog
@@ -1,3 +1,7 @@
+2013-12-12  Josh Stone  <jistone@redhat.com>
+
+       * dynamicsizehash.c (lookup): Add a shortcut around division.
+
 2013-04-30  Jan Kratochvil  <jan.kratochvil@redhat.com>
 
        * eu-config.h (COMPAT_VERSION_NEWPROTO): New.  Twice.

diff --git a/lib/dynamicsizehash.c b/lib/dynamicsizehash.c
index 40f48d5..1fdff1b 100644 (file)
--- a/lib/dynamicsizehash.c
+++ b/lib/dynamicsizehash.c
@@ -49,8 +49,9 @@ lookup (htab, hval, val)
      HASHTYPE hval;
      TYPE val __attribute__ ((unused));
 {
-  /* First hash function: simply take the modul but prevent zero.  */
-  size_t idx = 1 + hval % htab->size;
+  /* First hash function: simply take the modul but prevent zero.  Small values
+     can skip the division, which helps performance when this is common.  */
+  size_t idx = 1 + (hval < htab->size ? hval : hval % htab->size);
 
   if (htab->table[idx].hashval != 0)
     {