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 <[email protected]> --- lib/ChangeLog | 4 ++++ lib/dynamicsizehash.c | 5 +++-- 2 files changed, 7 insertions(+), 2 deletions(-) diff --git a/lib/ChangeLog b/lib/ChangeLog index 6ce3168f0ae6..9b8899e2ca5c 100644 --- a/lib/ChangeLog +++ b/lib/ChangeLog @@ -1,3 +1,7 @@ +2013-12-12 Josh Stone <[email protected]> + + * dynamicsizehash.c (lookup): Add a shortcut around division. + 2013-04-30 Jan Kratochvil <[email protected]> * eu-config.h (COMPAT_VERSION_NEWPROTO): New. Twice. diff --git a/lib/dynamicsizehash.c b/lib/dynamicsizehash.c index 40f48d5ed119..acb360030376 100644 --- 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) { -- 1.8.4.2
