From: Philippe Waroquiers <philippe.waroquiers@skynet.be>
Date: Sat, 25 Apr 2015 14:53:35 +0000 (+0000)
Subject: Replace adler32 by sdbm_hash in m_deduppoolalloc.c
X-Git-Tag: svn/VALGRIND_3_11_0~457
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=f6ab65fc2a9bd47c1e0823d0ac099dc83c13cba3;p=thirdparty%2Fvalgrind.git

Replace adler32 by sdbm_hash in m_deduppoolalloc.c
adler32 is not very good as a hash function.
sdbm_hash gives more different keys that adler32,
and in a large majority of the cases, shorter chains.



git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15142
---

diff --git a/coregrind/m_deduppoolalloc.c b/coregrind/m_deduppoolalloc.c
index 2456e1b65e..bf6d1e3c88 100644
--- a/coregrind/m_deduppoolalloc.c
+++ b/coregrind/m_deduppoolalloc.c
@@ -231,6 +231,19 @@ void VG_(freezeDedupPA) (DedupPoolAlloc *ddpa,
    ddpa->ht_node_pa = NULL;
 }
 
+
+// hash function used by gawk and SDBM.
+static UInt sdbm_hash (const UChar* buf, UInt len )
+{
+  UInt h;
+  UInt i;
+
+  h = 0;
+  for (i = 0; i < len; i++)
+    h = *buf++ + (h<<6) + (h<<16) - h;
+  return h;
+}
+
 const void* VG_(allocEltDedupPA) (DedupPoolAlloc *ddpa, SizeT eltSzB,
                                   const void *elt)
 {
@@ -243,14 +256,7 @@ const void* VG_(allocEltDedupPA) (DedupPoolAlloc *ddpa, SizeT eltSzB,
 
    ddpa->nr_alloc_calls++;
 
-   // Currently using adler32 as hash function.
-   // Many references tells adler32 is bad as a hash function.
-   // And effectively, some tests on dwarf debug string shows
-   // a lot of collisions (at least for short elements).
-   // (A lot can be 10% of the elements colliding, even on
-   // small nr of elements such as 10_000).
-   ht_elt.key = VG_(adler32) (0, NULL, 0);
-   ht_elt.key = VG_(adler32) (ht_elt.key, elt, eltSzB);
+   ht_elt.key = sdbm_hash (elt, eltSzB);
 
    ht_elt.eltSzB = eltSzB;
    ht_elt.elt = elt;