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[msan] A comment for the chained-origin-depot hash function. 

llvm-svn: 212801
This commit is contained in:
Evgeniy Stepanov 2014-07-11 09:09:37 +00:00
parent e8162928c8
commit 9c62621c4b
1 changed files with 13 additions and 0 deletions
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13
compiler-rt/lib/msan/msan_chained_origin_depot.cc
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@ -19,6 +19,19 @@ namespace __msan {
struct ChainedOriginDepotDesc { struct ChainedOriginDepotDesc {
u32 here_id; u32 here_id;
u32 prev_id; u32 prev_id;
/* This is murmur2 hash for the 64->32 bit case.
It does not behave all that well because the keys have a very biased
distribution (I've seen 7-element buckets with the table only 14% full).
here_id is built of
* (1 bits) Reserved, zero.
* (8 bits) Part id = bits 13..20 of the hash value of here_id's key.
* (23 bits) Sequential number (each part has each own sequence).
prev_id has either the same distribution as here_id (but with 3:8:21)
split, or one of two reserved values (-1) or (-2). Either case can
dominate depending on the workload.
*/
u32 hash() const { u32 hash() const {
const u32 m = 0x5bd1e995; const u32 m = 0x5bd1e995;
const u32 seed = 0x9747b28c; const u32 seed = 0x9747b28c;