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[Support] Replace HashString with djbHash.

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This removes the HashString function from StringExtraces and replaces
its uses with calls to djbHash from DJB.h

This is *almost* NFC. While the algorithm is identical, the djbHash
implementation in StringExtras used 0 as its seed while the
implementation in DJB uses 5381. The latter has been shown to result in
less collisions and improved avalanching.

https://reviews.llvm.org/D43615
(cherry picked from commit 77f7f965bc9499a9ae768a296ca5a1f7347d1d2c)

llvm-svn: 326081
This commit is contained in:
Jonas Devlieghere 2018-02-26 11:30:13 +00:00
parent c3fa426be7
commit e8b48cad10
2 changed files with 20 additions and 32 deletions
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13
include/llvm/ADT/StringExtras.h
View File

@ -232,19 +232,6 @@ void SplitString(StringRef Source,
SmallVectorImpl<StringRef> &OutFragments,
StringRef Delimiters = " \t\n\v\f\r");
/// HashString - Hash function for strings.
///
/// This is the Bernstein hash function.
//
// FIXME: Investigate whether a modified bernstein hash function performs
// better: http://eternallyconfuzzled.com/tuts/algorithms/jsw_tut_hashing.aspx
// X*33+c -> X*33^c
inline unsigned HashString(StringRef Str, unsigned Result = 0) {
for (StringRef::size_type i = 0, e = Str.size(); i != e; ++i)
Result = Result * 33 + (unsigned char)Str[i];
return Result;
}
/// Returns the English suffix for an ordinal integer (-st, -nd, -rd, -th).
inline StringRef getOrdinalSuffix(unsigned Val) {
// It is critically important that we do this perfectly for

5
lib/Support/StringMap.cpp
View File

@ -14,6 +14,7 @@
#include "llvm/ADT/StringMap.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/DJB.h"
#include "llvm/Support/MathExtras.h"
#include <cassert>
@ -82,7 +83,7 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
init(16);
HTSize = NumBuckets;
}
unsigned FullHashValue = HashString(Name);
unsigned FullHashValue = djbHash(Name);
unsigned BucketNo = FullHashValue & (HTSize-1);
unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
@ -136,7 +137,7 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
int StringMapImpl::FindKey(StringRef Key) const {
unsigned HTSize = NumBuckets;
if (HTSize == 0) return -1; // Really empty table?
unsigned FullHashValue = HashString(Key);
unsigned FullHashValue = djbHash(Key);
unsigned BucketNo = FullHashValue & (HTSize-1);
unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);