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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

39
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>
@ -32,7 +33,7 @@ static unsigned getMinBucketToReserveForEntries(unsigned NumEntries) {
StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
ItemSize = itemSize;
// If a size is specified, initialize the table with that many buckets.
if (InitSize) {
// The table will grow when the number of entries reach 3/4 of the number of
@ -41,7 +42,7 @@ StringMapImpl::StringMapImpl(unsigned InitSize, unsigned itemSize) {
init(getMinBucketToReserveForEntries(InitSize));
return;
}
// Otherwise, initialize it with zero buckets to avoid the allocation.
TheTable = nullptr;
NumBuckets = 0;
@ -56,7 +57,7 @@ void StringMapImpl::init(unsigned InitSize) {
unsigned NewNumBuckets = InitSize ? InitSize : 16;
NumItems = 0;
NumTombstones = 0;
TheTable = static_cast<StringMapEntryBase **>(
std::calloc(NewNumBuckets+1,
sizeof(StringMapEntryBase **) + sizeof(unsigned)));
@ -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);
@ -98,11 +99,11 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
HashTable[FirstTombstone] = FullHashValue;
return FirstTombstone;
}
HashTable[BucketNo] = FullHashValue;
return BucketNo;
}
if (BucketItem == getTombstoneVal()) {
// Skip over tombstones. However, remember the first one we see.
if (FirstTombstone == -1) FirstTombstone = BucketNo;
@ -111,7 +112,7 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
// case here is that we are only looking at the buckets (for item info
// being non-null and for the full hash value) not at the items. This
// is important for cache locality.
// Do the comparison like this because Name isn't necessarily
// null-terminated!
char *ItemStr = (char*)BucketItem+ItemSize;
@ -120,10 +121,10 @@ unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
return BucketNo;
}
}
// Okay, we didn't find the item. Probe to the next bucket.
BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
// Use quadratic probing, it has fewer clumping artifacts than linear
// probing and has good cache behavior in the common case.
++ProbeAmt;
@ -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);
@ -146,7 +147,7 @@ int StringMapImpl::FindKey(StringRef Key) const {
// If we found an empty bucket, this key isn't in the table yet, return.
if (LLVM_LIKELY(!BucketItem))
return -1;
if (BucketItem == getTombstoneVal()) {
// Ignore tombstones.
} else if (LLVM_LIKELY(HashTable[BucketNo] == FullHashValue)) {
@ -154,7 +155,7 @@ int StringMapImpl::FindKey(StringRef Key) const {
// case here is that we are only looking at the buckets (for item info
// being non-null and for the full hash value) not at the items. This
// is important for cache locality.
// Do the comparison like this because NameStart isn't necessarily
// null-terminated!
char *ItemStr = (char*)BucketItem+ItemSize;
@ -163,10 +164,10 @@ int StringMapImpl::FindKey(StringRef Key) const {
return BucketNo;
}
}
// Okay, we didn't find the item. Probe to the next bucket.
BucketNo = (BucketNo+ProbeAmt) & (HTSize-1);
// Use quadratic probing, it has fewer clumping artifacts than linear
// probing and has good cache behavior in the common case.
++ProbeAmt;
@ -187,7 +188,7 @@ void StringMapImpl::RemoveKey(StringMapEntryBase *V) {
StringMapEntryBase *StringMapImpl::RemoveKey(StringRef Key) {
int Bucket = FindKey(Key);
if (Bucket == -1) return nullptr;
StringMapEntryBase *Result = TheTable[Bucket];
TheTable[Bucket] = getTombstoneVal();
--NumItems;
@ -241,13 +242,13 @@ unsigned StringMapImpl::RehashTable(unsigned BucketNo) {
NewBucketNo = NewBucket;
continue;
}
// Otherwise probe for a spot.
unsigned ProbeSize = 1;
do {
NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
} while (NewTableArray[NewBucket]);
// Finally found a slot. Fill it in.
NewTableArray[NewBucket] = Bucket;
NewHashArray[NewBucket] = FullHash;
@ -255,9 +256,9 @@ unsigned StringMapImpl::RehashTable(unsigned BucketNo) {
NewBucketNo = NewBucket;
}
}
free(TheTable);
TheTable = NewTableArray;
NumBuckets = NewSize;
NumTombstones = 0;