add a highly efficient hash table that is specialized for mapping C strings

to some other type.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@31286 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Support/CStringMap.cpp

@@ -0,0 +1,134 @@
+//===--- CStringMap.cpp - CString Hash table map implementation -----------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file was developed by Chris Lattner and is distributed under
+// the University of Illinois Open Source License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This file implements the CStringMap class.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/CStringMap.h"
+#include <cassert>
+using namespace llvm;
+
+CStringMapVisitor::~CStringMapVisitor() {
+}
+
+CStringMapImpl::CStringMapImpl(unsigned InitSize, unsigned itemSize) {
+  assert((InitSize & (InitSize-1)) == 0 &&
+         "Init Size must be a power of 2 or zero!");
+  NumBuckets = InitSize ? InitSize : 512;
+  ItemSize = itemSize;
+  NumItems = 0;
+  
+  TheTable = new ItemBucket[NumBuckets]();
+  memset(TheTable, 0, NumBuckets*sizeof(ItemBucket));
+}
+
+
+/// HashString - Compute a hash code for the specified string.
+///
+static unsigned HashString(const char *Start, const char *End) {
+  unsigned int Result = 0;
+  // Perl hash function.
+  while (Start != End)
+    Result = Result * 33 + *Start++;
+  Result = Result + (Result >> 5);
+  return Result;
+}
+
+/// LookupBucketFor - Look up the bucket that the specified string should end
+/// up in.  If it already exists as a key in the map, the Item pointer for the
+/// specified bucket will be non-null.  Otherwise, it will be null.  In either
+/// case, the FullHashValue field of the bucket will be set to the hash value
+/// of the string.
+unsigned CStringMapImpl::LookupBucketFor(const char *NameStart,
+                                         const char *NameEnd) {
+  unsigned HTSize = NumBuckets;
+  unsigned FullHashValue = HashString(NameStart, NameEnd);
+  unsigned BucketNo = FullHashValue & (HTSize-1);
+  
+  unsigned ProbeAmt = 1;
+  while (1) {
+    ItemBucket &Bucket = TheTable[BucketNo];
+    void *BucketItem = Bucket.Item;
+    // If we found an empty bucket, this key isn't in the table yet, return it.
+    if (BucketItem == 0) {
+      Bucket.FullHashValue = FullHashValue;
+      return BucketNo;
+    }
+    
+    // If the full hash value matches, check deeply for a match.  The common
+    // 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.
+    if (Bucket.FullHashValue == FullHashValue) {
+      // Do the comparison like this because NameStart isn't necessarily
+      // null-terminated!
+      char *ItemStr = (char*)BucketItem+ItemSize;
+      if (strlen(ItemStr) == unsigned(NameEnd-NameStart) &&
+          memcmp(ItemStr, NameStart, (NameEnd-NameStart)) == 0) {
+        // We found a match!
+        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;
+  }
+}
+
+/// RehashTable - Grow the table, redistributing values into the buckets with
+/// the appropriate mod-of-hashtable-size.
+void CStringMapImpl::RehashTable() {
+  unsigned NewSize = NumBuckets*2;
+  ItemBucket *NewTableArray = new ItemBucket[NewSize]();
+  memset(NewTableArray, 0, NewSize*sizeof(ItemBucket));
+  
+  // Rehash all the items into their new buckets.  Luckily :) we already have
+  // the hash values available, so we don't have to rehash any strings.
+  for (ItemBucket *IB = TheTable, *E = TheTable+NumBuckets; IB != E; ++IB) {
+    if (IB->Item) {
+      // Fast case, bucket available.
+      unsigned FullHash = IB->FullHashValue;
+      unsigned NewBucket = FullHash & (NewSize-1);
+      if (NewTableArray[NewBucket].Item == 0) {
+        NewTableArray[FullHash & (NewSize-1)].Item = IB->Item;
+        NewTableArray[FullHash & (NewSize-1)].FullHashValue = FullHash;
+        continue;
+      }
+      
+      unsigned ProbeSize = 1;
+      do {
+        NewBucket = (NewBucket + ProbeSize++) & (NewSize-1);
+      } while (NewTableArray[NewBucket].Item);
+      
+      // Finally found a slot.  Fill it in.
+      NewTableArray[NewBucket].Item = IB->Item;
+      NewTableArray[NewBucket].FullHashValue = FullHash;
+    }
+  }
+  
+  delete[] TheTable;
+  
+  TheTable = NewTableArray;
+  NumBuckets = NewSize;
+}
+
+
+/// VisitEntries - This method walks through all of the items,
+/// invoking Visitor.Visit for each of them.
+void CStringMapImpl::VisitEntries(const CStringMapVisitor &Visitor) const {
+  for (ItemBucket *IB = TheTable, *E = TheTable+NumBuckets; IB != E; ++IB) {
+    if (void *Id = IB->Item)
+      Visitor.Visit((char*)Id + ItemSize, Id);
+  }
+}