[Support] Replace StringRef hashing algorithm with DJB

[ekatz]

Summary

DJB hashing has entered in r326091 (https://reviews.llvm.org/D43615), for performance purposes (as described in the commit message). I am unaware of a reason for not to replace the StringRef's default hashing algorithm with DJB, instead of using it specifically for StringMap. Using StringRef as the Key for DenseMap should have the same performance boost.

[JDevlieghere]

This change looks sensible but I'd like to verify the performance impact. Could you provide some numbers that compare performance before and after the change?

llvm/lib/Support/StringMap.cpp

 //===--- StringMap.cpp - String Hash table map implementation -------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the StringMap class.
 //
 //===----------------------------------------------------------------------===//
 
 #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>
 
 using namespace llvm;
 
 /// Returns the number of buckets to allocate to ensure that the DenseMap can
 /// accommodate \p NumEntries without need to grow().
 static unsigned getMinBucketToReserveForEntries(unsigned NumEntries) {

 /// case, the FullHashValue field of the bucket will be set to the hash value
 /// of the string.
 unsigned StringMapImpl::LookupBucketFor(StringRef Name) {
   unsigned HTSize = NumBuckets;
   if (HTSize == 0) {  // Hash table unallocated so far?
     init(16);
     HTSize = NumBuckets;
   }
-  unsigned FullHashValue = djbHash(Name, 0);
+  unsigned FullHashValue = hash_value(Name);
   unsigned BucketNo = FullHashValue & (HTSize-1);
   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
 
   unsigned ProbeAmt = 1;
   int FirstTombstone = -1;
   while (true) {
     StringMapEntryBase *BucketItem = TheTable[BucketNo];
     // If we found an empty bucket, this key isn't in the table yet, return it.

 }
 
 /// FindKey - Look up the bucket that contains the specified key. If it exists
 /// in the map, return the bucket number of the key.  Otherwise return -1.
 /// This does not modify the map.
 int StringMapImpl::FindKey(StringRef Key) const {
   unsigned HTSize = NumBuckets;
   if (HTSize == 0) return -1;  // Really empty table?
-  unsigned FullHashValue = djbHash(Key, 0);
+  unsigned FullHashValue = hash_value(Key);
   unsigned BucketNo = FullHashValue & (HTSize-1);
   unsigned *HashTable = (unsigned *)(TheTable + NumBuckets + 1);
 
   unsigned ProbeAmt = 1;
   while (true) {
     StringMapEntryBase *BucketItem = TheTable[BucketNo];
     // If we found an empty bucket, this key isn't in the table yet, return.
     if (LLVM_LIKELY(!BucketItem))

llvm/lib/Support/StringRef.cpp

 //===-- StringRef.cpp - Lightweight String References ---------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/Hashing.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/edit_distance.h"
+#include "llvm/Support/DJB.h"
 #include <bitset>
 
 using namespace llvm;
 
 // MSVC emits references to this into the translation units which reference it.
 #ifndef _MSC_VER
 const size_t StringRef::npos;
 #endif
bool StringRef::getAsDouble(double &Result, bool AllowInexact) const {
   }
 
   Result = F.convertToDouble();
   return false;
 }
 
 // Implementation of StringRef hashing.
 hash_code llvm::hash_value(StringRef S) {
-  return hash_combine_range(S.begin(), S.end());
+  return djbHash(S, 0);
 }