Switch hash to rapidhash.

This is the currently fastest hash that passes SMHasher and does not
require special instructions (e.g. SIMD). Like emhash8, it is
MIT-licensed, and we include the .h file directly.

For a no-op build of Chromium (Linux, Zen 2),
this reduces time spent from 4.62 to 4.22 seconds.
(NOTE: This is a more difficult measurement than the previous ones,
as it necessarily involves removing the entire build log and doing
a clean build. However, just switching the HashMap hash takes
to 4.47 seconds or so.)

src/build_log.cc

@@ -53,63 +53,14 @@ using namespace std;
 namespace {
 
 const char kFileSignature[] = "# ninja log v%d\n";
-const int kOldestSupportedVersion = 6;
-const int kCurrentVersion = 6;
-
-// 64bit MurmurHash2, by Austin Appleby
-#if defined(_MSC_VER)
-#define BIG_CONSTANT(x) (x)
-#else   // defined(_MSC_VER)
-#define BIG_CONSTANT(x) (x##LLU)
-#endif // !defined(_MSC_VER)
-inline
-uint64_t MurmurHash64A(const void* key, size_t len) {
-  static const uint64_t seed = 0xDECAFBADDECAFBADull;
-  const uint64_t m = BIG_CONSTANT(0xc6a4a7935bd1e995);
-  const int r = 47;
-  uint64_t h = seed ^ (len * m);
-  const unsigned char* data = static_cast<const unsigned char*>(key);
-  while (len >= 8) {
-    uint64_t k;
-    memcpy(&k, data, sizeof k);
-    k *= m;
-    k ^= k >> r;
-    k *= m;
-    h ^= k;
-    h *= m;
-    data += 8;
-    len -= 8;
-  }
-  switch (len & 7)
-  {
-  case 7: h ^= uint64_t(data[6]) << 48;
-          NINJA_FALLTHROUGH;
-  case 6: h ^= uint64_t(data[5]) << 40;
-          NINJA_FALLTHROUGH;
-  case 5: h ^= uint64_t(data[4]) << 32;
-          NINJA_FALLTHROUGH;
-  case 4: h ^= uint64_t(data[3]) << 24;
-          NINJA_FALLTHROUGH;
-  case 3: h ^= uint64_t(data[2]) << 16;
-          NINJA_FALLTHROUGH;
-  case 2: h ^= uint64_t(data[1]) << 8;
-          NINJA_FALLTHROUGH;
-  case 1: h ^= uint64_t(data[0]);
-          h *= m;
-  };
-  h ^= h >> r;
-  h *= m;
-  h ^= h >> r;
-  return h;
-}
-#undef BIG_CONSTANT
-
+const int kOldestSupportedVersion = 7;
+const int kCurrentVersion = 7;
 
 }  // namespace
 
 // static
 uint64_t BuildLog::LogEntry::HashCommand(StringPiece command) {
-  return MurmurHash64A(command.str_, command.len_);
+  return rapidhash(command.str_, command.len_);
 }
 
 BuildLog::LogEntry::LogEntry(const string& output)

src/hash_map.h

@@ -20,39 +20,7 @@
 #include "string_piece.h"
 #include "util.h"
 #include "hash_table8.hpp"
-
-// MurmurHash2, by Austin Appleby
-static inline
-unsigned int MurmurHash2(const void* key, size_t len) {
-  static const unsigned int seed = 0xDECAFBAD;
-  const unsigned int m = 0x5bd1e995;
-  const int r = 24;
-  unsigned int h = seed ^ len;
-  const unsigned char* data = static_cast<const unsigned char*>(key);
-  while (len >= 4) {
-    unsigned int k;
-    memcpy(&k, data, sizeof k);
-    k *= m;
-    k ^= k >> r;
-    k *= m;
-    h *= m;
-    h ^= k;
-    data += 4;
-    len -= 4;
-  }
-  switch (len) {
-  case 3: h ^= data[2] << 16;
-          NINJA_FALLTHROUGH;
-  case 2: h ^= data[1] << 8;
-          NINJA_FALLTHROUGH;
-  case 1: h ^= data[0];
-    h *= m;
-  };
-  h ^= h >> 13;
-  h *= m;
-  h ^= h >> 15;
-  return h;
-}
+#include "rapidhash.h"
 
 namespace std {
 template<>
@@ -61,7 +29,7 @@ struct hash<StringPiece> {
   typedef size_t result_type;
 
   size_t operator()(StringPiece key) const {
-    return MurmurHash2(key.str_, key.len_);
+    return rapidhash(key.str_, key.len_);
   }
 };
 }