lldb: Cache string hash during ConstString pool queries/insertions

lldb was rehashing the string 3 times (once to determine which StringMap
to use, once to query the StringMap, once to insert) on insertion (twice
on successful lookup).

This patch allows the lldb to benefit from hash improvements in LLVM
(from djbHash to xxh3).

Though further changes would be needed to cache this value to disk - we
shouldn't rely on the StringMap::hash remaining the same in the
future/this value should not be serialized to disk. If we want cache
this value StringMap should take a hashing template parameter to allow
for a fixed hash to be requested.

lldb/source/Utility/ConstString.cpp

@@ -77,10 +77,10 @@ class Pool {
     return 0;
   }
 
-  StringPoolValueType GetMangledCounterpart(const char *ccstr) const {
+  StringPoolValueType GetMangledCounterpart(const char *ccstr) {
     if (ccstr != nullptr) {
-      const uint8_t h = hash(llvm::StringRef(ccstr));
-      llvm::sys::SmartScopedReader<false> rlock(m_string_pools[h].m_mutex);
+      const PoolEntry &pool = selectPool(llvm::StringRef(ccstr));
+      llvm::sys::SmartScopedReader<false> rlock(pool.m_mutex);
       return GetStringMapEntryFromKeyData(ccstr).getValue();
     }
     return nullptr;
@@ -100,19 +100,20 @@ class Pool {
 
   const char *GetConstCStringWithStringRef(llvm::StringRef string_ref) {
     if (string_ref.data()) {
-      const uint8_t h = hash(string_ref);
+      const uint32_t string_hash = StringPool::hash(string_ref);
+      PoolEntry &pool = selectPool(string_hash);
 
       {
-        llvm::sys::SmartScopedReader<false> rlock(m_string_pools[h].m_mutex);
-        auto it = m_string_pools[h].m_string_map.find(string_ref);
-        if (it != m_string_pools[h].m_string_map.end())
+        llvm::sys::SmartScopedReader<false> rlock(pool.m_mutex);
+        auto it = pool.m_string_map.find(string_ref, string_hash);
+        if (it != pool.m_string_map.end())
           return it->getKeyData();
       }
 
-      llvm::sys::SmartScopedWriter<false> wlock(m_string_pools[h].m_mutex);
+      llvm::sys::SmartScopedWriter<false> wlock(pool.m_mutex);
       StringPoolEntryType &entry =
-          *m_string_pools[h]
-               .m_string_map.insert(std::make_pair(string_ref, nullptr))
+          *pool.m_string_map
+               .insert(std::make_pair(string_ref, nullptr), string_hash)
                .first;
       return entry.getKeyData();
     }
@@ -125,12 +126,14 @@ class Pool {
     const char *demangled_ccstr = nullptr;
 
     {
-      const uint8_t h = hash(demangled);
-      llvm::sys::SmartScopedWriter<false> wlock(m_string_pools[h].m_mutex);
+      const uint32_t demangled_hash = StringPool::hash(demangled);
+      PoolEntry &pool = selectPool(demangled_hash);
+      llvm::sys::SmartScopedWriter<false> wlock(pool.m_mutex);
 
       // Make or update string pool entry with the mangled counterpart
-      StringPool &map = m_string_pools[h].m_string_map;
-      StringPoolEntryType &entry = *map.try_emplace(demangled).first;
+      StringPool &map = pool.m_string_map;
+      StringPoolEntryType &entry =
+          *map.try_emplace_with_hash(demangled, demangled_hash).first;
 
       entry.second = mangled_ccstr;
 
@@ -141,8 +144,8 @@ class Pool {
     {
       // Now assign the demangled const string as the counterpart of the
       // mangled const string...
-      const uint8_t h = hash(llvm::StringRef(mangled_ccstr));
-      llvm::sys::SmartScopedWriter<false> wlock(m_string_pools[h].m_mutex);
+      PoolEntry &pool = selectPool(llvm::StringRef(mangled_ccstr));
+      llvm::sys::SmartScopedWriter<false> wlock(pool.m_mutex);
       GetStringMapEntryFromKeyData(mangled_ccstr).setValue(demangled_ccstr);
     }
 
@@ -171,17 +174,20 @@ class Pool {
   }
 
 protected:
-  uint8_t hash(llvm::StringRef s) const {
-    uint32_t h = llvm::djbHash(s);
-    return ((h >> 24) ^ (h >> 16) ^ (h >> 8) ^ h) & 0xff;
-  }
-
   struct PoolEntry {
     mutable llvm::sys::SmartRWMutex<false> m_mutex;
     StringPool m_string_map;
   };
 
   std::array<PoolEntry, 256> m_string_pools;
+
+  PoolEntry &selectPool(const llvm::StringRef &s) {
+    return selectPool(StringPool::hash(s));
+  }
+
+  PoolEntry &selectPool(uint32_t h) {
+    return m_string_pools[((h >> 24) ^ (h >> 16) ^ (h >> 8) ^ h) & 0xff];
+  }
 };
 
 // Frameworks and dylibs aren't supposed to have global C++ initializers so we
@@ -197,7 +203,7 @@ static Pool &StringPool() {
   static Pool *g_string_pool = nullptr;
 
   llvm::call_once(g_pool_initialization_flag,
-                 []() { g_string_pool = new Pool(); });
+                  []() { g_string_pool = new Pool(); });
 
   return *g_string_pool;
 }