Optimize the loading of an identifier from a PCH file when given the

identifier's ID. In this case, we know where the identifier's entry is
located in the hash table (it starts right before the identifier
string itself), so skip the hash table lookup and read the entry
directly. The performance improvement here is, gain, hard to quantify,
but it's the right thing to do.

llvm-svn: 70078

1 files changed, 35 insertions, 17 deletions

diff --git a/clang/lib/Frontend/PCHReader.cpp b/clang/lib/Frontend/PCHReader.cpp
index 6f1ff329772..40e5c0d6030 100644
--- a/clang/lib/Frontend/PCHReader.cpp
+++ b/clang/lib/Frontend/PCHReader.cpp
@@ -2842,6 +2842,7 @@ IdentifierInfo *PCHReader::DecodeIdentifierInfo(unsigned ID) {
   
   if (!IdentifiersLoaded[ID - 1]) {
     uint32_t Offset = IdentifierOffsets[ID - 1];
+    const char *Str = IdentifierTableData + Offset;
 
     // If there is an identifier lookup table, but the offset of this
     // string is after the identifier table itself, then we know that
@@ -2850,23 +2851,40 @@ IdentifierInfo *PCHReader::DecodeIdentifierInfo(unsigned ID) {
     // identifier.
     PCHIdentifierLookupTable *IdTable 
       = (PCHIdentifierLookupTable *)IdentifierLookupTable;
-    bool SkipHashTable = IdTable &&
-      Offset >= uint32_t(IdTable->getBuckets() - IdTable->getBase());
-
-    if (SkipHashTable)
-      PP.getIdentifierTable().setExternalIdentifierLookup(0);
-
-    // All of the strings in the PCH file are preceded by a 16-bit
-    // length. Extract that 16-bit length to avoid having to run
-    // strlen().
-    const char *Str = IdentifierTableData + Offset;
-    const char *StrLenPtr = Str - 2;
-    unsigned StrLen = (((unsigned) StrLenPtr[0])
-                       | (((unsigned) StrLenPtr[1]) << 8)) - 1;
-    IdentifiersLoaded[ID - 1] = &Context.Idents.get(Str, Str + StrLen);
-
-    if (SkipHashTable)
-      PP.getIdentifierTable().setExternalIdentifierLookup(this);
+    if (!IdTable ||
+        Offset >= uint32_t(IdTable->getBuckets() - IdTable->getBase())) {
+      // Turn off lookup into the on-disk hash table. We know that
+      // this identifier is not there.
+      if (IdTable)
+        PP.getIdentifierTable().setExternalIdentifierLookup(0);
+
+      // All of the strings in the PCH file are preceded by a 16-bit
+      // length. Extract that 16-bit length to avoid having to execute
+      // strlen().
+      const char *StrLenPtr = Str - 2;
+      unsigned StrLen = (((unsigned) StrLenPtr[0])
+                         | (((unsigned) StrLenPtr[1]) << 8)) - 1;
+      IdentifiersLoaded[ID - 1] = &Context.Idents.get(Str, Str + StrLen);
+
+      // Turn on lookup into the on-disk hash table, if we have an
+      // on-disk hash table.
+      if (IdTable)
+        PP.getIdentifierTable().setExternalIdentifierLookup(this);
+    } else {
+      // The identifier is a key in our on-disk hash table. Since we
+      // know where the hash table entry starts, just read in this
+      // (key, value) pair.
+      PCHIdentifierLookupTrait Trait(const_cast<PCHReader &>(*this));
+      const unsigned char *Pos = (const unsigned char *)Str - 4;
+      std::pair<unsigned, unsigned> KeyDataLengths
+        = Trait.ReadKeyDataLength(Pos);
+
+      PCHIdentifierLookupTrait::internal_key_type InternalKey
+        = Trait.ReadKey(Pos, KeyDataLengths.first);
+      Pos = (const unsigned char *)Str + KeyDataLengths.first;
+      IdentifiersLoaded[ID - 1] = Trait.ReadData(InternalKey, Pos,
+                                                 KeyDataLengths.second);
+    }
   }
   
   return IdentifiersLoaded[ID - 1];