PCH optimization for the identifier table, where we separate

"interesting" identifiers (e.g., those where the IdentifierInfo has
some useful information) from "uninteresting" identifiers (where the
IdentifierInfo is just a name). This makes the hash table smaller (so
searching in it should be faster) and, when loading "uninteresting"
identifiers, we skip the lookup in the hash table.

PCH file size is slightly smaller than before (since we don't emit the
contents of the uninteresting IdentifierInfo structures). The
Cocoa.h-prefixed "Hello, World" doesn't show any speedup, although
we're getting to the point where system noise is a bit issue.

llvm-svn: 70075

1 files changed, 26 insertions, 3 deletions

diff --git a/clang/lib/Frontend/PCHReader.cpp b/clang/lib/Frontend/PCHReader.cpp
index 005436dccf2..6f1ff329772 100644
--- a/clang/lib/Frontend/PCHReader.cpp
+++ b/clang/lib/Frontend/PCHReader.cpp
@@ -1245,8 +1245,8 @@ public:
   static std::pair<unsigned, unsigned>
   ReadKeyDataLength(const unsigned char*& d) {
     using namespace clang::io;
-    unsigned KeyLen = ReadUnalignedLE16(d);
     unsigned DataLen = ReadUnalignedLE16(d);
+    unsigned KeyLen = ReadUnalignedLE16(d);
     return std::make_pair(KeyLen, DataLen);
   }
     
@@ -2842,8 +2842,31 @@ IdentifierInfo *PCHReader::DecodeIdentifierInfo(unsigned ID) {
   
   if (!IdentifiersLoaded[ID - 1]) {
     uint32_t Offset = IdentifierOffsets[ID - 1];
-    IdentifiersLoaded[ID - 1] 
-      = &Context.Idents.get(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
+    // this string is not in the on-disk hash table. Therefore,
+    // disable lookup into the hash table when looking for this
+    // 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);
   }
   
   return IdentifiersLoaded[ID - 1];