From 17e1c5eba505d0c99e35aa6143631653db668fb6 Mon Sep 17 00:00:00 2001
From: Douglas Gregor <dgregor@apple.com>
Date: Sat, 25 Apr 2009 21:21:38 +0000
Subject: [PATCH] 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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70078 91177308-0d34-0410-b5e6-96231b3b80d8
---
 lib/Frontend/PCHReader.cpp | 52 +++++++++++++++++++++++++-------------
 1 file changed, 35 insertions(+), 17 deletions(-)

diff --git a/lib/Frontend/PCHReader.cpp b/lib/Frontend/PCHReader.cpp
index 6f1ff32977..40e5c0d603 100644
--- a/lib/Frontend/PCHReader.cpp
+++ b/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];
-- 
2.40.0