From e1b6498c41b94c3bc5cede17b0702282543385ef Mon Sep 17 00:00:00 2001
From: Ted Kremenek <kremenek@apple.com>
Date: Mon, 26 Jan 2009 21:43:14 +0000
Subject: [PATCH] Embed the offset of the PTH table inside the prologue of the
 PTH file.  This will help improve gradual versioning of PTH files instead of
 relying that the PTH table is at a fixed offset.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@63045 91177308-0d34-0410-b5e6-96231b3b80d8
---
 Driver/CacheTokens.cpp | 10 ++++++++++
 lib/Lex/PTHLexer.cpp   | 19 ++++++++++---------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/Driver/CacheTokens.cpp b/Driver/CacheTokens.cpp
index fcb4d2f12d..975d3648ee 100644
--- a/Driver/CacheTokens.cpp
+++ b/Driver/CacheTokens.cpp
@@ -454,6 +454,11 @@ void PTHWriter::EmitCachedSpellings() {
 }
 
 void PTHWriter::GeneratePTH() {
+  // Generate the prologue.
+  Out << "cfe-pth";
+  Offset JumpOffset = Out.tell();
+  Emit32(0);
+  
   // Iterate over all the files in SourceManager.  Create a lexer
   // for each file and cache the tokens.
   SourceManager &SM = PP.getSourceManager();
@@ -490,10 +495,15 @@ void PTHWriter::GeneratePTH() {
   Offset FileTableOff = EmitFileTable();  
   
   // Finally, write out the offset table at the end.
+  Offset JumpTargetOffset = Out.tell();    
   Emit32(IdTableOff.first);
   Emit32(IdTableOff.second.first);
   Emit32(IdTableOff.second.second);
   Emit32(FileTableOff);
+  
+  // Now write the offset in the prologue.
+  Out.seek(JumpOffset);
+  Emit32(JumpTargetOffset);
 }
 
 void clang::CacheTokens(Preprocessor& PP, const std::string& OutFile) {
diff --git a/lib/Lex/PTHLexer.cpp b/lib/Lex/PTHLexer.cpp
index f6994e0976..747ec10156 100644
--- a/lib/Lex/PTHLexer.cpp
+++ b/lib/Lex/PTHLexer.cpp
@@ -520,17 +520,18 @@ PTHManager* PTHManager::Create(const std::string& file) {
   // words at the end of the file.
   const unsigned char* BufBeg = (unsigned char*)File->getBufferStart();
   const unsigned char* BufEnd = (unsigned char*)File->getBufferEnd();
-  
-  if(!(BufEnd > BufBeg + sizeof(uint32_t)*3)) {
-    assert(false && "Invalid PTH file.");
-    return 0; // FIXME: Proper error diagnostic?
-  }
+
+  // Check the prologue of the file.
+  if ((BufEnd - BufBeg) < (unsigned) (sizeof("cfe-pth") + 3) ||
+      memcmp(BufBeg, "cfe-pth", sizeof("cfe-pth") - 1) != 0)
+    return 0;
   
   // Compute the address of the index table at the end of the PTH file.
-  // This table contains the offset of the file lookup table, the
-  // persistent ID -> identifer data table.
-  // FIXME: We should just embed this offset in the PTH file.
-  const unsigned char* EndTable = BufEnd - sizeof(uint32_t)*4;
+  const unsigned char *p = BufBeg + (sizeof("cfe-pth") - 1);
+  const unsigned char *EndTable = BufBeg + ReadLE32(p);
+  
+  if (EndTable >= BufEnd)
+    return 0;
   
   // Construct the file lookup table.  This will be used for mapping from
   // FileEntry*'s to cached tokens.
-- 
2.50.1