Add a "signature" to AST files to verify that they haven't changed

Since the order of the IDs in the AST file (e.g. DeclIDs, SelectorIDs)
is not stable, it is not safe to load an AST file that depends on
another AST file that has been rebuilt since the importer was built,
even if "nothing changed". We previously used size and modtime to check
this, but I've seen cases where a module rebuilt quickly enough to foil
this check and caused very hard to debug build errors.

To save cycles when we're loading the AST, we just generate a random
nonce value and check that it hasn't changed when we load an imported
module, rather than actually hash the whole file.

This is slightly complicated by the fact that we need to verify the
signature inside addModule, since we might otherwise consider that a
mdoule is "OutOfDate" when really it is the importer that is out of
date. I didn't see any regressions in module load time after this
change.

llvm-svn: 220493

1 files changed, 16 insertions, 0 deletions

diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 0681e3975d2..e39e5bb0222 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -51,6 +51,7 @@
 #include "llvm/Support/MemoryBuffer.h"
 #include "llvm/Support/OnDiskHashTable.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/Process.h"
 #include <algorithm>
 #include <cstdio>
 #include <string.h>
@@ -862,6 +863,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   // Control Block.
   BLOCK(CONTROL_BLOCK);
   RECORD(METADATA);
+  RECORD(SIGNATURE);
   RECORD(MODULE_NAME);
   RECORD(MODULE_MAP_FILE);
   RECORD(IMPORTS);
@@ -1092,6 +1094,14 @@ adjustFilenameForRelocatablePCH(const char *Filename, StringRef isysroot) {
   return Filename + Pos;
 }
 
+static ASTFileSignature getSignature() {
+  while (1) {
+    if (ASTFileSignature S = llvm::sys::Process::GetRandomNumber())
+      return S;
+    // Rely on GetRandomNumber to eventually return non-zero...
+  }
+}
+
 /// \brief Write the control block.
 void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
                                   StringRef isysroot,
@@ -1121,6 +1131,11 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
   Stream.EmitRecordWithBlob(MetadataAbbrevCode, Record,
                             getClangFullRepositoryVersion());
 
+  // Signature
+  Record.clear();
+  Record.push_back(getSignature());
+  Stream.EmitRecord(SIGNATURE, Record);
+
   // Module name
   if (WritingModule) {
     BitCodeAbbrev *Abbrev = new BitCodeAbbrev();
@@ -1173,6 +1188,7 @@ void ASTWriter::WriteControlBlock(Preprocessor &PP, ASTContext &Context,
       AddSourceLocation((*M)->ImportLoc, Record);
       Record.push_back((*M)->File->getSize());
       Record.push_back((*M)->File->getModificationTime());
+      Record.push_back((*M)->Signature);
       const std::string &FileName = (*M)->FileName;
       Record.push_back(FileName.size());
       Record.append(FileName.begin(), FileName.end());