Object: Add version and producer fields to the irsymtab header. NFCI.

[pcc]

Summary

These will be necessary in order to handle upgrades from old bitcode
files.

[tejohnson]

llvm/include/llvm/Object/IRSymtab.h

137

Thinking it would be good to print these when dumping the symtab. That would help on one of the tests added on D33974 (I'll note it there).

[pcc]

llvm/include/llvm/Object/IRSymtab.h

137

Yes, maybe. It would mean that dump-symtab would have to have its own copy of the code that reads the header because right now it always uses (via a few layers of indirection) irsymtab::readBitcode which will always upgrade mismatching versions. I could add that in D33974 I suppose.

[pcc]

llvm/include/llvm/Object/IRSymtab.h

137

Done in the version of D33974 that I just uploaded.

[tejohnson]

LGTM

llvm/include/llvm/Object/IRSymtab.h

struct Uncommon {
   Word CommonSize, CommonAlign;
 
   /// COFF-specific: the name of the symbol that a weak external resolves to
   /// if not defined.
   Str COFFWeakExternFallbackName;
 };
 
 struct Header {
+  /// Version number of the symtab format. This number should be incremented
+  /// when the format changes, but it does not need to be incremented if a
+  /// change to LLVM would cause it to create a different symbol table.
+  Word Version;
+  enum { kCurrentVersion = 0 };
+
+  /// The producer's version string (LLVM_VERSION_STRING " " LLVM_REVISION).
+  /// Consumers should rebuild the symbol table from IR if the producer's
+  /// version does not match the consumer's version due to potential differences
+  /// in symbol table format, symbol enumeration order and so on.
+  Str Producer;
+
   Range<Module> Modules;
   Range<Comdat> Comdats;
   Range<Symbol> Symbols;
   Range<Uncommon> Uncommons;
 
   Str TargetTriple, SourceFileName;
 
   /// COFF-specific: linker directives.

llvm/lib/Object/IRSymtab.cpp

 #include "llvm/MC/StringTableBuilder.h"
 #include "llvm/Object/IRObjectFile.h"
 #include "llvm/Object/ModuleSymbolTable.h"
 #include "llvm/Object/SymbolicFile.h"
 #include "llvm/Support/Allocator.h"
 #include "llvm/Support/Casting.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/StringSaver.h"
+#include "llvm/Support/VCSRevision.h"
 #include "llvm/Support/raw_ostream.h"
 #include <cassert>
 #include <string>
 #include <utility>
 #include <vector>
 
 using namespace llvm;
 using namespace irsymtab;
 
 namespace {
 
+const char *getExpectedProducerName() {
+  static char DefaultName[] = LLVM_VERSION_STRING
+#ifdef LLVM_REVISION
+      " " LLVM_REVISION
+#endif
+      ;
+  // Allows for testing of the irsymtab writer and upgrade mechanism. This
+  // environment variable should not be set by users.
+  if (char *OverrideName = getenv("LLVM_OVERRIDE_PRODUCER"))
+    return OverrideName;
+  return DefaultName;
+}
+
+const char *kExpectedProducerName = getExpectedProducerName();
+
 /// Stores the temporary state that is required to build an IR symbol table.
 struct Builder {
   SmallVector<char, 0> &Symtab;
   StringTableBuilder &StrtabBuilder;
 
   Builder(SmallVector<char, 0> &Symtab, StringTableBuilder &StrtabBuilder,
           BumpPtrAllocator &Alloc)
       : Symtab(Symtab), StrtabBuilder(StrtabBuilder), Saver(Alloc) {}
Error Builder::addSymbol(const ModuleSymbolTable &Msymtab,
 
   return Error::success();
 }
 
 Error Builder::build(ArrayRef<Module *> IRMods) {
   storage::Header Hdr;
 
   assert(!IRMods.empty());
+  Hdr.Version = storage::Header::kCurrentVersion;
+  setStr(Hdr.Producer, kExpectedProducerName);
   setStr(Hdr.TargetTriple, IRMods[0]->getTargetTriple());
   setStr(Hdr.SourceFileName, IRMods[0]->getSourceFileName());
   TT = Triple(IRMods[0]->getTargetTriple());
 
   for (auto *M : IRMods)
     if (Error Err = addModule(M))
       return Err;