Add an (optional) identification block in the bitcode

Processing bitcode from a different LLVM version can lead to
unexpected behavior. The LLVM project guarantees autoupdating
bitcode from a previous minor revision for the same major, but
can't make any promise when reading bitcode generated from a
either a non-released LLVM, a vendor toolchain, or a "future"
LLVM release. This patch aims at being more user-friendly and
allows a bitcode produce to emit an optional block at the
beginning of the bitcode that will contains an opaque string
intended to describe the bitcode producer information. The
bitcode reader will dump this information alongside any error it
reports.

The optional block also includes an "epoch" number, monotonically
increasing when incompatible changes are made to the bitcode. The
reader will reject bitcode whose epoch is different from the one
expected.

Differential Revision: http://reviews.llvm.org/D13666

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 251325

1 files changed, 25 insertions, 0 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 5b50a905e1b..5e90cd1bf10 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -2829,6 +2829,29 @@ static void WriteCombinedFunctionSummary(const FunctionInfoIndex &I,
   Stream.ExitBlock();
 }
 
+// Create the "IDENTIFICATION_BLOCK_ID" containing a single string with the
+// current llvm version, and a record for the epoch number.
+static void WriteIdentificationBlock(const Module *M, BitstreamWriter &Stream) {
+  Stream.EnterSubblock(bitc::IDENTIFICATION_BLOCK_ID, 5);
+
+  // Write the "user readable" string identifying the bitcode producer
+  BitCodeAbbrev *Abbv = new BitCodeAbbrev();
+  Abbv->Add(BitCodeAbbrevOp(bitc::IDENTIFICATION_CODE_STRING));
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Char6));
+  auto StringAbbrev = Stream.EmitAbbrev(Abbv);
+  WriteStringRecord(bitc::IDENTIFICATION_CODE_STRING,
+                    "LLVM" LLVM_VERSION_STRING, StringAbbrev, Stream);
+
+  // Write the epoch version
+  Abbv = new BitCodeAbbrev();
+  Abbv->Add(BitCodeAbbrevOp(bitc::IDENTIFICATION_CODE_EPOCH));
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));
+  SmallVector<unsigned, 1> Vals = {bitc::BITCODE_CURRENT_EPOCH};
+  Stream.EmitRecord(bitc::IDENTIFICATION_CODE_EPOCH, Vals);
+  Stream.ExitBlock();
+}
+
 /// WriteModule - Emit the specified module to the bitstream.
 static void WriteModule(const Module *M, BitstreamWriter &Stream,
                         bool ShouldPreserveUseListOrder,
@@ -3000,6 +3023,8 @@ void llvm::WriteBitcodeToFile(const Module *M, raw_ostream &Out,
     // Emit the file header.
     WriteBitcodeHeader(Stream);
 
+    WriteIdentificationBlock(M, Stream);
+
     // Emit the module.
     WriteModule(M, Stream, ShouldPreserveUseListOrder, BitcodeStartBit,
                 EmitFunctionSummary);