Bitcode: Collect all MDString records into a single blob

Optimize output of MDStrings in bitcode.  This emits them in big blocks
(currently 1024) in a pair of records:
  - BULK_STRING_SIZES: the sizes of the strings in the block, and
  - BULK_STRING_DATA: a single blob, which is the concatenation of all
    the strings.

Inspired by Mehdi's similar patch, http://reviews.llvm.org/D18342, this
should (a) slightly reduce bitcode size, since there is less record
overhead, and (b) greatly improve reading speed, since blobs are super
cheap to deserialize.

I needed to add support for blobs to streaming input to get the test
suite passing.
  - StreamingMemoryObject::getPointer reads ahead and returns the
    address of the blob.
  - To avoid a possible reallocation of StreamingMemoryObject::Bytes,
    BitstreamCursor::readRecord needs to move the call to JumpToEnd
    forward so that getPointer is the last bitstream operation.

llvm-svn: 264409

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1347,31 +1347,78 @@ static void writeNamedMetadata(const Module &M, const ValueEnumerator &VE,
   }
 }
 
+static unsigned createMDStringDataAbbrev(BitstreamWriter &Stream) {
+  BitCodeAbbrev *Abbv = new BitCodeAbbrev();
+  Abbv->Add(BitCodeAbbrevOp(bitc::METADATA_BULK_STRING_DATA));
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
+  return Stream.EmitAbbrev(Abbv);
+}
+
+static void emitMDStringBlob(unsigned DataAbbrev,
+                             ArrayRef<const Metadata *> Strings,
+                             BitstreamWriter &Stream,
+                             SmallVectorImpl<uint64_t> &Record,
+                             SmallString<4096> &Blob) {
+  for (const Metadata *MD : Strings) {
+    StringRef S = cast<MDString>(MD)->getString();
+    Record.push_back(S.size());
+    Blob.append(S.begin(), S.end());
+  }
+
+  Stream.EmitRecord(bitc::METADATA_BULK_STRING_SIZES, Record);
+  Record.clear();
+
+  Record.push_back(bitc::METADATA_BULK_STRING_DATA);
+  Stream.EmitRecordWithBlob(DataAbbrev, Record, Blob);
+  Record.clear();
+}
+
+/// Write out a section of records for MDString.
+///
+/// All the MDString elements in a metadata block are emitted in bulk.  They're
+/// grouped into blocks, and each block is emitted with pair of records:
+///
+///   - SIZES: a list of the sizes of the strings in the block.
+///   - DATA: the blob itself.
+static void writeMetadataStrings(ArrayRef<const Metadata *> Strings,
+                                 BitstreamWriter &Stream,
+                                 SmallVectorImpl<uint64_t> &Record) {
+  if (Strings.empty())
+    return;
+
+  // Emit strings in large blocks to reduce record overhead.  Somewhat
+  // arbitrarily, limit this to 512 strings per blob:
+  //   - big enough to eliminate overhead;
+  //   - small enough that the reader's SIZES record will stay within a page.
+  const size_t NumStringsPerBlob = 512;
+  Record.reserve(std::min(NumStringsPerBlob, Strings.size()));
+
+  unsigned DataAbbrev = createMDStringDataAbbrev(Stream);
+  SmallString<4096> Blob;
+  while (Strings.size() > NumStringsPerBlob) {
+    emitMDStringBlob(DataAbbrev, Strings.slice(0, NumStringsPerBlob), Stream,
+                     Record, Blob);
+    Strings = Strings.slice(NumStringsPerBlob);
+  }
+  if (!Strings.empty())
+    emitMDStringBlob(DataAbbrev, Strings, Stream, Record, Blob);
+}
+
 static void WriteModuleMetadata(const Module &M,
                                 const ValueEnumerator &VE,
                                 BitstreamWriter &Stream) {
-  const auto &MDs = VE.getMDs();
-  if (MDs.empty() && M.named_metadata_empty())
+  if (VE.getMDs().empty() && M.named_metadata_empty())
     return;
 
   Stream.EnterSubblock(bitc::METADATA_BLOCK_ID, 3);
 
-  unsigned MDSAbbrev = 0;
-  if (VE.hasMDString()) {
-    // Abbrev for METADATA_STRING.
-    BitCodeAbbrev *Abbv = new BitCodeAbbrev();
-    Abbv->Add(BitCodeAbbrevOp(bitc::METADATA_STRING));
-    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
-    Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 8));
-    MDSAbbrev = Stream.EmitAbbrev(Abbv);
-  }
-
   // Initialize MDNode abbreviations.
 #define HANDLE_MDNODE_LEAF(CLASS) unsigned CLASS##Abbrev = 0;
 #include "llvm/IR/Metadata.def"
 
   SmallVector<uint64_t, 64> Record;
-  for (const Metadata *MD : MDs) {
+  writeMetadataStrings(VE.getMDStrings(), Stream, Record);
+  for (const Metadata *MD : VE.getNonMDStrings()) {
     if (const MDNode *N = dyn_cast<MDNode>(MD)) {
       assert(N->isResolved() && "Expected forward references to be resolved");
 
@@ -1385,17 +1432,7 @@ static void WriteModuleMetadata(const Module &M,
 #include "llvm/IR/Metadata.def"
       }
     }
-    if (const auto *MDC = dyn_cast<ConstantAsMetadata>(MD)) {
-      WriteValueAsMetadata(MDC, VE, Stream, Record);
-      continue;
-    }
-    const MDString *MDS = cast<MDString>(MD);
-    // Code: [strchar x N]
-    Record.append(MDS->bytes_begin(), MDS->bytes_end());
-
-    // Emit the finished record.
-    Stream.EmitRecord(bitc::METADATA_STRING, Record, MDSAbbrev);
-    Record.clear();
+    WriteValueAsMetadata(cast<ConstantAsMetadata>(MD), VE, Stream, Record);
   }
 
   writeNamedMetadata(M, VE, Stream, Record);