Flush bitcode incrementally for LTO output

Bitcode writer does not flush buffer until the end by default. This is
fine to small bitcode files. When -flto,--plugin-opt=emit-llvm,-gmlt are
used, the final bitcode file is large, for example, >8G. Keeping all
data in memory consumes a lot of memory.

This change allows bitcode writer flush data to disk early when buffered
data size is above some threshold. This is only enabled when lld emits
LLVM bitcode.

One issue to address is backpatching bitcode: subblock length, function
body indexes, meta data indexes need to backfill. If buffer can be
flushed partially, we introduced raw_fd_stream that supports
read/seek/write, and enables backpatching bitcode flushed in disk.

Reviewed-by: tejohnson, MaskRay

Differential Revision: https://reviews.llvm.org/D86905

include/llvm/Bitcode/BitcodeWriter.h

@@ -47,7 +47,7 @@ class raw_ostream;
 
   public:
     /// Create a BitcodeWriter that writes to Buffer.
-    BitcodeWriter(SmallVectorImpl<char> &Buffer);
+    BitcodeWriter(SmallVectorImpl<char> &Buffer, raw_fd_stream *FS = nullptr);
 
     ~BitcodeWriter();
 

include/llvm/Bitstream/BitstreamWriter.h

@@ -20,17 +20,27 @@
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Bitstream/BitCodes.h"
 #include "llvm/Support/Endian.h"
+#include "llvm/Support/raw_ostream.h"
 #include <vector>
 
 namespace llvm {
 
 class BitstreamWriter {
+  /// Out - The buffer that keeps unflushed bytes.
   SmallVectorImpl<char> &Out;
 
+  /// FS - The file stream that Out flushes to. If FS is nullptr, it does not
+  /// support read or seek, Out cannot be flushed until all data are written.
+  raw_fd_stream *FS;
+
+  /// FlushThreshold - If FS is valid, this is the threshold (unit B) to flush
+  /// FS.
+  const uint64_t FlushThreshold;
+
   /// CurBit - Always between 0 and 31 inclusive, specifies the next bit to use.
   unsigned CurBit;
 
-  /// CurValue - The current value.  Only bits < CurBit are valid.
+  /// CurValue - The current value. Only bits < CurBit are valid.
   uint32_t CurValue;
 
   /// CurCodeSize - This is the declared size of code values used for the
@@ -64,15 +74,19 @@ class BitstreamWriter {
 
   void WriteByte(unsigned char Value) {
     Out.push_back(Value);
+    FlushToFile();
   }
 
   void WriteWord(unsigned Value) {
     Value = support::endian::byte_swap<uint32_t, support::little>(Value);
     Out.append(reinterpret_cast<const char *>(&Value),
                reinterpret_cast<const char *>(&Value + 1));
+    FlushToFile();
   }
 
-  size_t GetBufferOffset() const { return Out.size(); }
+  uint64_t GetNumOfFlushedBytes() const { return FS ? FS->tell() : 0; }
+
+  size_t GetBufferOffset() const { return Out.size() + GetNumOfFlushedBytes(); }
 
   size_t GetWordIndex() const {
     size_t Offset = GetBufferOffset();
@@ -80,9 +94,29 @@ class BitstreamWriter {
     return Offset / 4;
   }
 
+  /// If the related file stream supports reading, seeking and writing, flush
+  /// the buffer if its size is above a threshold.
+  void FlushToFile() {
+    if (!FS)
+      return;
+    if (Out.size() < FlushThreshold)
+      return;
+    FS->write((char *)&Out.front(), Out.size());
+    Out.clear();
+  }
+
 public:
-  explicit BitstreamWriter(SmallVectorImpl<char> &O)
-    : Out(O), CurBit(0), CurValue(0), CurCodeSize(2) {}
+  /// Create a BitstreamWriter that writes to Buffer \p O.
+  ///
+  /// \p FS is the file stream that \p O flushes to incrementally. If \p FS is
+  /// null, \p O does not flush incrementially, but writes to disk at the end.
+  ///
+  /// \p FlushThreshold is the threshold (unit M) to flush \p O if \p FS is
+  /// valid.
+  BitstreamWriter(SmallVectorImpl<char> &O, raw_fd_stream *FS = nullptr,
+                  uint32_t FlushThreshold = 512)
+      : Out(O), FS(FS), FlushThreshold(FlushThreshold << 20), CurBit(0),
+        CurValue(0), CurCodeSize(2) {}
 
   ~BitstreamWriter() {
     assert(CurBit == 0 && "Unflushed data remaining");
@@ -104,11 +138,59 @@ public:
   void BackpatchWord(uint64_t BitNo, unsigned NewWord) {
     using namespace llvm::support;
     uint64_t ByteNo = BitNo / 8;
-    assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
-               &Out[ByteNo], BitNo & 7)) &&
-           "Expected to be patching over 0-value placeholders");
-    endian::writeAtBitAlignment<uint32_t, little, unaligned>(
-        &Out[ByteNo], NewWord, BitNo & 7);
+    uint64_t StartBit = BitNo & 7;
+    uint64_t NumOfFlushedBytes = GetNumOfFlushedBytes();
+
+    if (ByteNo >= NumOfFlushedBytes) {
+      assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
+                 &Out[ByteNo - NumOfFlushedBytes], StartBit)) &&
+             "Expected to be patching over 0-value placeholders");
+      endian::writeAtBitAlignment<uint32_t, little, unaligned>(
+          &Out[ByteNo - NumOfFlushedBytes], NewWord, StartBit);
+      return;
+    }
+
+    // If the byte offset to backpatch is flushed, use seek to backfill data.
+    // First, save the file position to restore later.
+    uint64_t CurPos = FS->tell();
+
+    // Copy data to update into Bytes from the file FS and the buffer Out.
+    char Bytes[8];
+    size_t BytesNum = StartBit ? 8 : 4;
+    size_t BytesFromDisk = std::min(BytesNum, NumOfFlushedBytes - ByteNo);
+    size_t BytesFromBuffer = BytesNum - BytesFromDisk;
+
+    // When unaligned, copy existing data into Bytes from the file FS and the
+    // buffer Out so that it can be updated before writing. For debug builds
+    // read bytes unconditionally in order to check that the existing value is 0
+    // as expected.
+#ifdef NDEBUG
+    if (StartBit)
+#endif
+    {
+      FS->seek(ByteNo);
+      ssize_t BytesRead = FS->read(Bytes, BytesFromDisk);
+      (void)BytesRead; // silence warning
+      assert(BytesRead >= 0 && static_cast<size_t>(BytesRead) == BytesFromDisk);
+      for (size_t i = 0; i < BytesFromBuffer; ++i)
+        Bytes[BytesFromDisk + i] = Out[i];
+      assert((!endian::readAtBitAlignment<uint32_t, little, unaligned>(
+                 Bytes, StartBit)) &&
+             "Expected to be patching over 0-value placeholders");
+    }
+
+    // Update Bytes in terms of bit offset and value.
+    endian::writeAtBitAlignment<uint32_t, little, unaligned>(Bytes, NewWord,
+                                                             StartBit);
+
+    // Copy updated data back to the file FS and the buffer Out.
+    FS->seek(ByteNo);
+    FS->write(Bytes, BytesFromDisk);
+    for (size_t i = 0; i < BytesFromBuffer; ++i)
+      Out[i] = Bytes[BytesFromDisk + i];
+
+    // Restore the file position.
+    FS->seek(CurPos);
   }
 
   void BackpatchWord64(uint64_t BitNo, uint64_t Val) {

lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -86,6 +86,9 @@ static cl::opt<unsigned>
     IndexThreshold("bitcode-mdindex-threshold", cl::Hidden, cl::init(25),
                    cl::desc("Number of metadatas above which we emit an index "
                             "to enable lazy-loading"));
+static cl::opt<uint32_t> FlushThreshold(
+    "bitcode-flush-threshold", cl::Hidden, cl::init(512),
+    cl::desc("The threshold (unit M) for flushing LLVM bitcode."));
 
 static cl::opt<bool> WriteRelBFToSummary(
     "write-relbf-to-summary", cl::Hidden, cl::init(false),
@@ -4453,8 +4456,8 @@ static void writeBitcodeHeader(BitstreamWriter &Stream) {
   Stream.Emit(0xD, 4);
 }
 
-BitcodeWriter::BitcodeWriter(SmallVectorImpl<char> &Buffer)
-    : Buffer(Buffer), Stream(new BitstreamWriter(Buffer)) {
+BitcodeWriter::BitcodeWriter(SmallVectorImpl<char> &Buffer, raw_fd_stream *FS)
+    : Buffer(Buffer), Stream(new BitstreamWriter(Buffer, FS, FlushThreshold)) {
   writeBitcodeHeader(*Stream);
 }
 
@@ -4565,7 +4568,7 @@ void llvm::WriteBitcodeToFile(const Module &M, raw_ostream &Out,
   if (TT.isOSDarwin() || TT.isOSBinFormatMachO())
     Buffer.insert(Buffer.begin(), BWH_HeaderSize, 0);
 
-  BitcodeWriter Writer(Buffer);
+  BitcodeWriter Writer(Buffer, dyn_cast<raw_fd_stream>(&Out));
   Writer.writeModule(M, ShouldPreserveUseListOrder, Index, GenerateHash,
                      ModHash);
   Writer.writeSymtab();
@@ -4575,7 +4578,8 @@ void llvm::WriteBitcodeToFile(const Module &M, raw_ostream &Out,
     emitDarwinBCHeaderAndTrailer(Buffer, TT);
 
   // Write the generated bitstream to "Out".
-  Out.write((char*)&Buffer.front(), Buffer.size());
+  if (!Buffer.empty())
+    Out.write((char *)&Buffer.front(), Buffer.size());
 }
 
 void IndexBitcodeWriter::write() {