[ThinLTO] Add support for emitting minimized bitcode for thin link

Summary:
The cumulative size of the bitcode files for a very large application
can be huge, particularly with -g. In a distributed build environment,
all of these files must be sent to the remote build node that performs
the thin link step, and this can exceed size limits.

The thin link actually only needs the summary along with a bitcode
symbol table. Until we have a proper bitcode symbol table, simply
stripping the debug metadata results in significant size reduction.

Add support for an option to additionally emit minimized bitcode
modules, just for use in the thin link step, which for now just strips
all debug metadata. I plan to add a cc1 option so this can be invoked
easily during the compile step.

However, care must be taken to ensure that these minimized thin link
bitcode files produce the same index as with the original bitcode files,
as these original bitcode files will be used in the backends.

Specifically:
1) The module hash used for caching is typically produced by hashing the
written bitcode, and we want to include the hash that would correspond
to the original bitcode file. This is because we want to ensure that
changes in the stripped portions affect caching. Added plumbing to emit
the same module hash in the minimized thin link bitcode file.
2) The module paths in the index are constructed from the module ID of
each thin linked bitcode, and typically is automatically generated from
the input file path. This is the path used for finding the modules to
import from, and obviously we need this to point to the original bitcode
files. Added gold-plugin support to take a suffix replacement during the
thin link that is used to override the identifier on the MemoryBufferRef
constructed from the loaded thin link bitcode file. The assumption is
that the build system can specify that the minimized bitcode file has a
name that is similar but uses a different suffix (e.g. out.thinlink.bc
instead of out.o).

Added various tests to ensure that we get identical index files out of
the thin link step.

Reviewers: mehdi_amini, pcc

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D31027

llvm-svn: 298638

1 files changed, 37 insertions, 20 deletions

diff --git a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
index 10024ed..b120058 100644
--- a/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -108,6 +108,14 @@ class ModuleBitcodeWriter : public BitcodeWriterBase {
   /// True if a module hash record should be written.
   bool GenerateHash;
 
+  /// If non-null, when GenerateHash is true, the resulting hash is written
+  /// into ModHash. When GenerateHash is false, that specified value
+  /// is used as the hash instead of computing from the generated bitcode.
+  /// Can be used to produce the same module hash for a minimized bitcode
+  /// used just for the thin link as in the regular full bitcode that will
+  /// be used in the backend.
+  ModuleHash *ModHash;
+
   /// The start bit of the identification block.
   uint64_t BitcodeStartBit;
 
@@ -124,10 +132,12 @@ public:
   /// writing to the provided \p Buffer.
   ModuleBitcodeWriter(const Module *M, SmallVectorImpl<char> &Buffer,
                       BitstreamWriter &Stream, bool ShouldPreserveUseListOrder,
-                      const ModuleSummaryIndex *Index, bool GenerateHash)
+                      const ModuleSummaryIndex *Index, bool GenerateHash,
+                      ModuleHash *ModHash = nullptr)
       : BitcodeWriterBase(Stream), Buffer(Buffer), M(*M),
         VE(*M, ShouldPreserveUseListOrder), Index(Index),
-        GenerateHash(GenerateHash), BitcodeStartBit(Stream.GetCurrentBitNo()) {
+        GenerateHash(GenerateHash), ModHash(ModHash),
+        BitcodeStartBit(Stream.GetCurrentBitNo()) {
     // Assign ValueIds to any callee values in the index that came from
     // indirect call profiles and were recorded as a GUID not a Value*
     // (which would have been assigned an ID by the ValueEnumerator).
@@ -3778,17 +3788,24 @@ static void writeIdentificationBlock(BitstreamWriter &Stream) {
 void ModuleBitcodeWriter::writeModuleHash(size_t BlockStartPos) {
   // Emit the module's hash.
   // MODULE_CODE_HASH: [5*i32]
-  SHA1 Hasher;
-  Hasher.update(ArrayRef<uint8_t>((const uint8_t *)&(Buffer)[BlockStartPos],
-                                  Buffer.size() - BlockStartPos));
-  StringRef Hash = Hasher.result();
-  uint32_t Vals[5];
-  for (int Pos = 0; Pos < 20; Pos += 4) {
-    Vals[Pos / 4] = support::endian::read32be(Hash.data() + Pos);
-  }
+  if (GenerateHash) {
+    SHA1 Hasher;
+    uint32_t Vals[5];
+    Hasher.update(ArrayRef<uint8_t>((const uint8_t *)&(Buffer)[BlockStartPos],
+                                    Buffer.size() - BlockStartPos));
+    StringRef Hash = Hasher.result();
+    for (int Pos = 0; Pos < 20; Pos += 4) {
+      Vals[Pos / 4] = support::endian::read32be(Hash.data() + Pos);
+    }
 
-  // Emit the finished record.
-  Stream.EmitRecord(bitc::MODULE_CODE_HASH, Vals);
+    // Emit the finished record.
+    Stream.EmitRecord(bitc::MODULE_CODE_HASH, Vals);
+
+    if (ModHash)
+      // Save the written hash value.
+      std::copy(std::begin(Vals), std::end(Vals), std::begin(*ModHash));
+  } else if (ModHash)
+    Stream.EmitRecord(bitc::MODULE_CODE_HASH, ArrayRef<uint32_t>(*ModHash));
 }
 
 void ModuleBitcodeWriter::write() {
@@ -3849,9 +3866,7 @@ void ModuleBitcodeWriter::write() {
   writeValueSymbolTable(M.getValueSymbolTable(),
                         /* IsModuleLevel */ true, &FunctionToBitcodeIndex);
 
-  if (GenerateHash) {
-    writeModuleHash(BlockStartPos);
-  }
+  writeModuleHash(BlockStartPos);
 
   Stream.ExitBlock();
 }
@@ -3942,9 +3957,10 @@ BitcodeWriter::~BitcodeWriter() = default;
 void BitcodeWriter::writeModule(const Module *M,
                                 bool ShouldPreserveUseListOrder,
                                 const ModuleSummaryIndex *Index,
-                                bool GenerateHash) {
-  ModuleBitcodeWriter ModuleWriter(
-      M, Buffer, *Stream, ShouldPreserveUseListOrder, Index, GenerateHash);
+                                bool GenerateHash, ModuleHash *ModHash) {
+  ModuleBitcodeWriter ModuleWriter(M, Buffer, *Stream,
+                                   ShouldPreserveUseListOrder, Index,
+                                   GenerateHash, ModHash);
   ModuleWriter.write();
 }
 
@@ -3953,7 +3969,7 @@ void BitcodeWriter::writeModule(const Module *M,
 void llvm::WriteBitcodeToFile(const Module *M, raw_ostream &Out,
                               bool ShouldPreserveUseListOrder,
                               const ModuleSummaryIndex *Index,
-                              bool GenerateHash) {
+                              bool GenerateHash, ModuleHash *ModHash) {
   SmallVector<char, 0> Buffer;
   Buffer.reserve(256*1024);
 
@@ -3964,7 +3980,8 @@ void llvm::WriteBitcodeToFile(const Module *M, raw_ostream &Out,
     Buffer.insert(Buffer.begin(), BWH_HeaderSize, 0);
 
   BitcodeWriter Writer(Buffer);
-  Writer.writeModule(M, ShouldPreserveUseListOrder, Index, GenerateHash);
+  Writer.writeModule(M, ShouldPreserveUseListOrder, Index, GenerateHash,
+                     ModHash);
 
   if (TT.isOSDarwin() || TT.isOSBinFormatMachO())
     emitDarwinBCHeaderAndTrailer(Buffer, TT);