Revert D87970 "[ThinLTO] Avoid temporaries when loading global decl attachment metadata"

This reverts commit ab1b4810b55279bcf6fdd87be74a403440be3991.

It caused an issue in llvm::lto::thinBackend for a -fsanitize=cfi build.

```
AbbrevNo is 0 => "Invalid abbrev number"
0  llvm::BitstreamCursor::getAbbrev (this=0x9db4c8, AbbrevID=4) at llvm/include/llvm/Bitstream/BitstreamReader.h:528
1  0x00007f5f777a6eb4 in llvm::BitstreamCursor::readRecord (this=0x9db4c8, AbbrevID=4, Vals=llvm::SmallVector of Size 0, Capacity 64, Blob=0x7ffcd0e26558) at
usr/local/google/home/maskray/llvm/llvm/lib/Bitstream/Reader/BitstreamReader.cpp:228
2  0x00007f5f796bf633 in llvm::MetadataLoader::MetadataLoaderImpl::lazyLoadOneMetadata (this=0x9db3a0, ID=188, Placeholders=...) at /usr/local/google/home/mas
ray/llvm/llvm/lib/Bitcode/Reader/MetadataLoader.cpp:1091
3  0x00007f5f796c2527 in llvm::MetadataLoader::MetadataLoaderImpl::getMetadataFwdRefOrLoad (this=0x9db3a0, ID=188) at llvm
lib/Bitcode/Reader/MetadataLoader.cpp:668
4  0x00007f5f796bfff3 in llvm::MetadataLoader::getMetadataFwdRefOrLoad (this=0xd31580, Idx=188) at llvm/lib/Bitcode/Reader
MetadataLoader.cpp:2290
5  0x00007f5f79638265 in (anonymous namespace)::BitcodeReader::parseFunctionBody (this=0xd312e0, F=0x9de758) at llvm/lib/B
tcode/Reader/BitcodeReader.cpp:3938
6  0x00007f5f79635d32 in (anonymous namespace)::BitcodeReader::materialize (this=0xd312e0, GV=0x9de758) at llvm/lib/Bitcod
/Reader/BitcodeReader.cpp:5408
7  0x00007f5f7f8dbe3e in llvm::Module::materialize (this=0x9b92c0, GV=0x9de758) at llvm/lib/IR/Module.cpp:442
8  0x00007f5f7f7f8fbe in llvm::GlobalValue::materialize (this=0x9de758) at llvm/lib/IR/Globals.cpp:50
9  0x00007f5f83b9b5f5 in llvm::FunctionImporter::importFunctions (this=0x7ffcd0e2a730, DestModule=..., ImportList=...) at
llvm/lib/Transforms/IPO/FunctionImport.cpp:1182
```

1 files changed, 20 insertions, 106 deletions

diff --git a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
index 01ebad9..874bb84 100644
--- a/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
+++ b/llvm/lib/Bitcode/Reader/MetadataLoader.cpp
@@ -438,20 +438,6 @@ class MetadataLoader::MetadataLoaderImpl {
   /// Index that keeps track of where to find a metadata record in the stream.
   std::vector<uint64_t> GlobalMetadataBitPosIndex;
 
-  /// Cursor position of the start of the global decl attachments, to enable
-  /// loading using the index built for lazy loading, instead of forward
-  /// references.
-  uint64_t GlobalDeclAttachmentPos = 0;
-
-#ifndef NDEBUG
-  /// Sanity check that we end up parsing all of the global decl attachments.
-  unsigned NumGlobalDeclAttachSkipped = 0;
-  unsigned NumGlobalDeclAttachParsed = 0;
-#endif
-
-  /// Load the global decl attachments, using the index built for lazy loading.
-  Expected<bool> loadGlobalDeclAttachments();
-
   /// Populate the index above to enable lazily loading of metadata, and load
   /// the named metadata as well as the transitively referenced global
   /// Metadata.
@@ -695,10 +681,8 @@ Expected<bool>
 MetadataLoader::MetadataLoaderImpl::lazyLoadModuleMetadataBlock() {
   IndexCursor = Stream;
   SmallVector<uint64_t, 64> Record;
-  GlobalDeclAttachmentPos = 0;
   // Get the abbrevs, and preload record positions to make them lazy-loadable.
   while (true) {
-    uint64_t SavedPos = IndexCursor.GetCurrentBitNo();
     Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks(
         BitstreamCursor::AF_DontPopBlockAtEnd);
     if (!MaybeEntry)
@@ -833,11 +817,25 @@ MetadataLoader::MetadataLoaderImpl::lazyLoadModuleMetadataBlock() {
         break;
       }
       case bitc::METADATA_GLOBAL_DECL_ATTACHMENT: {
-        if (!GlobalDeclAttachmentPos)
-          GlobalDeclAttachmentPos = SavedPos;
-#ifndef NDEBUG
-        NumGlobalDeclAttachSkipped++;
-#endif
+        // FIXME: we need to do this early because we don't materialize global
+        // value explicitly.
+        if (Error Err = IndexCursor.JumpToBit(CurrentPos))
+          return std::move(Err);
+        Record.clear();
+        if (Expected<unsigned> MaybeRecord =
+                IndexCursor.readRecord(Entry.ID, Record))
+          ;
+        else
+          return MaybeRecord.takeError();
+        if (Record.size() % 2 == 0)
+          return error("Invalid record");
+        unsigned ValueID = Record[0];
+        if (ValueID >= ValueList.size())
+          return error("Invalid record");
+        if (auto *GO = dyn_cast<GlobalObject>(ValueList[ValueID]))
+          if (Error Err = parseGlobalObjectAttachment(
+                  *GO, ArrayRef<uint64_t>(Record).slice(1)))
+            return std::move(Err);
         break;
       }
       case bitc::METADATA_KIND:
@@ -887,81 +885,6 @@ MetadataLoader::MetadataLoaderImpl::lazyLoadModuleMetadataBlock() {
   }
 }
 
-// Load the global decl attachments after building the lazy loading index.
-// We don't load them "lazily" - all global decl attachments must be
-// parsed since they aren't materialized on demand. However, by delaying
-// their parsing until after the index is created, we can use the index
-// instead of creating temporaries.
-Expected<bool> MetadataLoader::MetadataLoaderImpl::loadGlobalDeclAttachments() {
-  // Nothing to do if we didn't find any of these metadata records.
-  if (!GlobalDeclAttachmentPos)
-    return true;
-  IndexCursor = Stream;
-  SmallVector<uint64_t, 64> Record;
-  // Jump to the position before the first global decl attachment, so we can
-  // scan for the first BitstreamEntry record.
-  if (Error Err = IndexCursor.JumpToBit(GlobalDeclAttachmentPos))
-    return std::move(Err);
-  while (true) {
-    Expected<BitstreamEntry> MaybeEntry = IndexCursor.advanceSkippingSubblocks(
-        BitstreamCursor::AF_DontPopBlockAtEnd);
-    if (!MaybeEntry)
-      return MaybeEntry.takeError();
-    BitstreamEntry Entry = MaybeEntry.get();
-
-    switch (Entry.Kind) {
-    case BitstreamEntry::SubBlock: // Handled for us already.
-    case BitstreamEntry::Error:
-      return error("Malformed block");
-    case BitstreamEntry::EndBlock:
-      // Sanity check that we parsed them all.
-      assert(NumGlobalDeclAttachSkipped == NumGlobalDeclAttachParsed);
-      return true;
-    case BitstreamEntry::Record:
-      break;
-    }
-    uint64_t CurrentPos = IndexCursor.GetCurrentBitNo();
-    Expected<unsigned> MaybeCode = IndexCursor.skipRecord(Entry.ID);
-    if (!MaybeCode)
-      return MaybeCode.takeError();
-    if (MaybeCode.get() != bitc::METADATA_GLOBAL_DECL_ATTACHMENT) {
-      // Anything other than a global decl attachment signals the end of
-      // these records. sanity check that we parsed them all.
-      assert(NumGlobalDeclAttachSkipped == NumGlobalDeclAttachParsed);
-      return true;
-    }
-#ifndef NDEBUG
-    NumGlobalDeclAttachParsed++;
-#endif
-    // FIXME: we need to do this early because we don't materialize global
-    // value explicitly.
-    if (Error Err = IndexCursor.JumpToBit(CurrentPos))
-      return std::move(Err);
-    Record.clear();
-    if (Expected<unsigned> MaybeRecord =
-            IndexCursor.readRecord(Entry.ID, Record))
-      ;
-    else
-      return MaybeRecord.takeError();
-    if (Record.size() % 2 == 0)
-      return error("Invalid record");
-    unsigned ValueID = Record[0];
-    if (ValueID >= ValueList.size())
-      return error("Invalid record");
-    if (auto *GO = dyn_cast<GlobalObject>(ValueList[ValueID])) {
-      // Need to save and restore the current position since
-      // parseGlobalObjectAttachment will resolve all forward references which
-      // would require parsing from locations stored in the index.
-      CurrentPos = IndexCursor.GetCurrentBitNo();
-      if (Error Err = parseGlobalObjectAttachment(
-              *GO, ArrayRef<uint64_t>(Record).slice(1)))
-        return std::move(Err);
-      if (Error Err = IndexCursor.JumpToBit(CurrentPos))
-        return std::move(Err);
-    }
-  }
-}
-
 /// Parse a METADATA_BLOCK. If ModuleLevel is true then we are parsing
 /// module level metadata.
 Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
@@ -991,14 +914,6 @@ Error MetadataLoader::MetadataLoaderImpl::parseMetadata(bool ModuleLevel) {
       MetadataList.resize(MDStringRef.size() +
                           GlobalMetadataBitPosIndex.size());
 
-      // Now that we have built the index, load the global decl attachments
-      // that were deferred during that process. This avoids creating
-      // temporaries.
-      SuccessOrErr = loadGlobalDeclAttachments();
-      if (!SuccessOrErr)
-        return SuccessOrErr.takeError();
-      assert(SuccessOrErr.get());
-
       // Reading the named metadata created forward references and/or
       // placeholders, that we flush here.
       resolveForwardRefsAndPlaceholders(Placeholders);
@@ -2106,8 +2021,7 @@ Error MetadataLoader::MetadataLoaderImpl::parseGlobalObjectAttachment(
     auto K = MDKindMap.find(Record[I]);
     if (K == MDKindMap.end())
       return error("Invalid ID");
-    MDNode *MD =
-        dyn_cast_or_null<MDNode>(getMetadataFwdRefOrLoad(Record[I + 1]));
+    MDNode *MD = MetadataList.getMDNodeFwdRefOrNull(Record[I + 1]);
     if (!MD)
       return error("Invalid metadata attachment: expect fwd ref to MDNode");
     GO.addMetadata(K->second, *MD);