[ThinLTO] Add an auto-hide feature

When a symbol is not exported outside of the
DSO, it is can be hidden. Usually we try to internalize
as much as possible, but it is not always possible, for
instance a symbol can be referenced outside of the LTO
unit, or there can be cross-module reference in ThinLTO.

This is a recommit of r293912 after fixing build failures.

Differential Revision: https://reviews.llvm.org/D28978

llvm-svn: 293918

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -800,13 +800,14 @@ static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
   // like getDecodedLinkage() above. Any future change to the linkage enum and
   // to getDecodedLinkage() will need to be taken into account here as above.
   auto Linkage = GlobalValue::LinkageTypes(RawFlags & 0xF); // 4 bits
-  RawFlags = RawFlags >> 4;
-  bool NotEligibleToImport = (RawFlags & 0x1) || Version < 3;
+  bool NotEligibleToImport = ((RawFlags >> 4) & 0x1) || Version < 3;
   // The LiveRoot flag wasn't introduced until version 3. For dead stripping
   // to work correctly on earlier versions, we must conservatively treat all
   // values as live.
-  bool LiveRoot = (RawFlags & 0x2) || Version < 3;
-  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, LiveRoot);
+  bool LiveRoot = ((RawFlags >> 5) & 0x1) || Version < 3;
+  bool AutoHide = (RawFlags >> 6) & 0x1;
+  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, LiveRoot,
+                                     AutoHide);
 }
 
 static GlobalValue::VisibilityTypes getDecodedVisibility(unsigned Val) {