[ThinLTO] Add Visibility bits to GlobalValueSummary::GVFlags

Imported functions and variable get the visibility from the module supplying the
definition.  However, non-imported definitions do not get the visibility from
(ELF) the most constraining visibility among all modules (Mach-O) the visibility
of the prevailing definition.

This patch

* adds visibility bits to GlobalValueSummary::GVFlags
* computes the result visibility and propagates it to all definitions

Protected/hidden can imply dso_local which can enable some optimizations (this
is stronger than GVFlags::DSOLocal because the implied dso_local can be
leveraged for ELF -shared while default visibility dso_local has to be cleared
for ELF -shared).

Note: we don't have summaries for declarations, so for ELF if a declaration has
the most constraining visibility, the result visibility may not be that one.

Differential Revision: https://reviews.llvm.org/D92900

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -968,13 +968,17 @@ static FunctionSummary::FFlags getDecodedFFlags(uint64_t RawFlags) {
   return Flags;
 }
 
-/// Decode the flags for GlobalValue in the summary.
+// Decode the flags for GlobalValue in the summary. The bits for each attribute:
+//
+// linkage: [0,4), notEligibleToImport: 4, live: 5, local: 6, canAutoHide: 7,
+// visibility: [8, 10).
 static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
                                                             uint64_t Version) {
   // Summary were not emitted before LLVM 3.9, we don't need to upgrade Linkage
   // 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
+  auto Visibility = GlobalValue::VisibilityTypes((RawFlags >> 8) & 3); // 2 bits
   RawFlags = RawFlags >> 4;
   bool NotEligibleToImport = (RawFlags & 0x1) || Version < 3;
   // The Live flag wasn't introduced until version 3. For dead stripping
@@ -984,7 +988,8 @@ static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
   bool Local = (RawFlags & 0x4);
   bool AutoHide = (RawFlags & 0x8);
 
-  return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, Live, Local, AutoHide);
+  return GlobalValueSummary::GVFlags(Linkage, Visibility, NotEligibleToImport,
+                                     Live, Local, AutoHide);
 }
 
 // Decode the flags for GlobalVariable in the summary

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -1044,7 +1044,8 @@ static uint64_t getEncodedFFlags(FunctionSummary::FFlags Flags) {
   return RawFlags;
 }
 
-// Decode the flags for GlobalValue in the summary
+// Decode the flags for GlobalValue in the summary. See getDecodedGVSummaryFlags
+// in BitcodeReader.cpp.
 static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   uint64_t RawFlags = 0;
 
@@ -1058,6 +1059,8 @@ static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   // account here as well.
   RawFlags = (RawFlags << 4) | Flags.Linkage; // 4 bits
 
+  RawFlags |= (Flags.Visibility << 8); // 2 bits
+
   return RawFlags;
 }