[ThinLTO] Double bits of module hash used for renaming

[tejohnson]

Summary

Use 64 instead of 32 bits of the module hash as the suffix when renaming
after promotion to reduce the likelihood of a collision (which we
observed in a binary when using 32 bits).

[pcc]

LGTM

include/llvm/IR/ModuleSummaryIndex.h

746

I'd write uint64_t(ModHash[0]) << 32 to make it unambiguous what is being extended.

[tejohnson]

Address comment

include/llvm/IR/ModuleSummaryIndex.h

const ModuleHash &getModuleHash(const StringRef ModPath) const {
     return It->second.second;
   }
 
   /// Convenience method for creating a promoted global name
   /// for the given value name of a local, and its original module's ID.
   static std::string getGlobalNameForLocal(StringRef Name, ModuleHash ModHash) {
     SmallString<256> NewName(Name);
     NewName += ".llvm.";
-    NewName += utostr(ModHash[0]); // Take the first 32 bits
+    NewName += utostr(((uint64_t)ModHash[0] << 32) |
+                      ModHash[1]); // Take the first 64 bits
     return NewName.str();
   }
 
   /// Helper to obtain the unpromoted name for a global value (or the original
   /// name if not promoted).
   static StringRef getOriginalNameBeforePromote(StringRef Name) {
     std::pair<StringRef, StringRef> Pair = Name.split(".llvm.");
     return Pair.first;