[ThinLTO] Fix printing of aliases for distributed backend indexes

[tejohnson]

Summary

When we import an alias (which will import a copy of the aliasee), but
aren't going to import the aliasee directly, the distributed backend
index will not contain the aliasee summary. Handle this in the summary
assembly printer by printing "null" as the aliasee.

[davidxl]

lgtm

lib/IR/AsmWriter.cpp

case GlobalValueSummary::FunctionKind:
     return "function";
   case GlobalValueSummary::GlobalVarKind:
     return "variable";
   }
   llvm_unreachable("invalid summary kind");
 }
 
 void AssemblyWriter::printAliasSummary(const AliasSummary *AS) {
-  Out << ", aliasee: ^"
-      << Machine.getGUIDSlot(SummaryToGUIDMap[&AS->getAliasee()]);
+  Out << ", aliasee: ";
+  // The indexes emitted for distributed backends may not include the
+  // aliasee summary (only if it is being imported directly). Handle
+  // that case by just emitting "null" as the aliasee.
+  if (AS->hasAliasee())
+    Out << "^" << Machine.getGUIDSlot(SummaryToGUIDMap[&AS->getAliasee()]);
+  else
+    Out << "null";
 }
 
 void AssemblyWriter::printGlobalVarSummary(const GlobalVarSummary *GS) {
   // Nothing for now
 }
 
 static std::string getLinkageName(GlobalValue::LinkageTypes LT) {
   switch (LT) {

test/ThinLTO/X86/distributed_indexes.ll

 ; BACKEND2-DAG: <VALUE_GUID op0={{.*}} op1=-5751648690987223394/>
 ; BACKEND2-DAG: <VALUE_GUID op0={{.*}} op1=-5300342847281564238/>
 ; BACKEND2-DAG: <VALUE_GUID op0={{.*}} op1=-1039159065113703048/>
 ; BACKEND2-NEXT: <COMBINED
 ; BACKEND2-NEXT: <COMBINED
 ; BACKEND2-NEXT: <COMBINED_ALIAS
 ; BACKEND2-NEXT: </GLOBALVAL_SUMMARY_BLOCK
 
+; Make sure that when the alias is imported as a copy of the aliasee, but the
+; aliasee is not being imported by itself, that we can still print the summary.
+; The aliasee should be "null".
+; RUN: llvm-dis %t1.bc.thinlto.bc -o - | FileCheck %s --check-prefix=DIS
+; DIS: aliasee: null
+
 declare void @g(...)
 declare void @analias(...)
 
 define void @f() {
 entry:
   call void (...) @g()
   call void (...) @analias()
   ret void
 }