[ThinLTO] Enable importing of aliases as copy of aliasee

Summary:
This implements a missing feature to allow importing of aliases, which
was previously disabled because alias cannot be available_externally.
We instead import an alias as a copy of its aliasee.

Some additional work was required in the IndexBitcodeWriter for the
distributed build case, to ensure that the aliasee has a value id
in the distributed index file (i.e. even when it is not being
imported directly).

This is a performance win in codes that have many aliases, e.g. C++
applications that have many constructor and destructor aliases.

Reviewers: pcc

Subscribers: mehdi_amini, inglorion, eraman, llvm-commits

Differential Revision: https://reviews.llvm.org/D40747

llvm-svn: 320895

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -413,7 +413,7 @@ public:
     // in writing out the call graph edges. Save the mapping from GUID
     // to the new global value id to use when writing those edges, which
     // are currently saved in the index in terms of GUID.
-    forEachSummary([&](GVInfo I) {
+    forEachSummary([&](GVInfo I, bool) {
       GUIDToValueIdMap[I.first] = ++GlobalValueId;
     });
   }
@@ -428,12 +428,18 @@ public:
   void forEachSummary(Functor Callback) {
     if (ModuleToSummariesForIndex) {
       for (auto &M : *ModuleToSummariesForIndex)
-        for (auto &Summary : M.second)
-          Callback(Summary);
+        for (auto &Summary : M.second) {
+          Callback(Summary, false);
+          // Ensure aliasee is handled, e.g. for assigning a valueId,
+          // even if we are not importing the aliasee directly (the
+          // imported alias will contain a copy of aliasee).
+          if (auto *AS = dyn_cast<AliasSummary>(Summary.getSecond()))
+            Callback({AS->getAliaseeGUID(), &AS->getAliasee()}, true);
+        }
     } else {
       for (auto &Summaries : Index)
         for (auto &Summary : Summaries.second.SummaryList)
-          Callback({Summaries.first, Summary.get()});
+          Callback({Summaries.first, Summary.get()}, false);
     }
   }
 
@@ -3604,7 +3610,7 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
     NameVals.clear();
   };
 
-  forEachSummary([&](GVInfo I) {
+  forEachSummary([&](GVInfo I, bool IsAliasee) {
     GlobalValueSummary *S = I.second;
     assert(S);
 
@@ -3612,6 +3618,12 @@ void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
     assert(ValueId);
     SummaryToValueIdMap[S] = *ValueId;
 
+    // If this is invoked for an aliasee, we want to record the above
+    // mapping, but then not emit a summary entry (if the aliasee is
+    // to be imported, we will invoke this separately with IsAliasee=false).
+    if (IsAliasee)
+      return;
+
     if (auto *AS = dyn_cast<AliasSummary>(S)) {
       // Will process aliases as a post-pass because the reader wants all
       // global to be loaded first.