[ThinLTO] Fix bug when importing writeonly variables

Patch enables import of write-only variables with non-trivial initializers
to fix linker errors. Initializers of imported variables are converted to
'zeroinitializer' to avoid promotion of referenced objects.

Differential revision: https://reviews.llvm.org/D70006

1 files changed, 15 insertions, 1 deletions

diff --git a/llvm/lib/IR/ModuleSummaryIndex.cpp b/llvm/lib/IR/ModuleSummaryIndex.cpp
index d82d2e9cb73..b575b5b6442 100644
--- a/llvm/lib/IR/ModuleSummaryIndex.cpp
+++ b/llvm/lib/IR/ModuleSummaryIndex.cpp
@@ -197,7 +197,21 @@ void ModuleSummaryIndex::propagateAttributes(
 bool ModuleSummaryIndex::canImportGlobalVar(GlobalValueSummary *S,
                                             bool AnalyzeRefs) const {
   auto HasRefsPreventingImport = [this](const GlobalVarSummary *GVS) {
-    return !isReadOnly(GVS) && GVS->refs().size();
+    // We don't analyze GV references during attribute propagation, so
+    // GV with non-trivial initializer can be marked either read or
+    // write-only.
+    // Importing definiton of readonly GV with non-trivial initializer
+    // allows us doing some extra optimizations (like converting indirect
+    // calls to direct).
+    // Definition of writeonly GV with non-trivial initializer should also
+    // be imported. Not doing so will result in:
+    // a) GV internalization in source module (because it's writeonly)
+    // b) Importing of GV declaration to destination module as a result
+    //    of promotion.
+    // c) Link error (external declaration with internal definition).
+    // However we do not promote objects referenced by writeonly GV
+    // initializer by means of converting it to 'zeroinitializer'
+    return !isReadOnly(GVS) && !isWriteOnly(GVS) && GVS->refs().size();
   };
   auto *GVS = cast<GlobalVarSummary>(S->getBaseObject());