[ThinLTO] Import local variables from the same module as caller

Summary:
We can sometimes end up with multiple copies of a local variable that
have the same GUID in the index. This happens when there are local
variables with the same name that are in different source files having the
same name/path at compile time (but compiled into different bitcode objects).

In this case make sure we import the copy in the caller's module.
This enables importing both of the variables having the same GUID
(but which will have different promoted names since the module paths,
and therefore the module hashes, will be distinct).

Importing the wrong copy is particularly problematic for read only
variables, since we must import them as a local copy whenever
referenced. Otherwise we get undefs at link time.

Note that the llvm-lto.cpp and ThinLTOCodeGenerator changes are needed
for testing the distributed index case via clang, which will be sent as
a separate clang-side patch shortly. We were previously not doing the
dead code/read only computation before computing imports when testing
distributed index generation (like it was for testing importing and
other ThinLTO mechanisms alone).

Reviewers: evgeny777

Subscribers: mehdi_amini, inglorion, eraman, steven_wu, dexonsmith, dang, llvm-commits

Differential Revision: https://reviews.llvm.org/D55047

llvm-svn: 347886

1 files changed, 13 insertions, 1 deletions

diff --git a/llvm/lib/Transforms/IPO/FunctionImport.cpp b/llvm/lib/Transforms/IPO/FunctionImport.cpp
index d8807f8a064..c5490f13d9a 100644
--- a/llvm/lib/Transforms/IPO/FunctionImport.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionImport.cpp
@@ -293,9 +293,21 @@ static void computeImportForReferencedGlobals(
 
     LLVM_DEBUG(dbgs() << " ref -> " << VI << "\n");
 
+    // If this is a local variable, make sure we import the copy
+    // in the caller's module. The only time a local variable can
+    // share an entry in the index is if there is a local with the same name
+    // in another module that had the same source file name (in a different
+    // directory), where each was compiled in their own directory so there
+    // was not distinguishing path.
+    auto LocalNotInModule = [&](const GlobalValueSummary *RefSummary) -> bool {
+      return GlobalValue::isLocalLinkage(RefSummary->linkage()) &&
+             RefSummary->modulePath() != Summary.modulePath();
+    };
+
     for (auto &RefSummary : VI.getSummaryList())
       if (isa<GlobalVarSummary>(RefSummary.get()) &&
-          canImportGlobalVar(RefSummary.get())) {
+          canImportGlobalVar(RefSummary.get()) &&
+          !LocalNotInModule(RefSummary.get())) {
         auto ILI = ImportList[RefSummary->modulePath()].insert(VI.getGUID());
         // Only update stat if we haven't already imported this variable.
         if (ILI.second)