Revert "Linker: Drop function pointers for overridden subprograms"

This reverts commit r233164 and its testcase follow-ups in r233165,
r233207, r233214, and r233221.  It apparently unleashed an LTO bootstrap
failure, at least on Darwin:

http://lab.llvm.org:8080/green/job/clang-stage2-configure-Rlto_build/3376/

I'm reproducing now.

llvm-svn: 233254

1 files changed, 13 insertions, 10 deletions

diff --git a/llvm/lib/Linker/LinkModules.cpp b/llvm/lib/Linker/LinkModules.cpp
index e884f452f03..d1000dc9a9f 100644
--- a/llvm/lib/Linker/LinkModules.cpp
+++ b/llvm/lib/Linker/LinkModules.cpp
@@ -1250,10 +1250,9 @@ void ModuleLinker::linkNamedMDNodes() {
 
 /// Drop DISubprograms that have been superseded.
 ///
-/// FIXME: this creates an asymmetric result: we strip functions from losing
-/// subprograms in DstM, but leave losing subprograms in SrcM.
-/// TODO: Remove this logic once the backend can correctly determine canonical
-/// subprograms.
+/// FIXME: this creates an asymmetric result: we strip losing subprograms from
+/// DstM, but leave losing subprograms in SrcM.  Instead we should also strip
+/// losers from SrcM, but this requires extra plumbing in MapMetadata.
 void ModuleLinker::stripReplacedSubprograms() {
   // Avoid quadratic runtime by returning early when there's nothing to do.
   if (OverridingFunctions.empty())
@@ -1263,8 +1262,8 @@ void ModuleLinker::stripReplacedSubprograms() {
   auto Functions = std::move(OverridingFunctions);
   OverridingFunctions.clear();
 
-  // Drop functions from subprograms if they've been overridden by the new
-  // compile unit.
+  // Drop subprograms whose functions have been overridden by the new compile
+  // unit.
   NamedMDNode *CompileUnits = DstM->getNamedMetadata("llvm.dbg.cu");
   if (!CompileUnits)
     return;
@@ -1275,15 +1274,19 @@ void ModuleLinker::stripReplacedSubprograms() {
     DITypedArray<DISubprogram> SPs(CU.getSubprograms());
     assert(SPs && "Expected valid subprogram array");
 
+    SmallVector<Metadata *, 16> NewSPs;
+    NewSPs.reserve(SPs.getNumElements());
     for (unsigned S = 0, SE = SPs.getNumElements(); S != SE; ++S) {
       DISubprogram SP = SPs.getElement(S);
-      if (!SP || !SP.getFunction() || !Functions.count(SP.getFunction()))
+      if (SP && SP.getFunction() && Functions.count(SP.getFunction()))
         continue;
 
-      // Prevent DebugInfoFinder from tagging this as the canonical subprogram,
-      // since the canonical one is in the incoming module.
-      SP->replaceFunction(nullptr);
+      NewSPs.push_back(SP);
     }
+
+    // Redirect operand to the overriding subprogram.
+    if (NewSPs.size() != SPs.getNumElements())
+      CU.replaceSubprograms(DIArray(MDNode::get(DstM->getContext(), NewSPs)));
   }
 }