[Linker] Support weak symbols in nodeduplicate COMDAT group

When a nodeduplicate COMDAT group contains a weak symbol, choose
a non-weak symbol (or one of the weak ones) rather than reporting
an error. This should address issue PR51394.

With the current IR representation, a generic comdat nodeduplicate
semantics is not representable for LTO. In the linker, sections and
symbols are separate concepts. A dropped weak symbol does not force the
defining input section to be dropped as well (though it can be collected
by GC). In the IR, when a weak linkage symbol is dropped, its associate
section content is dropped as well.

For InstrProfiling, which is where ran into this issue in PR51394, the
deduplication semantic is a sufficient workaround.

Differential Revision: https://reviews.llvm.org/D108689

1 files changed, 19 insertions, 3 deletions

diff --git a/llvm/lib/Linker/LinkModules.cpp b/llvm/lib/Linker/LinkModules.cpp
index 555aeea..2a44800 100644
--- a/llvm/lib/Linker/LinkModules.cpp
+++ b/llvm/lib/Linker/LinkModules.cpp
@@ -179,9 +179,25 @@ bool ModuleLinker::computeResultingSelectionKind(StringRef ComdatName,
     // Go with Dst.
     From = LinkFrom::Dst;
     break;
-  case Comdat::SelectionKind::NoDeduplicate:
-    return emitError("Linking COMDATs named '" + ComdatName +
-                     "': nodeduplicate has been violated!");
+  case Comdat::SelectionKind::NoDeduplicate: {
+    const GlobalVariable *DstGV;
+    const GlobalVariable *SrcGV;
+    if (getComdatLeader(DstM, ComdatName, DstGV) ||
+        getComdatLeader(*SrcM, ComdatName, SrcGV))
+      return true;
+
+    if (SrcGV->isWeakForLinker()) {
+      // Go with Dst.
+      From = LinkFrom::Dst;
+    } else if (DstGV->isWeakForLinker()) {
+      // Go with Src.
+      From = LinkFrom::Src;
+    } else {
+      return emitError("Linking COMDATs named '" + ComdatName +
+                       "': nodeduplicate has been violated!");
+    }
+    break;
+  }
   case Comdat::SelectionKind::ExactMatch:
   case Comdat::SelectionKind::Largest:
   case Comdat::SelectionKind::SameSize: {