[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

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: {

llvm/test/Linker/comdat-nodeduplicate.ll

@@ -3,14 +3,16 @@
 
 ; CHECK: error: Linking COMDATs named 'foo': nodeduplicate has been violated!
 
-; RUN: not llvm-link -S %t/2.ll %t/2-aux.ll 2>&1 | FileCheck %s --check-prefix=CHECK2
-; RUN: not llvm-link -S %t/2-aux.ll %t/2.ll 2>&1 | FileCheck %s --check-prefix=CHECK2
+; RUN: llvm-link -S %t/2.ll %t/2-aux.ll | FileCheck %s --check-prefix=CHECK2
+; RUN: llvm-link -S %t/2-aux.ll %t/2.ll | FileCheck %s --check-prefix=CHECK2
 
-; CHECK2: error: Linking COMDATs named 'foo'
+; CHECK2-DAG: @foo = global i64 2, section "data", comdat, align 8
+; CHECK2-DAG: @bar = weak global i64 0, section "cnts", comdat($foo)
+; CHECK2-DAG: @qux = weak_odr global i64 4, comdat($foo)
 
 ; RUN: not llvm-link -S %t/non-var.ll %t/non-var.ll 2>&1 | FileCheck %s --check-prefix=NONVAR
 
-; NONVAR: error: Linking COMDATs named 'foo': nodeduplicate has been violated!
+; NONVAR: error: Linking COMDATs named 'foo': GlobalVariable required for data dependent selection!
 
 ;--- 1.ll
 $foo = comdat nodeduplicate