IRGen: be less aggressive about applying COMDAT

COMDAT can only be applied to definitions, not declarations.  This
manifested in builds of llbuild with SwiftPM on Windows.  The nominal
type descriptor accessor declaration was marked as COMDAT.

Author: compnerd

include/swift/IRGen/Linking.h

@@ -1231,7 +1231,7 @@ class ApplyIRLinkage {
   IRLinkage IRL;
 public:
   ApplyIRLinkage(IRLinkage IRL) : IRL(IRL) {}
-  void to(llvm::GlobalValue *GV) const {
+  void to(llvm::GlobalValue *GV, bool definition = true) const {
     llvm::Module *M = GV->getParent();
     const llvm::Triple Triple(M->getTargetTriple());
 
@@ -1244,11 +1244,14 @@ class ApplyIRLinkage {
     if (Triple.isOSBinFormatELF())
       return;
 
-    if (IRL.Linkage == llvm::GlobalValue::LinkOnceODRLinkage ||
-        IRL.Linkage == llvm::GlobalValue::WeakODRLinkage)
-      if (Triple.supportsCOMDAT())
-        if (llvm::GlobalObject *GO = dyn_cast<llvm::GlobalObject>(GV))
-          GO->setComdat(M->getOrInsertComdat(GV->getName()));
+    // COMDATs cannot be applied to declarations.  If we have a definition,
+    // apply the COMDAT.
+    if (definition)
+      if (IRL.Linkage == llvm::GlobalValue::LinkOnceODRLinkage ||
+          IRL.Linkage == llvm::GlobalValue::WeakODRLinkage)
+        if (Triple.supportsCOMDAT())
+          if (llvm::GlobalObject *GO = dyn_cast<llvm::GlobalObject>(GV))
+            GO->setComdat(M->getOrInsertComdat(GV->getName()));
   }
 };
 

lib/IRGen/GenDecl.cpp

@@ -2024,8 +2024,10 @@ llvm::Function *irgen::createFunction(IRGenModule &IGM,
     IGM.Module.getFunctionList().push_back(fn);
   }
 
-  ApplyIRLinkage({linkInfo.getLinkage(), linkInfo.getVisibility(), linkInfo.getDLLStorage()})
-      .to(fn);
+  ApplyIRLinkage({linkInfo.getLinkage(),
+                  linkInfo.getVisibility(),
+                  linkInfo.getDLLStorage()})
+      .to(fn, linkInfo.isForDefinition());
 
   llvm::AttrBuilder initialAttrs;
   IGM.constructInitialFnAttributes(initialAttrs, FuncOptMode);
@@ -2082,7 +2084,7 @@ llvm::GlobalVariable *swift::irgen::createVariable(
   ApplyIRLinkage({linkInfo.getLinkage(),
                   linkInfo.getVisibility(),
                   linkInfo.getDLLStorage()})
-      .to(var);
+      .to(var, linkInfo.isForDefinition());
   var->setAlignment(llvm::MaybeAlign(alignment.getValue()));
 
   // Everything externally visible is considered used in Swift.
@@ -4742,7 +4744,7 @@ IRGenModule::getAddrOfWitnessTableLazyAccessFunction(
   LinkInfo link = LinkInfo::get(*this, entity, forDefinition);
   entry = createFunction(*this, link, signature);
   ApplyIRLinkage({link.getLinkage(), link.getVisibility(), link.getDLLStorage()})
-      .to(entry);
+      .to(entry, link.isForDefinition());
   return entry;
 }
 

lib/IRGen/GenKeyPath.cpp

@@ -1329,7 +1329,7 @@ void IRGenModule::emitSILProperty(SILProperty *prop) {
       ApplyIRLinkage({linkInfo.getLinkage(),
                       linkInfo.getVisibility(),
                       llvm::GlobalValue::DLLExportStorageClass})
-          .to(GA);
+          .to(GA, linkInfo.isForDefinition());
     }
     return;
   }

lib/IRGen/GenMeta.cpp

@@ -3635,7 +3635,7 @@ static void emitObjCClassSymbol(IRGenModule &IGM,
       ptrTy->getElementType(), ptrTy->getAddressSpace(), link.getLinkage(),
       link.getName(), metadata, &IGM.Module);
   ApplyIRLinkage({link.getLinkage(), link.getVisibility(), link.getDLLStorage()})
-      .to(alias);
+      .to(alias, link.isForDefinition());
 }
 
 /// Emit the type metadata or metadata template for a class.

test/IRGen/Inputs/comdat1.swift

@@ -0,0 +1,5 @@
+private final class C {}
+public func f() {
+  var cs: [C] = []
+  cs.append(C())
+}

test/IRGen/Inputs/comdat2.swift

@@ -0,0 +1,4 @@
+public func g() {
+  var a: [Int] = []
+  a.append(1)
+}

test/IRGen/comdat.swift

@@ -0,0 +1,14 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-swift-frontend -emit-ir %S/Inputs/comdat1.swift %S/Inputs/comdat2.swift -O -num-threads 1 -module-name comdat -o %t/comdat1.ll -o %t/comdat2.ll
+// RUN: %FileCheck -check-prefix CHECK-1 %s < %t/comdat1.ll
+// RUN: %FileCheck -check-prefix CHECK-2 %s < %t/comdat2.ll
+
+// REQUIRES: OS=windows-msvc
+
+// Ensure that the definition is marked as COMDAT
+// CHECK-1: "$s6comdat1C33_{{.*}}LLCMa" = comdat any
+// CHECK-1: "$s6comdat1C33_{{.*}}LLCMn" = comdat any
+
+// Ensure that no foward declaration is emitted
+// CHECK-2-NOT: "$s6comdat1C33_{{.*}}LLCMa" = comdat any
+// CHECK-2-NOT: "$s6comdat1C33_{{.*}}LLCMn" = comdat any