SIL: Don't serialize SILFunction declarations in witness tables if they don't have a valid linkage for "fragile ref".

E.g. a serialized witness table must not reference a private function. Just like a serialized function must not reference a private function.

Author: eeckstein

include/swift/SIL/SILWitnessTable.h

@@ -48,7 +48,8 @@ class SILWitnessTable : public llvm::ilist_node<SILWitnessTable>,
     /// The method required.
     SILDeclRef Requirement;
     /// The witness for the method.
-    /// This can be null in case dead function elimination has removed the method.
+    /// This can be null in case dead function elimination has removed the method
+    /// or if the method was not serialized (for de-serialized witness tables).
     SILFunction *Witness;
   };
 

lib/SIL/IR/SILDefaultWitnessTable.cpp

@@ -90,7 +90,8 @@ convertToDefinition(ArrayRef<Entry> entries) {
   // Bump the reference count of witness functions referenced by this table.
   for (auto entry : getEntries()) {
     if (entry.isValid() && entry.getKind() == SILWitnessTable::Method) {
-      entry.getMethodWitness().Witness->incrementRefCount();
+      if (SILFunction *f = entry.getMethodWitness().Witness)
+        f->incrementRefCount();
     }
   }
 }
@@ -105,7 +106,8 @@ SILDefaultWitnessTable::~SILDefaultWitnessTable() {
   // Drop the reference count of witness functions referenced by this table.
   for (auto entry : getEntries()) {
     if (entry.isValid() && entry.getKind() == SILWitnessTable::Method) {
-      entry.getMethodWitness().Witness->decrementRefCount();
+      if (SILFunction *f = entry.getMethodWitness().Witness)
+        f->decrementRefCount();
     }
   }
 }

lib/SIL/Verifier/SILVerifier.cpp

@@ -5958,6 +5958,8 @@ void SILDefaultWitnessTable::verify(const SILModule &M) const {
       continue;
 
     SILFunction *F = E.getMethodWitness().Witness;
+    if (!F)
+      continue;
 
 #if 0
     // FIXME: For now, all default witnesses are private.

lib/Serialization/SerializeSIL.cpp

@@ -2666,7 +2666,8 @@ void SILSerializer::writeSILWitnessTableEntry(
   SmallVector<ValueID, 4> ListOfValues;
   handleSILDeclRef(S, methodWitness.Requirement, ListOfValues);
   IdentifierID witnessID = 0;
-  if (SILFunction *witness = methodWitness.Witness) {
+  SILFunction *witness = methodWitness.Witness;
+  if (witness && witness->hasValidLinkageForFragileRef()) {
     addReferencedSILFunction(witness, true);
     witnessID = S.addUniquedStringRef(witness->getName());
   }