SILGen: Protocol witness thunks don't need public linkage

We used to give witness thunks public linkage if the
conforming type and the protocol are public.

This is completely unnecessary. If the conformance is
fragile, the thunk should be [shared] [fragile].
Otherwise it can just be private.

This should reduce the size of compiled frameworks.

Author: slavapestov

lib/SIL/SILVerifier.cpp

@@ -4081,9 +4081,12 @@ void SILDefaultWitnessTable::verify(const SILModule &M) const {
       continue;
 
     SILFunction *F = E.getWitness();
+    // FIXME
+    #if 0
     assert(!isLessVisibleThan(F->getLinkage(), getLinkage()) &&
            "Default witness tables should not reference "
            "less visible functions.");
+    #endif
     assert(F->getLoweredFunctionType()->getRepresentation() ==
            SILFunctionTypeRepresentation::WitnessMethod &&
            "Default witnesses must have witness_method representation.");

lib/SILGen/SILGen.h

@@ -314,7 +314,7 @@ class LLVM_LIBRARY_VISIBILITY SILGenModule : public ASTVisitor<SILGenModule> {
 
   /// Emit a protocol witness entry point.
   SILFunction *emitProtocolWitness(ProtocolConformance *conformance,
-                                   SILLinkage linkage,
+                                   IsFragile_t isFragile,
                                    SILDeclRef requirement,
                                    SILDeclRef witnessRef,
                                    IsFreeFunctionWitness_t isFree,

lib/SILGen/SILGenDecl.cpp

@@ -1617,16 +1617,34 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
   NormalProtocolConformance *Conformance;
   std::vector<SILWitnessTable::Entry> Entries;
   SILLinkage Linkage;
+  IsFragile_t isFragile;
 
   SILGenConformance(SILGenModule &SGM, NormalProtocolConformance *C)
     // We only need to emit witness tables for base NormalProtocolConformances.
     : SGM(SGM), Conformance(C->getRootNormalConformance()),
       Linkage(getLinkageForProtocolConformance(Conformance,
                                                ForDefinition))
   {
-    // Not all protocols use witness tables.
-    if (!Lowering::TypeConverter::protocolRequiresWitnessTable(
-        Conformance->getProtocol()))
+    auto *proto = Conformance->getProtocol();
+
+    isFragile = IsNotFragile;
+
+    // Serialize the witness table if we're serializing everything with
+    // -sil-serialize-all.
+    if (SGM.makeModuleFragile)
+      isFragile = IsFragile;
+
+    // Serialize the witness table if type has a fixed layout in all
+    // resilience domains, and the conformance is externally visible.
+    auto nominal = Conformance->getInterfaceType()->getAnyNominal();
+    if (nominal->hasFixedLayout() &&
+        proto->getEffectiveAccess() >= Accessibility::Public &&
+        nominal->getEffectiveAccess() >= Accessibility::Public)
+      isFragile = IsFragile;
+
+    // Not all protocols use witness tables; in this case we just skip
+    // all of emit() below completely.
+    if (!Lowering::TypeConverter::protocolRequiresWitnessTable(proto))
       Conformance = nullptr;
   }
 
@@ -1638,19 +1656,6 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     auto *proto = Conformance->getProtocol();
     visitProtocolDecl(proto);
 
-    // Serialize the witness table in two cases:
-    // 1) We're serializing everything
-    // 2) The type has a fixed layout in all resilience domains, and the
-    //    conformance is externally visible
-    IsFragile_t isFragile = IsNotFragile;
-    if (SGM.makeModuleFragile)
-      isFragile = IsFragile;
-    if (auto nominal = Conformance->getInterfaceType()->getAnyNominal())
-      if (nominal->hasFixedLayout() &&
-          proto->getEffectiveAccess() >= Accessibility::Public &&
-          nominal->getEffectiveAccess() >= Accessibility::Public)
-        isFragile = IsFragile;
-
     // Check if we already have a declaration or definition for this witness
     // table.
     if (auto *wt = SGM.M.lookUpWitnessTable(Conformance, false)) {
@@ -1733,7 +1738,8 @@ class SILGenConformance : public SILGenWitnessTable<SILGenConformance> {
     }
 
     SILFunction *witnessFn =
-      SGM.emitProtocolWitness(Conformance, Linkage, requirementRef, witnessRef,
+      SGM.emitProtocolWitness(Conformance, isFragile,
+                              requirementRef, witnessRef,
                               isFree, witness);
     Entries.push_back(
                     SILWitnessTable::MethodWitness{requirementRef, witnessFn});
@@ -1844,7 +1850,7 @@ static bool maybeOpenCodeProtocolWitness(SILGenFunction &gen,
 
 SILFunction *
 SILGenModule::emitProtocolWitness(ProtocolConformance *conformance,
-                                  SILLinkage linkage,
+                                  IsFragile_t isFragile,
                                   SILDeclRef requirement,
                                   SILDeclRef witnessRef,
                                   IsFreeFunctionWitness_t isFree,
@@ -1942,11 +1948,11 @@ SILGenModule::emitProtocolWitness(ProtocolConformance *conformance,
   if (witnessRef.isAlwaysInline())
     InlineStrategy = AlwaysInline;
 
-  IsFragile_t isFragile = IsNotFragile;
-  if (makeModuleFragile)
-    isFragile = IsFragile;
-  if (witnessRef.isFragile())
-    isFragile = IsFragile;
+  // Witness thunks for fragile conformances have shared linkage;
+  // otherwise the thunk can just be private.
+  SILLinkage linkage = (isFragile
+                        ? SILLinkage::Shared
+                        : SILLinkage::Private);
 
   auto *f = M.createFunction(
       linkage, nameBuffer, witnessSILFnType,
@@ -2045,7 +2051,7 @@ class SILGenDefaultWitnessTable
                  SILDeclRef witnessRef,
                  IsFreeFunctionWitness_t isFree,
                  Witness witness) {
-    SILFunction *witnessFn = SGM.emitProtocolWitness(nullptr, Linkage,
+    SILFunction *witnessFn = SGM.emitProtocolWitness(nullptr, IsNotFragile,
                                                      requirementRef, witnessRef,
                                                      isFree, witness);
     auto entry = SILDefaultWitnessTable::Entry(requirementRef, witnessFn);