IRGen: lazily emit shared witness tables

This avoids emitting synthesized witness tables if they are not needed.

Author: eeckstein

lib/IRGen/IRGenModule.cpp

@@ -711,6 +711,12 @@ bool IRGenerator::canEmitWitnessTableLazily(SILWitnessTable *wt) {
   if (Opts.UseJIT)
     return false;
 
+  // Regardless of the access level, if the witness table is shared it means
+  // we can safely not emit it. Every other module which needs it will generate
+  // its own shared copy of it.
+  if (wt->getLinkage() == SILLinkage::Shared)
+    return true;
+
   NominalTypeDecl *ConformingTy =
     wt->getConformance()->getType()->getNominalOrBoundGenericNominal();
 

test/IRGen/objc_ns_enum.swift

@@ -12,7 +12,7 @@ import gizmo
 // CHECK: @"$sSo16NSRuncingOptionsVN" = linkonce_odr hidden constant
 //   CHECK-SAME: @"$sBi{{[0-9]+}}_WV"
 // CHECK: @"$sSo16NSRuncingOptionsVSQSCMc" = linkonce_odr hidden constant %swift.protocol_conformance_descriptor { {{.*}}@"$sSo16NSRuncingOptionsVSQSCWa
-// CHECK: @"$sSo28NeverActuallyMentionedByNameVSQSCWp" = linkonce_odr hidden constant
+// CHECK-NOT: @"$sSo28NeverActuallyMentionedByNameVSQSCWp" = linkonce_odr hidden constant
 
 // CHECK-LABEL: define{{( dllexport)?}}{{( protected)?}} i32 @main
 // CHECK:         call swiftcc %swift.metadata_response @"$sSo16NSRuncingOptionsVMa"(i64 0)