Initial runtime changes to support relative protocol witness tables

include/swift/ABI/Metadata.h

@@ -1642,6 +1642,42 @@ using AssociatedWitnessTableAccessFunction =
                                  const Metadata *self,
                                  const WitnessTable *selfConformance);
 
+template<typename Runtime>
+using TargetRelativeProtocolConformanceDescriptorPointer =
+  RelativeIndirectablePointer<const TargetProtocolConformanceDescriptor<Runtime>,
+                              /*nullable*/ false>;
+
+/// A relative witness table for a protocol.
+///
+/// With the exception of the initial protocol conformance descriptor,
+/// the layout of a witness table is dependent on the protocol being
+/// represented.
+/// Entries are relative pointers.
+template <typename Runtime>
+class TargetRelativeWitnessTable {
+  /// The protocol conformance descriptor from which this witness table
+  /// was generated.
+  TargetRelativeProtocolConformanceDescriptorPointer<Runtime> Description;
+
+public:
+  const TargetProtocolConformanceDescriptor<Runtime> *getDescription() const {
+    // "Look through" dynamically allocated witness tables.
+    uintptr_t selfValue = (uintptr_t)this;
+    if (selfValue & 0x1) {
+      selfValue = selfValue & ~((uintptr_t)0x1);
+      auto selfAddr = (TargetRelativeWitnessTable<Runtime> **)selfValue;
+      return (*selfAddr)->Description;
+    }
+    return Description;
+  }
+};
+using RelativeWitnessTable = TargetRelativeWitnessTable<InProcess>;
+
+using AssociatedRelativeWitnessTableAccessFunction =
+  SWIFT_CC(swift) RelativeWitnessTable *(const Metadata *associatedType,
+                                 const Metadata *self,
+                                 const RelativeWitnessTable *selfConformance);
+
 /// The possible physical representations of existential types.
 enum class ExistentialTypeRepresentation {
   /// The type uses an opaque existential representation.

include/swift/Runtime/Metadata.h

@@ -354,7 +354,8 @@ swift_getWitnessTable(const ProtocolConformanceDescriptor *conformance,
                       const Metadata *type,
                       const void * const *instantiationArgs);
 
-const WitnessTable *
+SWIFT_RUNTIME_EXPORT
+const RelativeWitnessTable *
 swift_getWitnessTableRelative(const ProtocolConformanceDescriptor *conformance,
                       const Metadata *type,
                       const void * const *instantiationArgs);
@@ -377,7 +378,7 @@ MetadataResponse swift_getAssociatedTypeWitness(
 SWIFT_RUNTIME_EXPORT SWIFT_CC(swift)
 MetadataResponse swift_getAssociatedTypeWitnessRelative(
                                           MetadataRequest request,
-                                          WitnessTable *wtable,
+                                          RelativeWitnessTable *wtable,
                                           const Metadata *conformingType,
                                           const ProtocolRequirement *reqBase,
                                           const ProtocolRequirement *assocType);
@@ -401,8 +402,8 @@ const WitnessTable *swift_getAssociatedConformanceWitness(
                                   const ProtocolRequirement *assocConformance);
 
 SWIFT_RUNTIME_EXPORT SWIFT_CC(swift)
-const WitnessTable *swift_getAssociatedConformanceWitnessRelative(
-                                  WitnessTable *wtable,
+const RelativeWitnessTable *swift_getAssociatedConformanceWitnessRelative(
+                                  RelativeWitnessTable *wtable,
                                   const Metadata *conformingType,
                                   const Metadata *assocType,
                                   const ProtocolRequirement *reqBase,

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -414,6 +414,10 @@ function(_add_target_variant_c_compile_flags)
     list(APPEND result "-DSWIFT_STDLIB_CONCURRENCY_TRACING")
   endif()
 
+  if(SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES)
+    list(APPEND result "-DSWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES")
+  endif()
+
   list(APPEND result ${SWIFT_STDLIB_EXTRA_C_COMPILE_FLAGS})
 
   set("${CFLAGS_RESULT_VAR_NAME}" "${result}" PARENT_SCOPE)

stdlib/cmake/modules/StdlibOptions.cmake

@@ -230,3 +230,7 @@ option(SWIFT_STDLIB_CONCURRENCY_TRACING
   "Enable concurrency tracing in the runtime; assumes the presence of os_log(3)
    and the os_signpost(3) API."
   "${SWIFT_STDLIB_CONCURRENCY_TRACING_default}")
+
+option(SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES
+       "Use relative protocol witness tables"
+       FALSE)

stdlib/cmake/modules/SwiftSource.cmake

@@ -568,6 +568,10 @@ function(_compile_swift_files
     list(APPEND swift_flags "-experimental-hermetic-seal-at-link")
   endif()
 
+  if (SWIFT_STDLIB_USE_RELATIVE_PROTOCOL_WITNESS_TABLES)
+    list(APPEND swift_flags "-Xfrontend" "-enable-relative-protocol-witness-tables")
+  endif()
+
   if(SWIFT_STDLIB_DISABLE_INSTANTIATION_CACHES)
     list(APPEND swift_flags "-Xfrontend" "-disable-preallocated-instantiation-caches")
   endif()