[ABI] Optimize out the witness table pattern for resilient conformances.

Resilient conformances now put all witnesses into the resilient witness
table, so optimize away the witness table pattern for such cases. Teach
the runtime to fill in the protocol conformance descriptor, because that’s
the only bit of real information that would be in the pattern.

This is a minor optimization enabled by rdar://problem/46282080.

(cherry picked from commit f2dd852)

Author: DougGregor

lib/IRGen/GenProto.cpp

@@ -1918,9 +1918,7 @@ namespace {
       Flags = Flags.withNumConditionalRequirements(numConditional);
 
       // Relative reference to the witness table.
-      auto witnessTableRef =
-        ConstantReference(Description.pattern, ConstantReference::Direct);
-      B.addRelativeAddress(witnessTableRef);
+      B.addRelativeAddressOrNull(Description.pattern);
     }
 
     void addFlags() {
@@ -2244,19 +2242,27 @@ void IRGenModule::emitSILWitnessTable(SILWitnessTable *wt) {
   // Produce the initializer value.
   auto initializer = wtableContents.finishAndCreateFuture();
 
-  bool isDependent = isDependentConformance(conf, /*considerResilience=*/true);
-  auto global = cast<llvm::GlobalVariable>(
-    isDependent
-      ? getAddrOfWitnessTablePattern(cast<NormalProtocolConformance>(conf),
-                                     initializer)
-      : getAddrOfWitnessTable(conf, initializer));
-  global->setConstant(isConstantWitnessTable(wt));
-  global->setAlignment(getWitnessTableAlignment().getValue());
+  llvm::GlobalVariable *global = nullptr;
+  unsigned tableSize;
+  if (!isResilientConformance(conf)) {
+    bool isDependent =
+      isDependentConformance(conf, /*considerResilience=*/false);
+    global = cast<llvm::GlobalVariable>(
+      isDependent
+        ? getAddrOfWitnessTablePattern(cast<NormalProtocolConformance>(conf),
+                                       initializer)
+        : getAddrOfWitnessTable(conf, initializer));
+    global->setConstant(isConstantWitnessTable(wt));
+    global->setAlignment(getWitnessTableAlignment().getValue());
+    tableSize = wtableBuilder.getTableSize();
+  } else {
+    initializer.abandon();
+    tableSize = 0;
+  }
 
   // Collect the information that will go into the protocol conformance
   // descriptor.
-  ConformanceDescription description(conf, wt, global,
-                                     wtableBuilder.getTableSize(),
+  ConformanceDescription description(conf, wt, global, tableSize,
                                      wtableBuilder.getTablePrivateSize(),
                                      wtableBuilder.requiresSpecialization(),
                                      isDependentConformance(

stdlib/public/runtime/Metadata.cpp

@@ -4040,12 +4040,18 @@ WitnessTableCacheEntry::allocate(
   // Advance the address point; the private storage area is accessed via
   // negative offsets.
   auto table = fullTable + privateSizeInWords;
-  auto pattern = reinterpret_cast<void * const *>(
-                   &*conformance->getWitnessTablePattern());
-
-  // Fill in the provided part of the requirements from the pattern.
-  for (size_t i = 0, e = numPatternWitnesses; i < e; ++i) {
-    table[i] = pattern[i];
+  if (auto pattern =
+          reinterpret_cast<void * const *>(
+            &*conformance->getWitnessTablePattern())) {
+    // Fill in the provided part of the requirements from the pattern.
+    for (size_t i = 0, e = numPatternWitnesses; i < e; ++i) {
+      table[i] = pattern[i];
+    }
+  } else {
+    // Put the conformance descriptor in place. Instantiation will fill in the
+    // rest.
+    assert(numPatternWitnesses == 0);
+    table[0] = (void *)conformance;
   }
 
   // Copy any instantiation arguments that correspond to conditional

test/IRGen/newtype.swift

@@ -8,8 +8,8 @@ import Newtype
 
 // REQUIRES: objc_interop
 
-// Witness table for synthesized ClosedEnums : _ObjectiveCBridgeable.
-// CHECK: @"$sSo13SNTClosedEnumas21_ObjectiveCBridgeableSCWp" = linkonce_odr
+// Conformance descriptor for synthesized ClosedEnums : _ObjectiveCBridgeable.
+// CHECK: @"$sSo13SNTClosedEnumas21_ObjectiveCBridgeableSCMc" =
 
 // CHECK-LABEL: define swiftcc %TSo8NSStringC* @"$s7newtype14getErrorDomainSo08SNTErrorD0ayF"()
 public func getErrorDomain() -> ErrorDomain {

test/IRGen/objc_extensions.swift

@@ -180,7 +180,7 @@ extension NSDogcow {
   @NSManaged var woof: Int
 }
 
-// CHECK: @"$sSo8NSObjectC15objc_extensionsE8SomeEnum33_1F05E59585E0BB585FCA206FBFF1A92DLLOSQACWp" =
+// CHECK: @"$sSo8NSObjectC15objc_extensionsE8SomeEnum33_1F05E59585E0BB585FCA206FBFF1A92DLLOSQACMc" =
 
 class SwiftSubGizmo : SwiftBaseGizmo {
 

test/IRGen/protocol_conformance_records.swift

@@ -119,7 +119,7 @@ extension NativeGenericType : Spoon where T: Spoon {
 // -- nominal type descriptor
 // CHECK-SAME:           @"{{got.|__imp_}}$sSiMn"
 // -- witness table pattern
-// CHECK-SAME:           @"$sSi18resilient_protocol22OtherResilientProtocol0B20_conformance_recordsWp"
+// CHECK-SAME:           i32 0,
 // -- flags
 // CHECK-SAME:           i32 131144,
 // -- module context for retroactive conformance

test/IRGen/protocol_resilience.sil

@@ -78,33 +78,24 @@ protocol InternalProtocol {
   func f()
 }
 
-// Generic witness table pattern for ConformsWithRequirements : ProtocolWithRequirements
+// No generic witness table pattern for ConformsWithRequirements : ProtocolWithRequirements; it's all resilient
 
-// CHECK: @"$s19protocol_resilience24ConformsWithRequirementsV010resilient_A008ProtocoldE0AAWp" =
+// CHECK-NOT: @"$s19protocol_resilience24ConformsWithRequirementsV010resilient_A008ProtocoldE0AAWp" =
 
-// CHECK-SAME: internal global [1 x i8*] [
 
-// -- conformance descriptor
-// CHECK-SAME: "$s19protocol_resilience24ConformsWithRequirementsV010resilient_A008ProtocoldE0AAMc"
+// No witness table pattern for conformance with resilient associated type
 
-// CHECK-SAME: ]
+// CHECK-NOT: @"$s19protocol_resilience26ConformsWithResilientAssocVAA03HaseF0AAWp" = {{(protected )?}}internal 
 
 
-// Witness table pattern for conformance with resilient associated type
-
-// CHECK: @"$s19protocol_resilience26ConformsWithResilientAssocVAA03HaseF0AAWp" = {{(protected )?}}internal global [3 x i8*] [
-// CHECK-SAME:   @"associated conformance 19protocol_resilience26ConformsWithResilientAssocVAA03HaseF0AA1TAaDP_010resilient_A005OtherE8Protocol"
-// CHECK-SAME:   @"symbolic 19protocol_resilience23ResilientConformingTypeV"
-// CHECK-SAME: ]
-
 // Protocol conformance descriptor for ResilientConformingType : OtherResilientProtocol
 
 // CHECK: @"$s19protocol_resilience23ResilientConformingTypeV010resilient_A005OtherC8ProtocolAAMc" =
 
 // CHECK-SAME: i32 131072,
 
 // -- number of witness table entries
-// CHECK-SAME:   i16 1,
+// CHECK-SAME:   i16 0,
 
 // -- size of private area + 'requires instantiation' bit (not set)
 // CHECK-SAME:   i16 0,
@@ -134,7 +125,7 @@ protocol InternalProtocol {
 // CHECK-SAME: @secondWitness
 
 // -- number of witness table entries
-// CHECK-SAME:   i16 1,
+// CHECK-SAME:   i16 0,
 
 // -- size of private area + 'requires instantiation' bit (not set)
 // CHECK-SAME:   i16 0,

test/IRGen/protocol_resilience_descriptors.swift

@@ -61,8 +61,9 @@ public struct Y { }
 
 // CHECK-USAGE-LABEL: @"$s31protocol_resilience_descriptors1YV010resilient_A022OtherResilientProtocolAAMc" =
 // CHECK-USAGE-SAME: i32 131072,
-// CHECK-USAGE-SAME: i16 1,
-// CHECK-USAGE-SAME: i16 0
+// CHECK-USAGE-SAME: i16 0,
+// CHECK-USAGE-SAME: i16 0,
+// CHECK-USAGE-SAME: i32 0
 extension Y: OtherResilientProtocol { }
 
 // CHECK-USAGE: @"$s31protocol_resilience_descriptors29ConformsWithAssocRequirementsV010resilient_A008ProtocoleF12TypeDefaultsAAMc" =