[Runtime] Add a disabled workaround for protocol conformance checking to check conformances in reverse order.

include/swift/Runtime/Bincompat.h

@@ -0,0 +1,31 @@
+//===--- Bincompat.h - Binary compatibility checks. -------------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Checks for enabling binary compatibility workarounds.
+//
+//===----------------------------------------------------------------------===//
+
+namespace swift {
+
+namespace runtime {
+
+namespace bincompat {
+
+/// Whether protocol conformance iteration should be reversed, to prefer
+/// conformances from images that are later in the list over earlier ones.
+bool workaroundProtocolConformanceReverseIteration();
+
+} // namespace bincompat
+
+} // namespace runtime
+
+} // namespace swift

stdlib/public/runtime/Bincompat.cpp

@@ -0,0 +1,31 @@
+//===--- Bincompat.cpp - Binary compatibility checks. -----------*- C++ -*-===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2020 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+//
+// Checks for enabling binary compatibility workarounds.
+//
+//===----------------------------------------------------------------------===//
+
+#include "swift/Runtime/Bincompat.h"
+
+namespace swift {
+
+namespace runtime {
+
+namespace bincompat {
+
+bool workaroundProtocolConformanceReverseIteration() { return false; }
+
+} // namespace bincompat
+
+} // namespace runtime
+
+} // namespace swift

stdlib/public/runtime/CMakeLists.txt

@@ -30,6 +30,7 @@ set(swift_runtime_sources
     Array.cpp
     AutoDiffSupport.cpp
     BackDeployment.cpp
+    Bincompat.cpp
     Casting.cpp
     CompatibilityOverride.cpp
     CygwinPort.cpp

stdlib/public/runtime/ProtocolConformance.cpp

@@ -16,6 +16,7 @@
 
 #include "swift/Basic/Lazy.h"
 #include "swift/Demangling/Demangle.h"
+#include "swift/Runtime/Bincompat.h"
 #include "swift/Runtime/Casting.h"
 #include "swift/Runtime/Concurrent.h"
 #include "swift/Runtime/HeapObject.h"
@@ -238,8 +239,11 @@ namespace {
 struct ConformanceState {
   ConcurrentReadableHashMap<ConformanceCacheEntry> Cache;
   ConcurrentReadableArray<ConformanceSection> SectionsToScan;
-
+  bool scanSectionsBackwards;
+
   ConformanceState() {
+    scanSectionsBackwards =
+        runtime::bincompat::workaroundProtocolConformanceReverseIteration();
     initializeProtocolConformanceLookup();
   }
 
@@ -466,8 +470,7 @@ swift_conformsToProtocolImpl(const Metadata *const type,
     return found.second;
 
   // Scan conformance records.
-  auto snapshot = C.SectionsToScan.snapshot();
-  for (auto &section : snapshot) {
+  auto processSection = [&](const ConformanceSection &section) {
     // Eagerly pull records for nondependent witnesses into our cache.
     for (const auto &record : section) {
       auto &descriptor = *record.get();
@@ -485,6 +488,15 @@ swift_conformsToProtocolImpl(const Metadata *const type,
         C.cacheResult(matchingType, protocol, witness, /*always cache*/ 0);
       }
     }
+  };
+
+  auto snapshot = C.SectionsToScan.snapshot();
+  if (C.scanSectionsBackwards) {
+    for (auto &section : llvm::reverse(snapshot))
+      processSection(section);
+  } else {
+    for (auto &section : snapshot)
+      processSection(section);
   }
 
   // Try the search again to look for the most specific cached conformance.

test/Runtime/protocol_conformance_collision.swift

@@ -0,0 +1,76 @@
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -o %t/newSDK %target-link-sdk-future-version
+// RUN: %target-codesign %t/newSDK
+// RUN: %target-run %t/newSDK newSDK
+// RUN: %target-build-swift %s -o %t/oldSDK %target-link-sdk-2020-version
+// RUN: %target-codesign %t/oldSDK
+// RUN: %target-run %t/oldSDK oldSDK
+
+// REQUIRES: VENDOR=apple
+
+// Simulators refuse to run binaries built with an SDK newer than the simulator.
+// UNSUPPORTED: DARWIN_SIMULATOR=ios
+// UNSUPPORTED: DARWIN_SIMULATOR=tvos
+// UNSUPPORTED: DARWIN_SIMULATOR=watchos
+
+import Accelerate
+import Foundation
+import StdlibUnittest
+import SwiftShims
+
+
+extension CFString: Hashable {
+  static var localHashableCallCount = 0
+  public var hashValue: Int {
+    Self.localHashableCallCount += 1
+    return (self as String).hashValue
+  }
+}
+
+protocol P {
+  func firstHashValue() -> Int
+}
+
+extension Set: P {
+  func firstHashValue() -> Int {
+    return first!.hashValue
+  }
+}
+
+@_optimize(none)
+func firstHashValue(_ x: P) -> Int {
+  x.firstHashValue()
+}
+
+let osHasWorkaround: Bool
+// These are deliberately NOT version 9999, as we don't want to hit the special
+// case where development runtimes always return true for 9999. This check needs
+// to be false until real version numbers are put in.
+if #available(macOS 99990, iOS 99990, tvOS 99990, watchOS 99990, *) {
+  osHasWorkaround = true
+} else {
+  osHasWorkaround = false
+}
+
+let testingOldSDK = CommandLine.arguments.last == "oldSDK"
+
+var tests: TestSuite
+
+if testingOldSDK {
+  tests = TestSuite("old SDK protocol conformance collision")
+  tests.test("CFString: Hashable conformance") {
+    _ = firstHashValue(NSSet(object: "Whatever") as! Set<CFString>)
+
+    let expectedCallCount = osHasWorkaround ? 1 : 0
+    expectEqual(expectedCallCount, CFString.localHashableCallCount)
+  }
+} else {
+  tests = TestSuite("new SDK protocol conformance collision")
+  tests.test("CFString: Hashable conformance") {
+    _ = firstHashValue(NSSet(object: "Whatever") as! Set<CFString>)
+
+    expectEqual(0, CFString.localHashableCallCount)
+  }
+}
+
+runAllTests()

test/lit.cfg

@@ -1107,6 +1107,31 @@ if run_vendor == 'apple':
 
     config.otool_classic = ("%s otool-classic" % (xcrun_prefix))
 
+    SDK_2020_VERSION = {
+        'macosx': '11.0',
+        'ios': '14.0',
+        'maccatalyst': '14.0',
+        'tvos': '14.0',
+        'watchos': '7.0'
+    }
+    sdk_2020_version = SDK_2020_VERSION.get(run_os, '')
+    linker_os = {
+        'iphoneos': 'ios',
+        'appletvos': 'tvos',
+        'watchos': 'watchos',
+        'iphonesimulator': 'ios-simulator',
+        'watchsimulator': 'watchos-simulator',
+        'appletvsimulator': 'tvos-simulator',
+        'macosx': 'macos'
+    }.get(config.target_sdk_name, run_os)
+
+    config.target_link_sdk_2020_version = (
+      "-Xlinker -platform_version -Xlinker %s -Xlinker %s -Xlinker %s" %
+      (linker_os, sdk_2020_version, sdk_2020_version))
+    config.target_link_sdk_future_version = (
+      "-Xlinker -platform_version -Xlinker %s -Xlinker %s -Xlinker %s" %
+      (linker_os, target_future_version, target_future_version))
+
 elif run_os in ['windows-msvc']:
     lit_config.note('Testing Windows ' + config.variant_triple)
     config.environment['NUMBER_OF_PROCESSORS'] = os.environ['NUMBER_OF_PROCESSORS']
@@ -2021,6 +2046,13 @@ config.substitutions.append(('%llvm-cov', config.llvm_cov))
 if hasattr(config, 'otool_classic'):
     config.substitutions.append(('%otool-classic', config.otool_classic))
 
+if hasattr(config, 'target_link_sdk_2020_version'):
+    config.substitutions.append(('%target-link-sdk-2020-version',
+                                 config.target_link_sdk_2020_version))
+if hasattr(config, 'target_link_sdk_future_version'):
+    config.substitutions.append(('%target-link-sdk-future-version',
+                                 config.target_link_sdk_future_version))
+
 run_filecheck = '%s %s --sanitize BUILD_DIR=%s --sanitize SOURCE_DIR=%s --use-filecheck %s %s' % (
         shell_quote(sys.executable),
         shell_quote(config.PathSanitizingFileCheck),