[ModuleInterfaces] Combine the normalized target triple into the cache hash

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -391,12 +391,15 @@ class ModuleInterfaceLoaderImpl {
     // anyways.
     H = hash_combine(H, interfacePath);
 
-    // Include the target CPU architecture. In practice, .swiftinterface files
-    // will be in architecture-specific subdirectories and would have
-    // architecture-specific pieces #if'd out. However, it doesn't hurt to
+    // Include the normalized target triple. In practice, .swiftinterface files
+    // will be in target-specific subdirectories and would have
+    // target-specific pieces #if'd out. However, it doesn't hurt to
     // include it, and it guards against mistakenly reusing cached modules
-    // across architectures.
-    H = hash_combine(H, SubInvocation.getLangOptions().Target.getArchName());
+    // across targets. Note that this normalization explicitly doesn't
+    // include the minimum deployment target (e.g. the '12.0' in 'ios12.0').
+    auto normalizedTargetTriple =
+        getTargetSpecificModuleTriple(SubInvocation.getLangOptions().Target);
+    H = hash_combine(H, normalizedTargetTriple.str());
 
     // The SDK path is going to affect how this module is imported, so include
     // it.

test/ModuleInterface/multiple-targets-same-interface.swift

@@ -0,0 +1,28 @@
+// RUN: %empty-directory(%t)
+// RUN: %empty-directory(%t/MCP)
+
+// This test makes sure that the module cache hash properly distinguishes
+// between different targets, even when the path to the module interface is
+// the same for multiple targets.
+
+// 1. Build a .swiftinterface for a dummy module.
+
+// RUN: %target-swift-frontend -typecheck -target x86_64-apple-macosx10.9 -emit-module-interface-path %t/SwiftModule.swiftinterface.tmp -parse-stdlib %s -module-name SwiftModule
+
+// 2. Remove the -target line from the .swiftinterface. Clients will build using their target.
+
+// RUN: sed -E 's/-target [^ ]+//g' %t/SwiftModule.swiftinterface.tmp > %t/SwiftModule.swiftinterface
+
+// 3. Build for a bunch of different x86_64 targets, and ensure they all succeed by putting something else in the module cache.
+
+// RUN: echo 'import SwiftModule' > %t/test.swift
+
+// RUN: %target-swift-frontend -typecheck -sdk '' -target x86_64-apple-macosx10.9 -module-cache-path %t/MCP -parse-stdlib -I %t %t/test.swift
+// RUN: %target-swift-frontend -typecheck -sdk '' -target x86_64-apple-tvos13.0   -module-cache-path %t/MCP -parse-stdlib -I %t %t/test.swift
+// RUN: %target-swift-frontend -typecheck -sdk '' -target x86_64-apple-ios10.0 -module-cache-path %t/MCP -parse-stdlib -I %t %t/test.swift
+
+// 4. Test iOS again, but with a newer minimum deployment target
+// RUN: %target-swift-frontend -typecheck -sdk '' -target x86_64-apple-ios13.0    -module-cache-path %t/MCP -parse-stdlib -I %t %t/test.swift
+
+// 5. Make sure there are only 3 .swiftmodules in the cache path (because iOS was reused)
+// RUN: ls %t/MCP/*.swiftmodule | count 3