IRGen: Emit reflection metadata version into Swift binaries

Emit a 16-bit constant that tracks the version of the reflection
metadata emitted into binaries. This can be used to cross-check
what is supported by the SwiftRemoteMirror library with the new
version API.

rdar://problem/27251582

Author: bitjammer

lib/IRGen/GenReflection.cpp

@@ -930,3 +930,13 @@ void IRGenModule::emitFieldMetadataRecord(const NominalTypeDecl *Decl) {
   if (var)
     addUsedGlobal(var);
 }
+
+void IRGenModule::emitReflectionMetadataVersion() {
+  auto Init =
+    llvm::ConstantInt::get(Int16Ty, SWIFT_REFLECTION_METADATA_VERSION);
+  auto Version = new llvm::GlobalVariable(Module, Int16Ty, /*constant*/ true,
+                                          llvm::GlobalValue::LinkOnceODRLinkage,
+                                          Init,
+                                          "__swift_reflection_version");
+  addUsedGlobal(Version);
+}

lib/IRGen/IRGen.cpp

@@ -623,6 +623,7 @@ static std::unique_ptr<llvm::Module> performIRGeneration(IRGenOptions &Opts,
       IGM.emitProtocolConformances();
       IGM.emitTypeMetadataRecords();
       IGM.emitBuiltinReflectionMetadata();
+      IGM.emitReflectionMetadataVersion();
     }
 
     // Okay, emit any definitions that we suddenly need.

lib/IRGen/IRGenModule.h

@@ -743,6 +743,7 @@ class IRGenModule {
   void emitAssociatedTypeMetadataRecord(const ProtocolConformance *Conformance);
   void emitFieldMetadataRecord(const NominalTypeDecl *Decl);
   void emitBuiltinReflectionMetadata();
+  void emitReflectionMetadataVersion();
   std::string getBuiltinTypeMetadataSectionName();
   std::string getFieldTypeMetadataSectionName();
   std::string getAssociatedTypeMetadataSectionName();

test/IRGen/unused.sil

@@ -45,8 +45,8 @@ bb0:
   return %1 : $()
 }
 
-// CHECK-macho: @llvm.used = appending global [1 x i8*] [i8* bitcast (void ()* @frieda to i8*)], section "llvm.metadata", align 8
-// CHECK-elf: @llvm.used = appending global [2 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* getelementptr inbounds ([0 x i8], [0 x i8]* @_swift1_autolink_entries, i32 0, i32 0)], section "llvm.metadata", align 8
+// CHECK-macho: @llvm.used = appending global [2 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*)], section "llvm.metadata"
+// CHECK-elf: @llvm.used = appending global [3 x i8*] [i8* bitcast (void ()* @frieda to i8*), i8* bitcast (i16* @__swift_reflection_version to i8*), i8* getelementptr inbounds ([0 x i8], [0 x i8]* @_swift1_autolink_entries, i32 0, i32 0)], section "llvm.metadata"
 
 // CHECK: define linkonce_odr hidden void @qux()
 // CHECK: define hidden void @fred()