[Driver/IRGen] Put backward-deployment libraries into a table.

Describe the backward-deployment libraries via a preprocessor-driven
table. Macro-metaprogramming the two places in the code base---the
driver and IRGen---to use this tabble to determine which
backward-compatibility libraries to link against.

Author: DougGregor

include/swift/Frontend/BackDeploymentLibs.def

@@ -0,0 +1,31 @@
+//===------ BackDeploymentLibs.def - Backward Deployment Libraries --------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2014 - 2018 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
+//
+//===----------------------------------------------------------------------===//
+// Enumerates the backward deployment libraries that need to be linked
+// into Swift targets. Clients of this file must define the macro
+//
+//   BACK_DEPLOYMENT_LIB(Version, Filter, LibraryName)
+//
+// where:
+//   Version is a maximum Swift version written like a tuple, e.g., (5, 1)
+//   Filter is one of executable or all.
+//   LibraryName is the name of the library, e.g., "swiftCompatibility51"
+//===----------------------------------------------------------------------===//
+
+#ifndef BACK_DEPLOYMENT_LIB
+#  error "Must define BACK_DEPLOYMENT_LIB(Version, Filter, Library)"
+#endif
+
+BACK_DEPLOYMENT_LIB((5, 0), all, "swiftCompatibility50")
+BACK_DEPLOYMENT_LIB((5, 1), all, "swiftCompatibility51")
+BACK_DEPLOYMENT_LIB((5, 0), executable, "swiftCompatibilityDynamicReplacements")
+                    
+#undef BACK_DEPLOYMENT_LIB

lib/Driver/DarwinToolChains.cpp

@@ -330,6 +330,26 @@ toolchains::Darwin::addSanitizerArgs(ArgStringList &Arguments,
                                      /*shared=*/false);
 }
 
+namespace {
+
+enum class BackDeployLibFilter {
+  executable,
+  all
+};
+
+// Whether the given job matches the backward-deployment library filter.
+bool jobMatchesFilter(LinkKind jobKind, BackDeployLibFilter filter) {
+  switch (filter) {
+  case BackDeployLibFilter::executable:
+    return jobKind == LinkKind::Executable;
+    
+  case BackDeployLibFilter::all:
+    return true;
+  }
+}
+
+}
+
 void
 toolchains::Darwin::addArgsToLinkStdlib(ArgStringList &Arguments,
                                         const DynamicLinkJobAction &job,
@@ -359,47 +379,31 @@ toolchains::Darwin::addArgsToLinkStdlib(ArgStringList &Arguments,
   }
 
   if (runtimeCompatibilityVersion) {
-    if (*runtimeCompatibilityVersion <= llvm::VersionTuple(5, 0)) {
-      // Swift 5.0 compatibility library
-      SmallString<128> BackDeployLib;
-      BackDeployLib.append(SharedResourceDirPath);
-      llvm::sys::path::append(BackDeployLib, "libswiftCompatibility50.a");
-      
-      if (llvm::sys::fs::exists(BackDeployLib)) {
-        Arguments.push_back("-force_load");
-        Arguments.push_back(context.Args.MakeArgString(BackDeployLib));
-      }
-    }
+    auto addBackDeployLib = [&](llvm::VersionTuple version,
+                                BackDeployLibFilter filter,
+                                StringRef libraryName) {
+      if (*runtimeCompatibilityVersion > version)
+        return;
 
-    if (*runtimeCompatibilityVersion <= llvm::VersionTuple(5, 1)) {
-      // Swift 5.1 compatibility library
+      if (!jobMatchesFilter(job.getKind(), filter))
+        return;
+      
       SmallString<128> BackDeployLib;
       BackDeployLib.append(SharedResourceDirPath);
-      llvm::sys::path::append(BackDeployLib, "libswiftCompatibility51.a");
+      llvm::sys::path::append(BackDeployLib, "lib" + libraryName + ".a");
 
       if (llvm::sys::fs::exists(BackDeployLib)) {
         Arguments.push_back("-force_load");
         Arguments.push_back(context.Args.MakeArgString(BackDeployLib));
       }
-    }
+    };
+
+    #define BACK_DEPLOYMENT_LIB(Version, Filter, LibraryName) \
+      addBackDeployLib(                                       \
+          llvm::VersionTuple Version, BackDeployLibFilter::Filter, LibraryName);
+    #include "swift/Frontend/BackDeploymentLibs.def"
   }
 
-  if (job.getKind() == LinkKind::Executable) {
-    if (runtimeCompatibilityVersion)
-      if (*runtimeCompatibilityVersion <= llvm::VersionTuple(5, 0)) {
-        // Swift 5.0 dynamic replacement compatibility library.
-        SmallString<128> BackDeployLib;
-        BackDeployLib.append(SharedResourceDirPath);
-        llvm::sys::path::append(BackDeployLib,
-                                "libswiftCompatibilityDynamicReplacements.a");
-
-        if (llvm::sys::fs::exists(BackDeployLib)) {
-          Arguments.push_back("-force_load");
-          Arguments.push_back(context.Args.MakeArgString(BackDeployLib));
-        }
-      }
-  }
-
   // Add the runtime library link path, which is platform-specific and found
   // relative to the compiler.
   SmallVector<std::string, 4> RuntimeLibPaths;

lib/IRGen/GenDecl.cpp

@@ -471,28 +471,31 @@ void IRGenModule::emitSourceFile(SourceFile &SF) {
   // situations where it isn't useful, such as for dylibs, though this is
   // harmless aside from code size.
   if (!IRGen.Opts.UseJIT) {
-    if (auto compatibilityVersion
-          = IRGen.Opts.AutolinkRuntimeCompatibilityLibraryVersion) {
-      if (*compatibilityVersion <= llvm::VersionTuple(5, 0)) {
-        this->addLinkLibrary(LinkLibrary("swiftCompatibility50",
-                                         LibraryKind::Library,
-                                         /*forceLoad*/ true));
-      }
-      if (*compatibilityVersion <= llvm::VersionTuple(5, 1)) {
-        this->addLinkLibrary(LinkLibrary("swiftCompatibility51",
-                                         LibraryKind::Library,
-                                         /*forceLoad*/ true));
+    auto addBackDeployLib = [&](llvm::VersionTuple version,
+                                StringRef libraryName) {
+      Optional<llvm::VersionTuple> compatibilityVersion;
+      if (libraryName == "swiftCompatibilityDynamicReplacements") {
+        compatibilityVersion = IRGen.Opts.
+            AutolinkRuntimeCompatibilityDynamicReplacementLibraryVersion;
+      } else {
+        compatibilityVersion = IRGen.Opts.
+            AutolinkRuntimeCompatibilityLibraryVersion;
       }
-    }
 
-    if (auto compatibilityVersion =
-            IRGen.Opts.AutolinkRuntimeCompatibilityDynamicReplacementLibraryVersion) {
-      if (*compatibilityVersion <= llvm::VersionTuple(5, 0)) {
-        this->addLinkLibrary(LinkLibrary("swiftCompatibilityDynamicReplacements",
-                                         LibraryKind::Library,
-                                         /*forceLoad*/ true));
-      }
-    }
+      if (!compatibilityVersion)
+        return;
+
+      if (*compatibilityVersion > version)
+        return;
+
+      this->addLinkLibrary(LinkLibrary(libraryName,
+                                       LibraryKind::Library,
+                                       /*forceLoad*/ true));
+    };
+
+    #define BACK_DEPLOYMENT_LIB(Version, Filter, LibraryName)         \
+      addBackDeployLib(llvm::VersionTuple Version, LibraryName);
+    #include "swift/Frontend/BackDeploymentLibs.def"
   }
 }