[Driver/Frontend] Add -print-target-triple

Add a -print-target-triple command line option to the Swift frontend
and driver to allow other tools (e.g., SwiftPM) to query the host
triple as it is understood by the Swift compiler. This follows the
precedent set by Clang. Implements rdar://problem/57434967.

include/swift/Frontend/FrontendOptions.h

@@ -247,6 +247,10 @@ public:
   /// Indicates whether full help (including "hidden" options) should be shown.
   bool PrintHelpHidden = false;
 
+  /// Indicates that the frontend should print the target triple and then
+  /// exit.
+  bool PrintTargetTriple = false;
+
   /// Should we sort SIL functions, vtables, witness tables, and global
   /// variables by name when we print it out. This eases diffing of SIL files.
   bool EmitSortedSIL = false;

include/swift/Option/Options.td

@@ -884,6 +884,9 @@ def target : Separate<["-"], "target">,
   HelpText<"Generate code for the given target <triple>, such as x86_64-apple-macos10.9">, MetaVarName<"<triple>">;
 def target_legacy_spelling : Joined<["--"], "target=">,
   Flags<[FrontendOption]>, Alias<target>;
+def print_target_triple : Flag<["-"], "print-target-triple">,
+    Flags<[FrontendOption]>,
+    HelpText<"Generate code for the given target <triple>, such as x86_64-apple-macos10.9">, MetaVarName<"<triple>">;
 
 def target_cpu : Separate<["-"], "target-cpu">, Flags<[FrontendOption, ModuleInterfaceOption]>,
   HelpText<"Generate code for a particular CPU variant">;

lib/Driver/Driver.cpp

@@ -2122,6 +2122,31 @@ bool Driver::handleImmediateArgs(const ArgList &Args, const ToolChain &TC) {
                                 std::end(commandArgs));
   }
 
+  if (Args.hasArg(options::OPT_print_target_triple)) {
+    SmallVector<const char *, 5> commandLine;
+    commandLine.push_back("-frontend");
+    commandLine.push_back("-print-target-triple");
+    if (const Arg *TargetArg = Args.getLastArg(options::OPT_target)) {
+      commandLine.push_back("-target");
+      commandLine.push_back(TargetArg->getValue());
+    }
+
+    std::string executable = getSwiftProgramPath();
+
+    sys::TaskQueue queue;
+    queue.addTask(executable.c_str(), commandLine);
+    queue.execute(nullptr,
+                  [](sys::ProcessId PID, int returnCode,
+                      StringRef output, StringRef errors,
+                      sys::TaskProcessInformation ProcInfo,
+                      void *unused) -> sys::TaskFinishedResponse {
+      llvm::outs() << output;
+      llvm::errs() << errors;
+      return sys::TaskFinishedResponse::ContinueExecution;
+    });
+    return false;
+  }
+
   return true;
 }
 

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -104,6 +104,10 @@ bool ArgsToFrontendOptionsConverter::convert(
   Opts.IgnoreSwiftSourceInfo |= Args.hasArg(OPT_ignore_module_source_info);
   computeHelpOptions();
 
+  if (Args.hasArg(OPT_print_target_triple)) {
+    Opts.PrintTargetTriple = true;
+  }
+
   if (const Arg *A = Args.getLastArg(OPT_verify_generic_signatures)) {
     Opts.VerifyGenericSignaturesInModule = A->getValue();
   }

lib/FrontendTool/FrontendTool.cpp

@@ -1987,6 +1987,11 @@ int swift::performFrontend(ArrayRef<const char *> Args,
     return finishDiagProcessing(0);
   }
 
+  if (Invocation.getFrontendOptions().PrintTargetTriple) {
+    llvm::outs() << Invocation.getLangOptions().Target.getTriple() << "\n";
+    return finishDiagProcessing(0);
+  }
+
   if (Invocation.getFrontendOptions().RequestedAction ==
       FrontendOptions::ActionType::NoneAction) {
     Instance->getDiags().diagnose(SourceLoc(),

test/Driver/print_target_triple.swift

@@ -0,0 +1,8 @@
+// RUN: %swift_driver -print-target-triple -target arm64-apple-ios12.0 | %FileCheck -check-prefix CHECK-IOS %s
+// RUN: %target-swift-frontend -print-target-triple -target arm64-apple-ios12.0 | %FileCheck -check-prefix CHECK-IOS %s
+
+// RUN: %swift_driver -print-target-triple -target x86_64-unknown-linux | %FileCheck -check-prefix CHECK-LINUX %s
+// RUN: %target-swift-frontend -print-target-triple -target x86_64-unknown-linux | %FileCheck -check-prefix CHECK-LINUX %s
+
+// CHECK-IOS: arm64-apple-ios12.0
+// CHECK-LINUX: x86_64-unknown-linux

test/Driver/print_target_triple_macos.swift

@@ -0,0 +1,10 @@
+// Test that -print-target-triple infers the host OS when that host OS is macOS
+//
+// We could duplicate this test for other host platforms.
+
+// RUN: %swift_driver -print-target-triple  | %FileCheck %s
+// RUN: %target-swift-frontend -print-target-triple | %FileCheck %s
+
+// REQUIRES: OS=macosx
+
+// CHECK: x86_64-apple-macosx