Driver: add support for baremetal targets

This allows building the Swift standard library for targets which may
not have an actual OS running.  This is identified by the OS field in
the target triple being set to `none`.

Author: compnerd

lib/Basic/LangOptions.cpp

@@ -327,6 +327,12 @@ std::pair<bool, bool> LangOptions::setTarget(llvm::Triple triple) {
   case llvm::Triple::WASI:
     addPlatformConditionValue(PlatformConditionKind::OS, "WASI");
     break;
+  case llvm::Triple::UnknownOS:
+    if (Target.getOSName() == "none") {
+      addPlatformConditionValue(PlatformConditionKind::OS, "none");
+      break;
+    }
+    LLVM_FALLTHROUGH;
   default:
     UnsupportedOS = true;
     break;

lib/Basic/Platform.cpp

@@ -175,8 +175,6 @@ static StringRef getPlatformNameForDarwin(const DarwinPlatformKind platform) {
 
 StringRef swift::getPlatformNameForTriple(const llvm::Triple &triple) {
   switch (triple.getOS()) {
-  case llvm::Triple::UnknownOS:
-    llvm_unreachable("unknown OS");
   case llvm::Triple::ZOS:
   case llvm::Triple::Ananas:
   case llvm::Triple::CloudABI:
@@ -234,6 +232,8 @@ StringRef swift::getPlatformNameForTriple(const llvm::Triple &triple) {
     return "haiku";
   case llvm::Triple::WASI:
     return "wasi";
+  case llvm::Triple::UnknownOS:
+    return "none";
   }
   llvm_unreachable("unsupported OS");
 }

lib/Driver/Driver.cpp

@@ -368,6 +368,8 @@ Driver::buildToolChain(const llvm::opt::InputArgList &ArgList) {
     return std::make_unique<toolchains::GenericUnix>(*this, target);
   case llvm::Triple::WASI:
     return std::make_unique<toolchains::GenericUnix>(*this, target);
+  case llvm::Triple::UnknownOS:
+    return std::make_unique<toolchains::GenericUnix>(*this, target);
   default:
     Diags.diagnose(SourceLoc(), diag::error_unknown_target,
                    ArgList.getLastArg(options::OPT_target)->getValue());

test/Driver/baremetal-target.swift

@@ -0,0 +1,4 @@
+// RUN: %swiftc_driver_plain -target aarch64-unknwon-none-none -driver-print-jobs %s 2>&1 | %FileCheck %s
+
+// CHECK: {{.*}}swift{{c|-frontend}}{{(.exe)?"?}} -frontend -c
+// CHECK: {{.*}}clang{{(.exe)?"?}} -fuse-ld=gold

test/Driver/options.swift

@@ -80,8 +80,8 @@
 // RUN: %swift_driver -g -### %s 2>&1 | %FileCheck -check-prefix=OPTIONS_BEFORE_FILE %s
 // OPTIONS_BEFORE_FILE: -g
 
-// RUN: not %swift_driver -target abc -### %s 2>&1 | %FileCheck -check-prefix=BAD_TARGET %s
-// BAD_TARGET: error: unknown target 'abc'
+// RUN: not %swiftc_driver_plain -target x86_64-unknown-hurd -### %s 2>&1 | %FileCheck -check-prefix=BAD_TARGET %s
+// BAD_TARGET: error: unknown target 'x86_64-unknown-hurd'
 
 // RUN: %swiftc_driver -incremental %s -### 2>&1 | %FileCheck -check-prefix=INCREMENTAL_WITHOUT_OFM %s
 // INCREMENTAL_WITHOUT_OFM: warning: ignoring -incremental (currently requires an output file map)