[clang][driver] set macOS as the target OS for -arch arm64 when clang

is running on an Apple Silicon mac

This change allows users to use `-arch arm64` to build for mac when
running it on Apple Silicon mac without explicit `-target` option.

Differential Revision: https://reviews.llvm.org/D82428

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -1672,8 +1672,16 @@ inferDeploymentTargetFromArch(DerivedArgList &Args, const Darwin &Toolchain,
   llvm::Triple::OSType OSTy = llvm::Triple::UnknownOS;
 
   StringRef MachOArchName = Toolchain.getMachOArchName(Args);
-  if (MachOArchName == "armv7" || MachOArchName == "armv7s" ||
-      MachOArchName == "arm64")
+  if (MachOArchName == "arm64") {
+#if __arm64__
+    // A clang running on an Apple Silicon mac defaults
+    // to building for mac when building for arm64 rather than
+    // defaulting to iOS.
+    OSTy = llvm::Triple::MacOSX;
+#else
+    OSTy = llvm::Triple::IOS;
+#endif
+  } else if (MachOArchName == "armv7" || MachOArchName == "armv7s")
     OSTy = llvm::Triple::IOS;
   else if (MachOArchName == "armv7k" || MachOArchName == "arm64_32")
     OSTy = llvm::Triple::WatchOS;

clang/test/Driver/apple-arm64-arch.c

@@ -0,0 +1,6 @@
+// RUN: env SDKROOT="/" %clang -arch arm64 -c -### %s 2>&1 | \
+// RUN:   FileCheck %s
+//
+// XFAIL: apple-silicon-mac
+//
+// CHECK: "-triple" "arm64-apple-ios{{[0-9.]+}}"

clang/test/Driver/apple-silicon-arch.c

@@ -0,0 +1,6 @@
+// RUN: env SDKROOT="/" %clang -arch arm64 -c -### %s 2>&1 | \
+// RUN:   FileCheck %s
+//
+// REQUIRES: apple-silicon-mac
+//
+// CHECK: "-triple" "arm64-apple-macosx{{[0-9.]+}}"

clang/test/lit.cfg.py

@@ -155,6 +155,10 @@ def is_filesystem_case_insensitive():
 if not re.match(r'.*-(cygwin)$', config.target_triple):
     config.available_features.add('clang-driver')
 
+# Tests that are specific to the Apple Silicon macOS.
+if re.match(r'^arm64(e)?-apple-(macos|darwin)', config.target_triple):
+    config.available_features.add('apple-silicon-mac')
+
 # [PR18856] Depends to remove opened file. On win32, a file could be removed
 # only if all handles were closed.
 if platform.system() not in ['Windows']: