Index: clang/lib/Driver/ToolChains/Darwin.cpp
===================================================================
--- clang/lib/Driver/ToolChains/Darwin.cpp
+++ clang/lib/Driver/ToolChains/Darwin.cpp
@@ -1672,8 +1672,16 @@
   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;
Index: clang/test/Driver/apple-arm64-arch.c
===================================================================
--- /dev/null
+++ 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.]+}}"
Index: clang/test/Driver/apple-silicon-arch.c
===================================================================
--- /dev/null
+++ 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.]+}}"
Index: clang/test/lit.cfg.py
===================================================================
--- clang/test/lit.cfg.py
+++ clang/test/lit.cfg.py
@@ -155,6 +155,10 @@
 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']:
Index: llvm/cmake/config.guess
===================================================================
--- llvm/cmake/config.guess
+++ llvm/cmake/config.guess
@@ -1263,6 +1263,23 @@
 		      UNAME_PROCESSOR="x86_64"
 		  fi
 		fi ;;
+		arm)
+		eval $set_cc_for_build
+		if [ "$CC_FOR_BUILD" != 'no_compiler_found' ]; then
+			if (echo '#ifdef __LP64__'; echo IS_64BIT_ARCH; echo '#endif') | \
+				(CCOPTS= $CC_FOR_BUILD -E - 2>/dev/null) | \
+				grep IS_64BIT_ARCH >/dev/null
+			then
+				if (echo '#ifdef __PTRAUTH_INTRINSICS__'; echo HAS_AUTH; echo '#endif') | \
+					(CCOPTS= $CC_FOR_BUILD -E - 2>/dev/null) | \
+					grep HAS_AUTH >/dev/null
+				then
+					UNAME_PROCESSOR="arm64e"
+				else
+					UNAME_PROCESSOR="arm64"
+				fi
+			fi
+		fi ;;
 	    unknown) UNAME_PROCESSOR=powerpc ;;
 	esac
 	echo ${UNAME_PROCESSOR}-apple-darwin${UNAME_RELEASE}