[clang][driver] allow `-arch arm64` to be used to build for mac when on Apple Silicon Mac without explicit `-target`

[arphaman]

Summary

This patch allows a user to compile for the arm64-apple-macos target when invoking a clang running on an Apple Silicon machine by passing -arch arm64 only.

[steven_wu]

LGTM.

Not sure if it makes more sense to break the patch into two commits:

• config.guess change is for building the correct host triple on apple silicon machine without explicitly specify it.
• the driver change is for better default on Apple silicon Mac.

[arphaman]

In D82428#2110506, @steven_wu wrote:

LGTM.

Not sure if it makes more sense to break the patch into two commits:

• config.guess change is for building the correct host triple on apple silicon machine without explicitly specify it.
• the driver change is for better default on Apple silicon Mac.

That's a good idea, I will do that.

clang/lib/Driver/ToolChains/Darwin.cpp

 /// Tries to infer the target OS from the -arch.
 Optional<DarwinPlatform>
 inferDeploymentTargetFromArch(DerivedArgList &Args, const Darwin &Toolchain,
                               const llvm::Triple &Triple,
                               const Driver &TheDriver) {
   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;
   else if (MachOArchName != "armv6m" && MachOArchName != "armv7m" &&
            MachOArchName != "armv7em")
     OSTy = llvm::Triple::MacOSX;
 
   if (OSTy == llvm::Triple::UnknownOS)

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

+// 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

+// 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

 # [PR8833] LLP64-incompatible tests
 if not re.match(r'^x86_64.*-(windows-msvc|windows-gnu)$', config.target_triple):
     config.available_features.add('LP64')
 
 # [PR12920] "clang-driver" -- set if gcc driver is not used.
 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']:
     config.available_features.add('can-remove-opened-file')
 
 
 def calculate_arch_features(arch_string):
     features = []

llvm/cmake/config.guess

case $UNAME_PROCESSOR in
     if [ "$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
           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}
   exit ;;
     *:procnto*:*:* | *:QNX:[0123456789]*:*)
   UNAME_PROCESSOR=`uname -p`
   if test "$UNAME_PROCESSOR" = "x86"; then
     UNAME_PROCESSOR=i386