[build] Detect the host architecture of an ARM64 Mac as ARM64 even through Rosetta.

Lets AppJIT training happen directly, instead of the absurdity of arm64 (hardware) running x64 (Rosetta) running arm64 (VM's simulator).

Change-Id: Idbf82530d946099db80c550070257c4c6ead31e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/214763
Commit-Queue: Ryan Macnak <rmacnak@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>

build/config/clang/clang.gni

@@ -2,8 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+_toolchain_cpu = host_cpu
+if (host_os == "mac") {
+  # TODO(https://fxbug.dev/73385): Use arm64 toolchain on arm64 when it exists.
+  _toolchain_cpu = "x64"
+}
+
 default_clang_prefix =
-    rebase_path("//buildtools/${host_os}-${host_cpu}/clang/bin", root_build_dir)
+    rebase_path("//buildtools/${host_os}-${_toolchain_cpu}/clang/bin",
+                root_build_dir)
 
 declare_args() {
   clang_prefix = default_clang_prefix

tests/standalone/check_for_aot_snapshot_jit_test.dart

@@ -19,10 +19,7 @@ main() {
       "Can't locate gen_kernel$_batchSuffix on this platform");
   Expect.isTrue(File(genKernel).existsSync(),
       "Can't locate gen_kernel$_batchSuffix on this platform");
-  // Currently gen_snapshot is only in buildDir/clang_x64 on Mac ARM64.
-  final genSnapshot = Platform.isMacOS && buildDir.endsWith('XARM64')
-      ? path.join(buildDir, 'clang_x64', 'gen_snapshot$_execSuffix')
-      : path.join(buildDir, 'gen_snapshot$_execSuffix');
+  final genSnapshot = path.join(buildDir, 'gen_snapshot$_execSuffix');
   Expect.isTrue(File(genSnapshot).existsSync(),
       "Can't locate gen_snapshot$_execSuffix on this platform");
 

tests/standalone_2/check_for_aot_snapshot_jit_test.dart

@@ -19,10 +19,7 @@ main() {
       path.join(sdkDir, 'pkg', 'vm', 'tool', 'gen_kernel$_batchSuffix');
   Expect.isTrue(File(genKernel).existsSync(),
       "Can't locate gen_kernel$_batchSuffix on this platform");
-  // Currently gen_snapshot is only in buildDir/clang_x64 on Mac ARM64.
-  final genSnapshot = Platform.isMacOS && buildDir.endsWith('XARM64')
-      ? path.join(buildDir, 'clang_x64', 'gen_snapshot$_execSuffix')
-      : path.join(buildDir, 'gen_snapshot$_execSuffix');
+  final genSnapshot = path.join(buildDir, 'gen_snapshot$_execSuffix');
   Expect.isTrue(File(genSnapshot).existsSync(),
       "Can't locate gen_snapshot$_execSuffix on this platform");
 

tools/bots/test_matrix.json

@@ -2936,7 +2936,7 @@
             "create_sdk"
           ],
           "environment": {
-            "DART_GN_ARGS": "mac_use_goma_rbe=true dart_snapshot_kind=\"app-jit\""
+            "DART_GN_ARGS": "mac_use_goma_rbe=true"
           }
         },
         {

tools/gn.py

@@ -5,6 +5,7 @@
 
 import argparse
 import os
+import platform
 import subprocess
 import sys
 import time
@@ -66,7 +67,39 @@ def ToCommandLine(gn_args):
     return [merge(x, y) for x, y in gn_args.items()]
 
 
+# Runs true if the currently executing python interpreter is running under
+# Rosetta. I.e., python3 is an x64 executable and we're on an arm64 Mac.
+def IsRosetta():
+    if platform.system() == 'Darwin':
+        p = subprocess.Popen(['sysctl', '-in', 'sysctl.proc_translated'],
+                             stdout=subprocess.PIPE,
+                             stderr=subprocess.STDOUT)
+        output, _ = p.communicate()
+        return output.decode('utf-8').strip() == '1'
+    return False
+
+
 def HostCpuForArch(arch):
+    # Check for Rosetta before checking platform.machine(), as the latter
+    # returns 'x86_64' when running under Rosetta.
+    if IsRosetta():
+        if arch in ['x64', 'x64c']:
+            # Without this case, we would try to build with
+            #   host_cpu="arm64"
+            #   target_cpu="x64"
+            #   dart_target_arch="x64"
+            # Which requires the VM to use an x64 simulator in the host
+            # arm64 binaries, and this simulator is unimplemented.
+            return 'x64'
+        else:
+            return 'arm64'
+
+    m = platform.machine()
+    if m == 'aarch64' or m == 'arm64':
+        return 'arm64'
+    if m == 'armv7l' or m == 'armv6l':
+        return 'arm'
+
     if arch in ['ia32', 'arm', 'armv6', 'simarm', 'simarmv6', 'simarm_x64']:
         return 'x86'
     if arch in [