[gn] Automatically derive build configs in test runner.

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index b1c71dd0a76d7e12222dbf85a2465ab4e5dd1075..09f301854a46a1006d949d26c103e5f8b2572720 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -30,6 +30,7 @@
 
 from collections import OrderedDict
 import itertools
+import json
 import multiprocessing
 import optparse
 import os
@@ -434,6 +435,34 @@ def ProcessOptions(options):
   global EXHAUSTIVE_VARIANTS
   global VARIANTS
 
+  # First try to auto-detect configurations based on the build if GN was
+  # used. This can't be overridden by cmd-line arguments.
+  options.auto_detect = False
+  build_config_path = os.path.join(
+      BASE_DIR, options.outdir, "v8_build_config.json")
+  if os.path.exists(build_config_path):
+    try:
+      with open(build_config_path) as f:
+        build_config = json.load(f)
+    except Exception:
+      print ("%s exists but contains invalid json. Is your build up-to-date?" %
+             build_config_path)
+      return False
+    options.auto_detect = True
+
+    options.arch_and_mode = None
+    options.arch = build_config["v8_target_cpu"]
+    if options.arch == 'x86':
+      # TODO(machenbach): Transform all to x86 eventually.
+      options.arch = 'ia32'
+    options.asan = build_config["is_asan"]
+    options.dcheck_always_on = build_config["dcheck_always_on"]
+    options.mode = 'debug' if build_config["is_debug"] else 'release'
+    options.msan = build_config["is_msan"]
+    options.no_i18n = not build_config["v8_enable_i18n_support"]
+    options.no_snap = not build_config["v8_use_snapshot"]
+    options.tsan = build_config["is_tsan"]
+
   # Architecture and mode related stuff.
   if options.arch_and_mode:
     options.arch_and_mode = [arch_and_mode.split(".")
@@ -667,6 +696,10 @@ def Execute(arch, mode, args, options, suites):
       # buildbot. Currently this is capitalized Release and Debug.
       shell_dir = os.path.join(BASE_DIR, options.outdir, mode)
       mode = BuildbotToV8Mode(mode)
+    elif options.auto_detect:
+      # If an output dir with a build was passed, test directly in that
+      # directory.
+      shell_dir = os.path.join(BASE_DIR, options.outdir)
     else:
       shell_dir = os.path.join(
           BASE_DIR,
@@ -712,6 +745,8 @@ def Execute(arch, mode, args, options, suites):
                         sancov_dir=options.sancov_dir)
 
   # TODO(all): Combine "simulator" and "simulator_run".
+  # TODO(machenbach): In GN we can derive simulator run from
+  # target_arch != v8_target_arch in the dumped build config.
   simulator_run = not options.dont_skip_simulator_slow_tests and \
       arch in ['arm64', 'arm', 'mipsel', 'mips', 'mips64', 'mips64el', \
                'ppc', 'ppc64'] and \