Rework handling of os and cpu_arch in GN.

tools/gn/args.cc

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "tools/gn/args.h"
 
 #include "base/sys_info.h"
 #include "build/build_config.h"
 #include "tools/gn/variables.h"
 
 const char kBuildArgs_Help[] =
     "Build Arguments Overview\n"
     "\n"
     "  Build arguments are variables passed in from outside of the build\n"
     "  that build files can query to determine how the build works.\n"
     "\n"
     "How build arguments are set\n"
     "\n"
     "  First, system default arguments are set based on the current system.\n"
     "  The built-in arguments are:\n"
-    "   - cpu_arch (by default this is the same as \"default_cpu_arch\")\n"
-    "   - default_cpu_arch\n"
-    "   - default_os\n"
-    "   - os (by default this is the same as \"default_os\")\n"
+    "   - host_cpu\n"
+    "   - host_os\n"
+    "   - current_cpu\n"
+    "   - current_os\n"
+    "   - target_cpu\n"
+    "   - target_os\n"
     "\n"
     "  If specified, arguments from the --args command line flag are used. If\n"
     "  that flag is not specified, args from previous builds in the build\n"
     "  directory will be used (this is in the file args.gn in the build\n"
     "  directory).\n"
     "\n"
     "  Last, for targets being compiled with a non-default toolchain, the\n"
     "  toolchain overrides are applied. These are specified in the\n"
     "  toolchain_args section of a toolchain definition. The use-case for\n"
     "  this is that a toolchain may be building code for a different\n"
@@ skipping 179 matching lines @@
   os = "win";
 #elif defined(OS_MACOSX)
   os = "mac";
 #elif defined(OS_LINUX)
   os = "linux";
 #elif defined(OS_ANDROID)
   os = "android";
 #else
   #error Unknown OS type.
 #endif
-  Value os_val(nullptr, std::string(os));
-  dest->SetValue(variables::kBuildOs, os_val, nullptr);
-  dest->SetValue(variables::kOs, os_val, nullptr);
 
   // Host architecture.
   static const char kX86[] = "x86";
   static const char kX64[] = "x64";
   static const char kArm[] = "arm";
   const char* arch = nullptr;
 
-  // Set the CPU architecture based on the underlying OS, not
+  // Set the host CPU architecture based on the underlying OS, not
   // whatever the current bit-tedness of the GN binary is.
   std::string os_arch = base::SysInfo::OperatingSystemArchitecture();
   if (os_arch == "x86")
     arch = kX86;
   else if (os_arch == "x86_64")
     arch = kX64;
   else if (os_arch.substr(3) == "arm")
     arch = kArm;
   else
     CHECK(false) << "OS architecture not handled.";
 
-  Value arch_val(nullptr, std::string(arch));
-  dest->SetValue(variables::kBuildCpuArch, arch_val, nullptr);
-  dest->SetValue(variables::kCpuArch, arch_val, nullptr);
-
   // Save the OS and architecture as build arguments that are implicitly
   // declared. This is so they can be overridden in a toolchain build args
   // override, and so that they will appear in the "gn args" output.
-  //
-  // Do not declare the build* variants since these shouldn't be changed.
-  //
+  Value empty_string(nullptr, std::string());
+
+  Value os_val(nullptr, std::string(os));
+  dest->SetValue(variables::kHostOs, os_val, nullptr);
+  dest->SetValue(variables::kTargetOs, empty_string, nullptr);
+  dest->SetValue(variables::kCurrentOs, empty_string, nullptr);
+
+  Value arch_val(nullptr, std::string(arch));
+  dest->SetValue(variables::kHostCpu, arch_val, nullptr);
+  dest->SetValue(variables::kTargetCpu, empty_string, nullptr);
+  dest->SetValue(variables::kCurrentCpu, empty_string, nullptr);
+
+  declared_arguments_[variables::kHostOs] = os_val;
+  declared_arguments_[variables::kCurrentOs] = empty_string;
+  declared_arguments_[variables::kTargetOs] = empty_string;
+  declared_arguments_[variables::kHostCpu] = arch_val;
+  declared_arguments_[variables::kCurrentCpu] = empty_string;
+  declared_arguments_[variables::kTargetCpu] = empty_string;
+
   // Mark these variables used so the build config file can override them
   // without geting a warning about overwriting an unused variable.
-  declared_arguments_[variables::kOs] = os_val;
-  declared_arguments_[variables::kCpuArch] = arch_val;
-  dest->MarkUsed(variables::kCpuArch);
-  dest->MarkUsed(variables::kOs);
+  dest->MarkUsed(variables::kHostCpu);
+  dest->MarkUsed(variables::kCurrentCpu);
+  dest->MarkUsed(variables::kTargetCpu);
+  dest->MarkUsed(variables::kHostOs);
+  dest->MarkUsed(variables::kCurrentOs);
+  dest->MarkUsed(variables::kTargetOs);
 }
 
 void Args::ApplyOverridesLocked(const Scope::KeyValueMap& values,
                                 Scope* scope) const {
   lock_.AssertAcquired();
   for (const auto& val : values)
     scope->SetValue(val.first, val.second, val.second.origin());
 }
 
 void Args::SaveOverrideRecordLocked(const Scope::KeyValueMap& values) const {
   lock_.AssertAcquired();
   for (const auto& val : values)
     all_overrides_[val.first] = val.second;
 }