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"
+    "   - build_cpu_arch\n"
+    "   - build_os\n"
+    "   - cpu_arch (by default this is the same as \"build_cpu_arch\")\n"
+    "   - os (by default this is the same as \"build_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 81 matching lines @@
   ApplyOverridesLocked(toolchain_overrides, dest);
   SaveOverrideRecordLocked(toolchain_overrides);
 }
 
 bool Args::DeclareArgs(const Scope::KeyValueMap& args,
                        Scope* scope_to_set,
                        Err* err) const {
   base::AutoLock lock(lock_);
 
   for (const auto& arg : args) {
+    std::string arg_name = arg.first.as_string();
+
+    // TODO(dpranke): Figure out how to allow a declare_args() block
+    // to update the value of a built-in variable. I think we want
+    // something like:
+    //
+    // if (arg_name == variables::kBuildCpuArch ||
+    //     arg_name == variables::kBuildOs ||
+    //     arg_name == variables::kCpuArch ||
+    //     arg_name == variables::kOs ||
+    //     arg_name == variables::kTargetCpuArch ||
+    //     arg_name == variables::kTargetOs) {
+    //   Scope::KeyValueMap::iterator previously_declared =
+    //     declared_arguments_.find(arg.first);
+    //   CHECK(previously_declared != declared_arguments_.end());
+    //   if (previously_declared->second.origin() == nullptr) {
+    //     previously_declared->second.set_origin(arg.second.origin());
+    //     // TODO: update the built-in variable's value somehow.
+    //   }
+    // }
+

[Dirk Pranke, 2015/02/11 02:34:31]

Brett: I couldn't figure out how to make this work. Would it be better to
"undeclare" the predefined variable and "redeclare" it like it was a normal
declare_args() variable, or do some magic surgery to evaluate the value and
store it back in the original location?  

     // Verify that the value hasn't already been declared. We want each value
     // to be declared only once.
     //
     // The tricky part is that a buildfile can be interpreted multiple times
     // when used from different toolchains, so we can't just check that we've
     // seen it before. Instead, we check that the location matches.
     Scope::KeyValueMap::iterator previously_declared =
         declared_arguments_.find(arg.first);
     if (previously_declared != declared_arguments_.end()) {
       if (previously_declared->second.origin() != arg.second.origin()) {
@@ skipping 81 matching lines @@
 #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);
+  dest->SetValue(variables::kTargetOs, 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
   // 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);
+  dest->SetValue(variables::kTargetCpuArch, 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.
-  //
   // Mark these variables used so the build config file can override them
   // without geting a warning about overwriting an unused variable.
+  declared_arguments_[variables::kBuildOs] = os_val;
   declared_arguments_[variables::kOs] = os_val;
+  declared_arguments_[variables::kTargetOs] = os_val;
+  declared_arguments_[variables::kBuildCpuArch] = arch_val;
   declared_arguments_[variables::kCpuArch] = arch_val;
+  declared_arguments_[variables::kTargetCpuArch] = arch_val;
+  dest->MarkUsed(variables::kBuildCpuArch);
   dest->MarkUsed(variables::kCpuArch);
+  dest->MarkUsed(variables::kTargetCpuArch);
+  dest->MarkUsed(variables::kBuildOs);
   dest->MarkUsed(variables::kOs);
+  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;
 }