Windows precompiled header support in GN

tools/gn/variables.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/variables.h"
 
 namespace variables {
 
 // Built-in variables ----------------------------------------------------------
 
@@ skipping 836 matching lines @@
     "\n"
     "  Outputs is valid for \"copy\", \"action\", and \"action_foreach\"\n"
     "  target types and indicates the resulting files. The values may contain\n"
     "  source expansions to generate the output names from the sources (see\n"
     "  \"gn help source_expansion\").\n"
     "\n"
     "  For copy targets, the outputs is the destination for the copied\n"
     "  file(s). For actions, the outputs should be the list of files\n"
     "  generated by the script.\n";
 
+const char kPrecompiledHeader[] = "precompiled_header";
+const char kPrecompiledHeader_HelpShort[] =
+    "precompiled_header: [string] Header file to precompile.";
+const char kPrecompiledHeader_Help[] =
+    "precompiled_header: [string] Header file to precompile.\n"
+    "\n"
+    "  Precompiled headers will be used when a target specifies this\n"
+    "  value, or a config applying to this target specifies this value.\n"
+    "  In addition, the tool corresponding to the source files must also\n"
+    "  specify precompiled headers (see \"gn help tool\"). The tool\n"
+    "  will also specify what type of precompiled headers to use.\n"
+    "\n"
+    "  The precompiled header/source variables can be specified on a target\n"
+    "  or a config, but must be the same for all configs applying to a given\n"
+    "  target since a target can only have one precompiled header.\n"
+    "\n"
+    "MSVC precompiled headers\n"
+    "\n"
+    "  When using MSVC-style precompiled headers, the \"precompiled_header\"\n"
+    "  value is a string corresponding to the header. This is NOT a path\n"
+    "  to a file that GN recognises, but rather the exact string that appears\n"
+    "  in quotes after an #include line in source code. The compiler will\n"
+    "  match this string against includes or forced includes (/FI).\n"
+    "\n"
+    "  MSVC also requires a source file to compile the header with. This must\n"
+    "  be specified by the \"precompiled_source\" value. In contrast to the\n"
+    "  header value, this IS a GN-style file name, and tells GN which source\n"
+    "  file to compile to make the .pch file used for subsequent compiles.\n"
+    "\n"
+    "  If you use both C and C++ sources, the precompiled header and source\n"
+    "  file will be compiled using both tools. You will want to make sure\n"
+    "  to wrap C++ includes in __cplusplus #ifdefs so the file will compile\n"
+    "  in C mode.\n"
+    "\n"
+    "  For example, if the toolchain specifies MSVC headers:\n"
+    "\n"
+    "    toolchain(\"vc_x64\") {\n"
+    "      ...\n"
+    "      tool(\"cxx\") {\n"
+    "        precompiled_header_type = \"msvc\"\n"
+    "        ...\n"
+    "\n"
+    "  You might make a config like this:\n"
+    "\n"
+    "    config(\"use_precompiled_headers\") {\n"
+    "      precompiled_header = \"build/precompile.h\"\n"
+    "      precompiled_source = \"//build/precompile.cc\"\n"
+    "\n"
+    "      # Either your source files should #include \"build/precompile.h\"\n"
+    "      # first, or you can do this to force-include the header.\n"
+    "      cflags = [ \"/FI$precompiled_header\" ]\n"
+    "    }\n"
+    "\n"
+    "  And then define a target that uses the config:\n"
+    "\n"
+    "    executable(\"doom_melon\") {\n"
+    "      configs += [ \":use_precompiled_headers\" ]\n"
+    "      ...\n"
+    "\n";
+
+const char kPrecompiledSource[] = "precompiled_source";
+const char kPrecompiledSource_HelpShort[] =
+    "precompiled_source: [file name] Source file to precompile.";
+const char kPrecompiledSource_Help[] =
+    "precompiled_source: [file name] Source file to precompile.\n"
+    "\n"
+    "  The source file that goes along with the precompiled_header when\n"
+    "  using \"msvc\"-style precompiled headers. It will be implicitly added\n"
+    "  to the sources of the target. See \"gn help precompiled_header\".\n";
+
 const char kPublic[] = "public";
 const char kPublic_HelpShort[] =
     "public: [file list] Declare public header files for a target.";
 const char kPublic_Help[] =
     "public: Declare public header files for a target.\n"
     "\n"
     "  A list of files that other targets can include. These permissions are\n"
     "  checked via the \"check\" command (see \"gn help check\").\n"
     "\n"
     "  If no public files are declared, other targets (assuming they have\n"
@@ skipping 247 matching lines @@
     INSERT_VARIABLE(Deps)
     INSERT_VARIABLE(ForwardDependentConfigsFrom)
     INSERT_VARIABLE(IncludeDirs)
     INSERT_VARIABLE(Inputs)
     INSERT_VARIABLE(Ldflags)
     INSERT_VARIABLE(Libs)
     INSERT_VARIABLE(LibDirs)
     INSERT_VARIABLE(OutputExtension)
     INSERT_VARIABLE(OutputName)
     INSERT_VARIABLE(Outputs)
+    INSERT_VARIABLE(PrecompiledHeader)
+    INSERT_VARIABLE(PrecompiledSource)
     INSERT_VARIABLE(Public)
     INSERT_VARIABLE(PublicConfigs)
     INSERT_VARIABLE(PublicDeps)
     INSERT_VARIABLE(Script)
     INSERT_VARIABLE(Sources)
     INSERT_VARIABLE(Testonly)
     INSERT_VARIABLE(Visibility)
   }
   return info_map;
 }
 
 #undef INSERT_VARIABLE
 
 }  // namespace variables