Windows precompiled header support in GN

tools/gn/variables.cc

Index: tools/gn/variables.cc
diff --git a/tools/gn/variables.cc b/tools/gn/variables.cc
index 51a68d5728779ad5039c6847b5e2a9c5e3c5704a..472554ae964f57c484f687bafbc14e249c50533d 100644
--- a/tools/gn/variables.cc
+++ b/tools/gn/variables.cc
@@ -854,6 +854,76 @@ const char kOutputs_Help[] =
     "  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.";
@@ -1121,6 +1191,8 @@ const VariableInfoMap& GetTargetVariables() {
     INSERT_VARIABLE(OutputExtension)
     INSERT_VARIABLE(OutputName)
     INSERT_VARIABLE(Outputs)
+    INSERT_VARIABLE(PrecompiledHeader)
+    INSERT_VARIABLE(PrecompiledSource)
     INSERT_VARIABLE(Public)
     INSERT_VARIABLE(PublicConfigs)
     INSERT_VARIABLE(PublicDeps)