Windows precompiled header support in GN

build/config/BUILD.gn

Index: build/config/BUILD.gn
diff --git a/build/config/BUILD.gn b/build/config/BUILD.gn
index 9fc86fd2703a36aa877d146a9b158b900e94a577..7c8c45a6978a51e4ed7a4c150a7e063c8d042933 100644
--- a/build/config/BUILD.gn
+++ b/build/config/BUILD.gn
@@ -396,3 +396,31 @@ config("default_libs") {
     libs = [ "dl" ]
   }
 }
+
+# Add this config to your target to enable precompiled headers.
+#
+# On Windows, precompiled headers are done on a per-target basis. If you have
+# just a couple of files, the time it takes to precompile (~2 seconds) can
+# actually be longer than the time saved. On a Z620, a 100 file target compiles
+# about 2 seconds faster with precompiled headers, with greater savings for
+# larger targets.
+#
+# Recommend precompiled headers for targets with more than 50 .cc files.
+config("precompiled_headers") {
+  # TODO(brettw) enable this when GN support in the binary has been rolled.
+  #if (is_win) {
+  if (false) {
+    # This is a string rather than a file GN knows about. It has to match
+    # exactly what's in the /FI flag below, and what might appear in the source
+    # code in quotes for an #include directive.
+    precompiled_header = "build/precompile.h"
+
+    # This is a file that GN will compile with the above header. It will be
+    # implicitly added to the sources (potentially multiple times, with one
+    # variant for each language used in the target).
+    precompiled_source = "//build/precompile.cc"
+
+    # Force include the header.
+    cflags = [ "/FI$precompiled_header" ]
+  }
+}