kbuild: Make NOSTDINC_FLAGS a simply expanded variable

dianders · masahir0y · commit 0c22be0712b8 · 2019-03-17T12:56:30.000+09:00
During a simple no-op (nothing changed) build I saw 39 invocations of the C compiler with the argument "-print-file-name=include". We don't need to call the C compiler 39 times for this--one time will suffice. Let's change NOSTDINC_FLAGS to a simply expanded variable to avoid this since there doesn't appear to be any reason it should be recursively expanded. On my build this shaved ~400 ms off my "no-op" build. Note that the recursive expansion seems to date back to the (really old) commit e8f5bdb ("[PATCH] Makefile include path ordering"). It's a little unclear to me if the point of that patch was to switch the variable to be recursively expanded (which it did) or to avoid directly assigning to NOSTDINC_FLAGS (AKA to switch to +=) because someone else (out of tree?) was setting it. I presume later since if the only goal was to switch to recursive expansion the patch would have just removed the ":". Signed-off-by: Douglas Anderson <dianders@chromium.org> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/Makefile b/Makefile
@@ -412,7 +412,7 @@ CHECK		= sparse
 
 CHECKFLAGS     := -D__linux__ -Dlinux -D__STDC__ -Dunix -D__unix__ \
 		  -Wbitwise -Wno-return-void -Wno-unknown-attribute $(CF)
-NOSTDINC_FLAGS  =
+NOSTDINC_FLAGS :=
 CFLAGS_MODULE   =
 AFLAGS_MODULE   =
 LDFLAGS_MODULE  =
