linux-kernel - [PATCH] kbuild: Make NOSTDINC_FLAGS a simply expanded variable

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Date:   Thu, 14 Mar 2019 16:41:59 -0700
From:   Douglas Anderson <dianders@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: [PATCH] kbuild: Make NOSTDINC_FLAGS a simply expanded variable

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 e8f5bdb02ce0 ("[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@...omium.org>
---

 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 9ef547fc7ffe..3034ba66ad51 100644
--- a/Makefile
+++ b/Makefile
@@ -402,7 +402,7 @@ 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  =
-- 
2.21.0.360.g471c308f928-goog