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

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Message-ID: <CAD=FV=XqH7knZwK+a+bsACwc-j2NgH914mLTc_POR_GAipVf7w@mail.gmail.com>
Date:   Fri, 15 Mar 2019 08:40:52 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Masahiro Yamada <yamada.masahiro@...ionext.com>
Cc:     Michal Marek <michal.lkml@...kovi.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH] kbuild: Make NOSTDINC_FLAGS a simply expanded variable

Hi,

On Fri, Mar 15, 2019 at 8:37 AM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> On Sat, Mar 16, 2019 at 12:27 AM Masahiro Yamada
> <yamada.masahiro@...ionext.com> wrote:
> >
> > On Fri, Mar 15, 2019 at 8:42 AM Douglas Anderson <dianders@...omium.org> wrote:
> > >
> > > 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>
> >
> > Applied to linux-kbuild.
> > Thanks.
>
>
> BTW, I noticed no one else
> appends NOSTDINC_FLAGS.
>
>
> So, does it make more sense to do as follows?
>
>
> NOSTDINC_FLAGS := -nostdinc -isystem $(shell $(CC) -print-file-name=include)

It might.  ...but go back to look at commit e8f5bdb02ce0 ("[PATCH]
Makefile include path ordering").  Why did they do a += there since
even then nobody was setting it.  Try:

$ git grep NOSTDINC_FLAGS e8f5bdb02ce0~


Do you understand what was going on in that commit?

-Doug