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SubjectRe: [PATCH] kbuild: Make NOSTDINC_FLAGS a simply expanded variable
Hi,

On Fri, Mar 15, 2019 at 8:37 AM Masahiro Yamada
<yamada.masahiro@socionext.com> wrote:
>
> On Sat, Mar 16, 2019 at 12:27 AM Masahiro Yamada
> <yamada.masahiro@socionext.com> wrote:
> >
> > On Fri, Mar 15, 2019 at 8:42 AM Douglas Anderson <dianders@chromium.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@chromium.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

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