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Message-ID: <e1cd904dc8973733fbb836bdabb28e2f13d8a5ab.camel@infradead.org>
Subject: Re: [PATCH] KVM: selftests: add -MP to CFLAGS
From:   David Woodhouse <dwmw2@infradead.org>
To:     David Laight <David.Laight@ACULAB.COM>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc:     Paolo Bonzini <pbonzini@redhat.com>, Shuah Khan <shuah@kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Date:   Mon, 30 Oct 2023 10:22:51 +0000
In-Reply-To: <aae76a24900444b7a812a21815a0b212@AcuMS.aculab.com>
References: <9fc8b5395321abbfcaf5d78477a9a7cd350b08e4.camel@infradead.org>
         <3fa5bdded7504d6582cf01f4db4cd6b4@AcuMS.aculab.com>
         <213451dcf8036133426aa04b6f377a16c6b53ff5.camel@infradead.org>
         <aae76a24900444b7a812a21815a0b212@AcuMS.aculab.com>
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On Mon, 2023-10-30 at 10:08 +0000, David Laight wrote:
> From: David Woodhouse
> > Sent: 30 October 2023 09:46
> >=20
> > On Sun, 2023-10-29 at 21:13 +0000, David Laight wrote:
> > > From: David Woodhouse
> > > > Sent: 28 October 2023 20:35
> > > >=20
> > > > Using -MD without -MP causes build failures when a header file is d=
eleted
> > > > or moved. With -MP, the compiler will emit phony targets for the he=
ader
> > > > files it lists as dependencies, and the Makefiles won't refuse to a=
ttempt
> > > > to rebuild a C unit which no longer includes the deleted header.
> > >=20
> > > Won't a phony target stop a header being built if there is
> > > an actual rule to build it?
> >=20
> > It probably would have taken you about the same time to find the answer
> > for yourself, as it took to write that email. Why don't you try it?
>=20
> I was sure that just adding
>=20
> foo.h:
>=20
> would generate a 'no rules to build' error.
> Maybe that was BSD make or SYS-V make.
>=20
> But calling the 'phony' is probably wrong.
> PHONY has a very specific meaning to make - and these aren't PHONY.

I recommend filing a bug against the GCC documentation then.

https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html#index-MP


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