linux-kernel - Re: [PATCH] KVM: selftests: add -MP to CFLAGS

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Date:   Mon, 30 Oct 2023 10:22:51 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     David Laight <David.Laight@...LAB.COM>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, Shuah Khan <shuah@...nel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] KVM: selftests: add -MP to CFLAGS

On Mon, 2023-10-30 at 10:08 +0000, David Laight wrote:
> From: David Woodhouse
> > Sent: 30 October 2023 09:46
> > 
> > On Sun, 2023-10-29 at 21:13 +0000, David Laight wrote:
> > > From: David Woodhouse
> > > > Sent: 28 October 2023 20:35
> > > > 
> > > > Using -MD without -MP causes build failures when a header file is deleted
> > > > or moved. With -MP, the compiler will emit phony targets for the header
> > > > files it lists as dependencies, and the Makefiles won't refuse to attempt
> > > > to rebuild a C unit which no longer includes the deleted header.
> > > 
> > > Won't a phony target stop a header being built if there is
> > > an actual rule to build it?
> > 
> > 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?
> 
> I was sure that just adding
> 
> foo.h:
> 
> would generate a 'no rules to build' error.
> Maybe that was BSD make or SYS-V make.
> 
> 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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