Re: [Xen-devel] [PATCH] Add -MP to CFLAGS along with -MMD.

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

David Woodhouse writes ("Re: [PATCH] Add -MP to CFLAGS along with -MMD."):
> On Tue, 2020-03-17 at 15:52 +0100, Jan Beulich wrote:
> > On 17.03.2020 15:34, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > > 
> > > This causes gcc (yes, and clang) to emit phony targets for each 
> > > dependency.
> > > 
> > > This means that when a header file is deleted, the C files which *used*
> > > to include it will no longer stop building with bogus out-of-date
> > > dependencies like this:
> > > 
> > >    make[5]: *** No rule to make target
> > >    '/home/dwmw2/git/xen/xen/include/asm/hvm/svm/amd-iommu-proto.h',
> > >    needed by 'p2m.o'. Stop.
> > 
> > In principle this would be nice, but there must be a reason this isn't
> > the default behavior. As the workaround for the issue at hand is quite
> > simple, I wouldn't like to treat addressing this one by some other
> > anomaly/quirk. Do you (or does anyone else) have insight into why this
> > isn't default behavior?
> 
> No.

I think this answer is:

I think it could interfere with other rules intended to build (or
rebuild) .h files.  This is particularly true for pattern or suffix
rules.  I would have to RTFM properly and think about it to understand
all the implications, to know what kind of nontrivial .h-rebuilding
rules might be affected (and therefore, to know whether we have any
such rules).

Sorry.

Ian.

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