Re: [Xen-devel] [XEN PATCH v3 15/23] xen/build: have the root Makefile generates the CFLAGS

From: Anthony PERARD <anthony.perard@citrix.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Julien Grall <julien@xen.org>, Wei Liu <wl@xen.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Jan Beulich <jbeulich@suse.com>,
	xen-devel@lists.xenproject.org,
	Daniel De Graaf <dgdegra@tycho.nsa.gov>,
	Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
Subject: Re: [Xen-devel] [XEN PATCH v3 15/23] xen/build: have the root Makefile generates the CFLAGS
Date: Thu, 19 Mar 2020 16:24:12 +0000	[thread overview]
Message-ID: <20200319162412.GH4088@perard.uk.xensource.com> (raw)
In-Reply-To: <20200317180524.GA4088@perard.uk.xensource.com>

On Tue, Mar 17, 2020 at 06:05:24PM +0000, Anthony PERARD wrote:
> On Thu, Feb 27, 2020 at 12:05:04PM +0100, Roger Pau Monné wrote:
> > On Wed, Feb 26, 2020 at 11:33:47AM +0000, Anthony PERARD wrote:
> > > +ifneq ($(CONFIG_CC_IS_CLANG),y)
> > > +# Clang doesn't understand this command line argument, and doesn't appear to
> > > +# have an suitable alternative.  The resulting compiled binary does function,
> > > +# but has an excessively large symbol table.
> > > +CFLAGS += -Wa,--strip-local-absolute
> > 
> > This is not really related to clang, but to the assembler. If clang is
> > used with -no-integrated-as it's quite likely that the GNU assembler
> > will be used, and hence this option would be available.
> > 
> > Can we use cc-option-add here in order to detect whether the build
> > toolchain support the option?
> 
> That can be done, but I think I'll do that as a follow up of this patch,
> to avoid too many changes when moving the cflags around.
> 
> > Ideally this should be done after the integrated assembler tests
> > performed in x86/Rules.mk.

So, testing for the -Wa,--strip-local-absolute flags turns out to be
more complicated than I though it would be.
 - cc-option-add doesn't work because it doesn't test with the current list
   of CFLAGS. And if I add the CFLAGS, clang says the option is unused,
   it doesn't matter if -no-integrated-as is present or not.
 - I tried to use as-option macro from Linux but that comes with issues
   as well. I don't think that enough work as been done to make it work
   well with clang. (I probably need to filter -Wall out of the CFLAGS
   when testing, and they are probably other issues.)

So I think I leave the enhancement for later. Having the flag depends
on GCC is good enough for now.

Thanks,

-- 
Anthony PERARD

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