Re: [Xen-devel] Ping: [PATCH v2] dom0-build: fix build with clang5

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jan 15, 2020 at 10:56:37AM +0100, Roger Pau Monné wrote:
> On Fri, Dec 20, 2019 at 05:26:34PM +0100, Jan Beulich wrote:
> > On 17.07.2019 08:47, Jan Beulich wrote:
> > > With non-empty CONFIG_DOM0_MEM clang5 produces
> > > 
> > > dom0_build.c:344:24: error: use of logical '&&' with constant operand 
> > > [-Werror,-Wconstant-logical-operand]
> > >      if ( !dom0_mem_set && CONFIG_DOM0_MEM[0] )
> > >                         ^  ~~~~~~~~~~~~~~~~~~
> > > dom0_build.c:344:24: note: use '&' for a bitwise operation
> > >      if ( !dom0_mem_set && CONFIG_DOM0_MEM[0] )
> > >                         ^~
> > >                         &
> > > dom0_build.c:344:24: note: remove constant to silence this warning
> > >      if ( !dom0_mem_set && CONFIG_DOM0_MEM[0] )
> > >                        ~^~~~~~~~~~~~~~~~~~~~~
> > > 1 error generated.
> > > 
> > > Obviously neither of the two suggestions are an option here. Oddly
> > > enough swapping the operands of the && helps, while e.g. casting or
> > > parenthesizing doesn't. Another workable variant looks to be the use of
> > > !! on the constant.
> > > 
> > > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > > ---
> > > v2: Also adjust the Arm incarnation of the same construct.
> > > ---
> > > I'm open to going the !! or yet some different route (but not really the
> > > suggested strlen() one). No matter which one we choose, I'm afraid it is
> > > going to remain guesswork what newer (and future) versions of clang will
> > > choke on.
> > 
> > I guess the disagreement on how to exactly address the issue has
> > stalled this. But I think we should rather have _some_ (e.g.
> > this) solution in the repo, than continue to ship versions which
> > don't build. People wanting to beautify the code further could
> > then submit incremental patches.
> 
> Acked-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> I'm not providing a RB because this is all guesswork, so it doesn't
> feel appropriate to review something that's based on undocumented
> compiler behavior.
> 
> Another option would be to pass -Wconstant-logical-operand but that
> would prevent caching some licit issues.

Forgot to mention, but could you please add a comment to note that the
condition is ordered this way to make clang5 happy?

Roger.

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