Re: [Xen-devel] [PATCH] x86/dom0-build: fix build with clang5

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Mon, Jul 15, 2019 at 8:13 AM Jan Beulich <JBeulich@xxxxxxxx> wrote:
>
> On 15.07.2019 15:41, Tamas K Lengyel wrote:
> > On Mon, Jul 15, 2019 at 4:36 AM Jan Beulich <JBeulich@xxxxxxxx> 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>
> >> ---
> >> I'm open to going the !! or yet some different route. 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.
> >
> > Is disabling the check itself not an option? Seems to me to be a more
> > sensible option then hacking around it.
>
> How would you envision to disable the check? It's there for a reason
> after all.

By passing -Wno-constant-logical-operand to the compiler. It looks
like a check for a non-common situation which evidently Xen uses, so
what's the point of keeping it but hacking around with it with tricks
that are fragile?

Tamas

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