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

[Jan Beulich <JBeulich@xxxxxxxx>]

On 29.07.2019 12:05, Julien Grall wrote:
> Hi,
> 
> On 7/17/19 7:47 AM, 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 quite like the strlen one, however looking around online this may not
> solve the problem. AFAIK, Clang is not happy because the constant is not
> a boolean.

I don't think it's as simple as "not a boolean", but I also haven't
played with it more than I had to in order to find possible
workarounds. As to the strlen() approach - I've expressed my dislike
before.

> So !! or != 0 should work here.

Well, with perhaps as much of a chance of breaking again as any
other of the possible solutions, unless they recognize and address
the oddity. Andrew - would you be willing to ack a v3 using !!
instead of re-ordering the && operands (seeing that you didn't ack
v2 yet, and perhaps you're also not meaning to)?

Jan
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