Re: [Xen-devel] [PATCH] xen/x86: Fix build with clang following c/s 4fa0105

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.09.16 at 20:21, <andrew.cooper3@xxxxxxxxxx> wrote:
> https://travis-ci.org/xen-project/xen/jobs/158494027#L2344 
> 
> Clang complains:
> 
>   emulate.c:2016:14: error: comparison of unsigned enum expression < 0
>   is always false [-Werror,-Wtautological-compare]
>       if ( seg < 0 || seg >= ARRAY_SIZE(hvmemul_ctxt->seg_reg) )
>            ~~~ ^ ~
> 
> Clang is wrong to raise a warning like this.  The signed-ness of an enum is
> implementation defined in C, and robust code must not assume the choices made
> by the compiler.

Indeed.

> In this case, dropping the < 0 check creates a latent bug which would result
> in an array underflow when compiled with a compiler which chooses a signed
> enum.
> 
> Work around the bug by explicitly pulling seg into an unsigned integer, and
> only perform the upper bounds check.
> 
> No functional change.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


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