Re: [PATCH rdma-core] mlx4: Fix 1<<31 expressions

[Jason Gunthorpe <jgg-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>]

On Fri, Feb 09, 2018 at 04:01:51PM +0000, Bart Van Assche wrote:
> On Thu, 2018-02-08 at 16:38 -0700, Jason Gunthorpe wrote:
> >  enum {
> > -	MLX4_WQE_BIND_TYPE_2		= (1<<31),
> > +	MLX4_WQE_BIND_TYPE_2		= (1UL<<31),
> >  	MLX4_WQE_BIND_ZERO_BASED	= (1<<30),
> >  };
> 
> Hello Jason,
> 
> A quote from the C11 standard:
> 
> "The expression that defines the value of an enumeration constant shall be an
> integer constant expression that has a value representable as an int. [ ... ]
> Each enumerated type shall be compatible with char, a signed integer type, or
> an unsigned integer type. The choice of type is implementation-defined but
> shall be capable of representing the values of all the members of the
> enumeration."
> 
> Does that mean that this change relies on extensions to the C standard?

Yes, this relies on a common compiler extension to promote the type of
enum constants beyond int. AFAIK all compilers have done this forever,
kinda bonkers it isn't in C11, IMHO.

As I understand it, the current code is also non-conforming as (1<<31)
is apparently undefined behavior, and at least gcc throws a warning
for it in -Wpedantic mode.

I did a patch that will make our public headers mostly strictly
conforming to C11 with -Wpedantic set, which I've been wondering if it
is worth sending..

Jason
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