Re: Fixing "cast truncates bits from constant value"

[Christopher Li <sparse@chrisli.org>]

On Mon, Aug 17, 2009 at 2:02 PM, Pavel Roskin<proski@gnu.org> wrote:
> static unsigned short test(void)
> {
>        return (unsigned short)~0x8000;
> }
>
> test.c:3:32: warning: cast truncates bits from constant value (ffff7fff
> becomes 7fff)

That warning is legit because you ARE truncating constant value here.

> In my opinion, an explicit cast should be enough to suppress the
> warning.  But it's not.  Moreover, it's a "superwarning" that cannot
> even be suppressed by the "force" attribute!  This source still
> generates a warning:
>
> static unsigned short test(void)
> {
>        return (__attribute__((force)) unsigned short)~0x8000;

Sparse currently does not thing than just mark it as forced.


>
> But this source doesn't:
>
> static unsigned short test(void)
> {
>        return 0xfffff000U;
> }

I think sparse consider it as signed extend.
Truncating all 0xffff is consider OK for the negative case.

Chris
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