Add macros for diagnostic control, use them in locale/weightwc.h

[Paul Eggert]

On 11/24/2014 05:29 PM, Joseph Myers wrote:
> We have the nscd/connections.c "ignoring return value of 'setuid', declared
> with attribute warn_unused_result [-Wunused-result]" (and similar for
> setgid), in a context where another id-setting call has already failed

Yes, we've run into that several times in GNU applications and we came 
up with a better way to fix it, in the Gnulib ignore-value module, which 
defines this:

# define ignore_value(x) \
     (__extension__ ({ __typeof__ (x) __x = (x); (void) __x; }))

One can then write "ignore_value (setuid (server_uid))" instead of 
"setuid (server_uid)".  So we shouldn't need a pragma for this one.

> We have, for some
> architecture/compiler combinations, "res_send.c:795:41: warning: 'resplen'
> may be used uninitialized in this function [-Wmaybe-uninitialized]" (a
> case where the relevant code only gets executed if the variable is
> initialized - the first time round a loop will execute a different if
> branch and cause it to be initialized).

Neither you nor Roland liked the IF_LINT approach that Gnulib uses for 
this sort of thing, but I hope we can figure out something else to 
satisfy glibc's constraints.  How about something like this?

#define ASSUME_MEM_INITIALIZED(v) asm ("" : "=m" (v))

and then write "ASSUME_MEM_INITIALIZED (resplen)" after declaring resplen.

> And that's it for warnings (on
> x86_64 / ARM / PowerPC, in my testing) for which suppression seems a
> sensible first approach.

In that case, it's not unreasonable to hope that we can avoid the 
pragmas entirely.