[PATCH] Fix maybe-uninitialized error on powerpc

[Joseph Myers]

On Thu, 9 Jan 2020, Matheus Castanho wrote:

> The build has been failing on powerpc64le-linux-gnu with GCC 10
> due to a maybe-uninitialized error:
> 
> ../sysdeps/ieee754/dbl-64/mpa.c:875:6: error: ‘w.e’ may be used
> uninitialized in this function [-Werror=maybe-uninitialized]
>   875 |   EY -= EX;
>       |      ^~
> 
> This commits adds proper initialization to avoid it.

glibc practice is not to add such initializations just to avoid warnings, 
if the warnings are false positives (rather, DIAG_* macros with 
appropriate comments, or __builtin_unreachable, are used instead).

I think what's going on here is that __dbl_mp may not set the exponent of 
a returned 0 value, but in fact a returned 0 value is impossible in the 
case where GCC is reporting uninitialized data.  If you agree with that 
analysis, does putting

  if (t == 0)
    __builtin_unreachable ();

(with a comment justifying *why* t cannot be 0), just before the __dbl_mp 
call in __inv, help with the warning?

-- 
Joseph S. Myers
joseph@codesourcery.com