[PATCH] Get rid of array-bounds warning in __kernel_rem_pio2[f] with gcc 6.1 -O3.

[Joseph Myers]

On Thu, 7 Jul 2016, Stefan Liebler wrote:

> Hi,
> 
> while testing, I've got the following werror on s390x when build with gcc 6.1
> (or current gcc head) and -O3:
> ../sysdeps/ieee754/dbl-64/k_rem_pio2.c: In function ‘__kernel_rem_pio2’:
> ../sysdeps/ieee754/dbl-64/k_rem_pio2.c:254:18: error: array subscript is below
> array bounds [-Werror=array-bounds]
>     for (k = 1; iq[jk - k] == 0; k++)
>                 ~~^~~~~~~~
> 
> I get the same error with sysdeps/ieee754/flt-32/k_rem_pio2f.c.
> 
> Does anybody else get this warning on another architecture, too?
> 
> This patch adds a check so that the index is always >= 0.

Such an unnecessary check is not how we deal with bogus warnings.  
Rather, use the DIAG_*_NEEDS_COMMENT macros from include/libc-internal.h, 
with a detailed comment explaining what the warning is and why it is a 
false positive.

-- 
Joseph S. Myers
joseph@codesourcery.com