clang gets numerical underflow wrong, please fix.

[Steve Kargl]

On Mon, Mar 14, 2016 at 08:23:33PM +0100, Dimitry Andric wrote:
> 
> Maybe this is a usable workaround for libm.
> 

Thanks for looking into this.  I just read the audit
trail at llvm.org.  Searching the clang user manual
turns up

   The support for standard C in clang is feature-complete
   except for the C99 floating-point pragmas.

There is no other statement concerning the implementation
defined behavior.  The understated assumption that FENV_ACCESS
is tacitly set to OFF should be documented.

It won't help possible libm issues.  The libm function is
trying to raise the FE_UNDERFLOW signal and return 0 to
a program.  As it is now, the libm function returns a
nonzero invalid result.

-- 
Steve