[PATCH] Add generic HAVE_RM_CTX implementation

[Wilco]

Marcus,

Can you check this in?

Wilco

ChangeLog:
2014-06-10  Wilco  <wdijkstr@arm.com>

	* sysdeps/generic/math_private.h: Add default HAVE_RM_CTX
	implementation. New function (libc_feholdsetround_noex_ctx).


Joseph wrote:
> On Tue, 20 May 2014, Wilco wrote:
> 
> > Ping
> >
> > As for an additional reviewer, Joseph could you please have a look?
> 
> OK, although my comments in
> <https://sourceware.org/ml/libc-alpha/2014-03/msg00267.html> apply.  All
> these macros / functions need careful review and refactoring; the present
> set makes it far from obvious that required invariants hold such as "if
> the function used at the start of a fixed-rounding-mode block has the
> feholdexcept effects of disabling traps or clearing existing set
> exceptions, then the function at the end of that block must have the
> reverse effects of restoring previously enabled traps and set exceptions
> (possibly with new exceptions merged in, depending on the semantics of the
> function at the end)".  I suspect things might be clearer if the functions
> / macros had extra arguments for such things as "clearing exceptions
> permitted / required / not permitted" or "new exceptions must be merged
> into the previous set / may be merged in / must be discarded", rather than
> having so many macros with similar names but different semantics.
> 
> --
> Joseph S. Myers
> joseph@codesourcery.com
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