[cfp-interest 3918] Setting the Dynamic Rounding Direction

[Damian McGuckin]

In the great "rounding" discussion, ....

Investigations by someone (far smarter than me) a while ago showed that
aggressive optimization by at least one main-stream compiler resulted in code
like

 	old_rounding_direction_mode = fegetround();
 	fesetround(new_rounding_direction_mode);
 	{
 		multiple statements;
 	}
 	fesetround(old_rounding_direction_mode);

not always guaranteeing that the new_rounding_direction_mode is set before any
of those "multiple statements" are executed and likewise the last of the
fesetround() statements might not always be done immediately those same
"multiple statements".

Recent re-investigations seem to show this problem has now been addressed by a
workaround. Sadly, its downside is that it no longer yields quite so optimal
code within the block. Ideally what the compiler should have done instead is
model the dependencies between rounding modes and instructions. Sub-optimal.
Undesirable.

Does demanding that this block structure is honored by the optimizer in terms of
the execution sequence a topic that either ourselves or even the WG14 itself can
raise?

I am curious - Damian

P.S. I will investigate what happens with static rounding #pragmas in my (ha-ha)
spare time.