[Cfp-interest] issues about rounding attributes

[Jim Thomas]

Here are some thoughts on two issues about rounding attributes that came 
up in our Aug meeting.


ISSUE 1: Must FENV_ACCESS ON be in effect where static rounding 
attributes appear?

No. The point of FENV_ACCESS ON is to inform the compiler of possible 
runtime controls (mode changes) which cannot be known at compile time 
but which must be accommodated. The static rounding attributes tell the 
compiler precisely what controls are to be used, so they do not require 
FENV_ACCESS ON to be in effect.

An implementation that supports static rounding attributes by saving, 
setting, and restoring dynamic rounding modes may have a static rounding 
attribute effectively imply FENV_ACCESS ON as a means of keeping its 
optimizer from undermining the static attribute. But this is not 
something the user should need to manage or be aware of.


ISSUE 2: Do static rounding attributes affect function calls within 
their scope? If so, which ones?

IEEE 754-2008 requires static attributes to affect the operations for 
which it specifies rounding. For C, some of these operations are (and 
more will be) implemented by standard library functions, including:
sqrt
fma
scalbn
formatOf operations (add, sub, mul, div, fma, sqrt)

The effect of the dynamic rounding mode on other standard library 
functions is not currently specified. Implementations of these standard 
library functions might honor directed roundings, guard against them, or 
ignore them (getting a haphazard effect). Thus, leaving the effect of 
static rounding attributes on these functions unspecified would seem to 
be acceptable by current expectations, and would make it easier for 
implementations. Greater consistency might be achieved by requiring that 
a static attribute either have (1) no effect or (2) the same effect 
(whatever it may be) as a dynamic rounding mode.

Having static rounding attributes affect calls to user functions seems 
inconsistent with the concept of static rounding attributes, turning 
them into syntactic sugar for save, set, and restore the global rounding 
mode. This would not free implementations (that might benefit from such 
freedom) from having to keep track of the global rounding mode, which 
was the purpose of static rounding attributes.

Proposal:

If a static rounding attribute is in effect, calls to functions for 
which IEEE 754-2008 specifies rounding shall be correctly rounded 
according to the static rounding attribute.
The effect of static rounding attributes on other standard library 
functions is unspecified.
Calls to user functions shall not be affected by static rounding attributes.


-Jim T