[Cfp-interest] Three round nearest modes and FLT_ROUNDS

[Fred J. Tydeman]

The current draft of 754 has three different rounding modes for midway

 ties to even (for binary and decimal)
 ties away from zero (for decimal; optional for binary)
 ties toward zero (for augmented operations; not required for any other operations)

WG14 paper N2124 is adding FE_TONEARESTFROMZERO to C

Should we be adding something like:
  FE_TONEARESTTOZERO
  FE_DEC_TONEARESTTOZERO
 to C?

Given that there could be three different meanings for Round to Nearest,
what should FLT_ROUNDS be?  Should FLT_ROUNDS have a value 
for each of the possible FE_* rounding macros?

C has (in 7.6.1)
216) Even though the rounding direction macros might expand to 
constants corresponding to the values of FLT_ROUNDS, they
are not required to do so.

F.2 has:
The value of FLT_ROUNDS applies to all IEC 60559 types 
supported by the implementation, but need not apply to non-IEC 60559 types.

F.3 has:
The fegetround and fesetround functions in <fenv.h> provide the facility to select among
the IEC 60559 directed rounding modes represented by the rounding direction macros in
<fenv.h> (FE_TONEAREST, FE_UPWARD, FE_DOWNWARD, FE_TOWARDZERO)
and the values 0, 1 , 2, and 3 of FLT_ROUNDS are the IEC 60559 directed rounding modes.


---
Fred J. Tydeman        Tydeman Consulting
tydeman at tybor.com      Testing, numerics, programming
+1 (702) 608-6093      Vice-chair of PL22.11 (ANSI "C")
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