[Cfp-interest] DFP: Conversions among FP types

[Fred J. Tydeman]

On Tue, 4 Dec 2012 09:16:29 -0800 Jim Thomas wrote:
>
>i believe what you're trying to do is separate conversions 
>to generic from conversions to decimal. Right? What do you 
>see that's wrong with the current spec?

Correct.

Current spec:

  Suggested change to C11:
  Add after 6.3.1.5#1.
  [2] When a value of real floating type is converted to a real 
  floating type and one or both of the types are decimal types, 
  if the value being converted cannot be represented exactly, 
  the result is correctly rounded with exceptions raised as 
  specified in IEC 60559.

Consider:
  float f = 0.1DF;  
Since one of the types is decimal, the above words require that
it be correctly rounded.  Yet, 'float' might be  IBM S/360 hex
FP which only supports truncate to zero rounding (if I remember
correctly).  So, it cannot always be correctly rounded.

Or, consider
  float f = DEC_INFINITY;
Again, the above words require correctly rounded results.  Yet,
if 'float' is IBM S/260 hex FP (which has no infinity), the 
correct thing is undefined behavior.

Only conversions to decimal FP must be correctly rounded.

Conversions to generic FP must allow for implementation-defined
rounding and undefined behavior.

The behavior (correct rounding, implementation rounding, undefined) 
depends upon the type being converted to, not the type being converted 
from.

---
Fred J. Tydeman        Tydeman Consulting
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