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Re: _Bool and trap representations

On Mon, 13 Jun 2016, Alexander Cherepanov wrote:

> Thanks for the info. IMHO this part of DR 260 has even more serious
> consequences than the part about pointer provenance. It effectively prohibits
> manual byte-by-byte (or any non-atomic) copying of objects for types like long
> double. If an implementation decides to normalize a value in a variable during
> copying it will see an inconsistent representation, e.g. a trap
> representation. It's a sure way to get total garbage. I don't know if allowing

No, that's not the case; even if representations can change during 
byte-by-byte copying, such copying of long double values is *still* safe.  
All long double values for x86 long double have exactly one valid 
representation in the value bits, and if the padding bits change during 
copying it doesn't matter; it's only representations that are already trap 
representations (unnormals, pseudo-* etc.) that might be interpreted 

Likewise for IBM long double; the only cases of more than one 
representation for a value are (a) a zero low part might have either sign 
(in which case an arbitrary choice of bytes from the two representations 
still gives a valid representation of the same value) and (b) the low part 
of a NaN is of no significance.

Joseph S. Myers

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