How to use _Generic with bit-fields

[Joseph Myers]

On Wed, 24 Feb 2016, Wink Saville wrote:

> Further more things like printing of "big" bit fields such as
> unsigned long long int b:33 doesn't issue any warnings with -Wall on clang

Of course, printing such a bit-field with %llu etc. isn't fully portable 
even with the C++ semantics for bit-field types.  With the C++ semantics, 
if int is 34 bits or more then it gets promoted to int; otherwise, if 
unsigned int is 33 bits or more then it gets promoted to unsigned int.  So 
as with many cases of using variadic functions, you need to include a cast 
to get the desired types.  It just so happens it's hard for warnings to 
tell how portable you want the code to be, or to tell whether an 
unportable format for a type was e.g. autoconf-detected to be correct.

> If someone were to supply a patch that changed the behavior to match
> what clang and apparently other compilers are doing, would you be likely
> to accept it?

Not without a clear direction from WG14 to require the C++ rules (in which 
case conditionals on the C standard version would be appropriate, given 
the previous direction from the C90 DRs).  You'd need to track the 
declared type for each bit-field alongside the reduced-width type, apply 
C++-style promotions and conversions to the declared type for relevant 
rvalue uses, and handle the types appropriately in _Generic and typeof - 
other changes could be needed if e.g. conversion from floating-point to 
bit-fields were defined to convert to the declared type and then convert 
from that to the bit-field.  (A narrower direction only defining types in 
_Generic might still require tracking declared types but not require so 
many other changes.)

-- 
Joseph S. Myers
joseph@codesourcery.com