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List:       gcc
Subject:    Re: How to use _Generic with bit-fields
From:       Wink Saville <wink () saville ! com>
Date:       2016-02-26 1:25:36
Message-ID: CAKk8isoFVU7=YNo0AaBB4sNDXLsVc3ukGx7uLm_8X_Q6hwceZw () mail ! gmail ! com
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Thanks for the info. What I'll probably do is file a bug and reply to this
thread and the other one when I do.


On Thu, Feb 25, 2016, 2:50 PM Joseph Myers <joseph@codesourcery.com> wrote:
>
> 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
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