Add command-line option to specify signedness of 'int' bit-fields.

[llvmbot]

Bugzilla Link	4493
Resolution	WONTFIX
Resolved on	Jun 13, 2012 13:24
Version	unspecified
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@efriedma-quic

Extended Description

According to the C99 standard (6.7.2p5):
"[...] it is implementation-defined whether the specifier 'int' designates the same type as 'signed int' or the same type as 'unsigned int'."

Similarly, in the C89 standard it is said:
"Each of the above comma-separated lists designates the same type,
except that for bit-field declarations, signed int (or signed ) may
differ from int (or no type specifiers)."

clang seems to be missing a language/target option to specify this implementation-defined behavior.

This should be added and exposed by supporting the GNU -fsignedbitfields -funsigned-bitfields command-line options.

[llvmbot]

Implementation-defined doesn't mean we have to support both ways; it is OK to pick one (but I believe there is a requirement about documentation that we may not meet). It is probably a good idea to support compatibility with gcc on this one, though.

[efriedma-quic]

(but I believe there is a requirement about documentation that we may
not meet).

C99 4p8, to be precise. And yes, we're missing documentation of all the implementation-defined choices.

[chandlerc]

There has been no movement on this in years.

No sense keeping a bug for one of the many implementation defined aspects of
Clang that are not documented.