[Bug c/39170] cannot silence -Wconversion warnings for bit-fields

["tom at atoptech dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #4 from tom at atoptech dot com  2009-03-10 17:40 -------
Manuel,

You miss understood what I meant by "old behavior was just fine". I was saying
that the previous behavior of "gcc" worked fine and I was NOT referring
specifically to the "-Wconversion" option. 

The previous version of "gcc" warned when implicit narrowing of doubles to
integral values, such as

    double n = 0.0000000005;
    int d = n;

when using the "-Wall" option. 

The behavior of "gcc" has changed. Moving all the conversion warnings to fall
under "-Wconversion" may make semantic sense, but it alters the behavior of
"gcc". 

We can fault ourselves for missing this change in the documentation, but there
a level of expectation that the fundamental behavior of the compiler is
consistent from release-to-release. And when fundamental behavior of "gcc"
changes, ample notice should be given. People need to change Makefiles, alter
code (if possible), etc... 

With regard to "-Wtraditional-conversion", it does not work when compiling
"C++" code.

> Do you think this would be an acceptable solution? (I don't know if this works
> now in GCC 4.4)

Absolutely not. It's not a portable solution. There is nothing in the "C++"
standard (that I'm aware of) that suggests that "anding" an integral value with
a "constant" value results in a truncated integral value. It's a bad hack.

As you say, its is unfortunate that "C" and "C++" do not have a bit-field
"cast-operators". But that is the reality. 

There is a lot of code written using "bit-fields". Look at "gcc" itself. Until
the "C" or "C++" language contains a "type-safe" construct, such as a
cast-operator, "gcc" should not issue a warning by default.

If you have ideas on how to solve this, please submit them to the "C" or "C++"
standards group.

The "gcc" 4.3 stream is not safe (for us) to use as is. We need the compiler to
issue warnings when implicit narrowing occurs (expect in the case of a
bit-field). As is, we get thousands of warnings from our code when using
"-Wconversion". Consequently, we are forced disable "-Wconversion" to suppress
the bit-field warnings at the risk of missing other narrowing warnings. And
this is not acceptable to us.

We've already moved back to the 4.2 "gcc".

Best Regards,

Tom Geocaris


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39170