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Re: ABI Issues: C++ PATCH: PR 12053

On Thursday, September 4, 2003, at 04:54 PM, Joe Buck wrote:

On Thu, Sep 04, 2003 at 04:45:25PM -0700, Mike Stump wrote:
Odd, you only test what people don't use, and don't test what people do
use. Maybe that is backward. Imagined compatibility isn't more
important than real compatibility.

That's why some of us pushed for -Wabi, with the goal that the compiler
would warn about all constructs known to cause ABI compatibility problems.
Since these problems are discovered as time goes on, the user would have
to run a -Wabi analysis with the newest available compiler.

At least this allows users to determine whether they are likely to run
into trouble. In practice, seeing that almost all the problems found so
far relate to bitfields and inheritance, I would avoid giving the compiler
any chance to mess up here, by avoiding leaving any sub-byte padding (or,
to be safer, sub-word padding) at the end of a class that is likely to be
derived from.

That reminds me, I've found another bug that has to do with bitfields---but bitfields and unions this time, not bitfields and inheritance. I've finally gotten around to filing it:

I don't think this relates to any subtleties in defining or implementing the new C++ ABI, I think it's just a garden variety bug. (And one that seems to have been there since roughly forever, and that nobody has noticed until now because it involves a feature combination that nobody uses or cares about.) But fixing could reasonable be considered to be an ABI-breaking change, because it'll change the sizes of some objects. Please read the bug description if you care about this sort of thing. Who knows, maybe you can show me that I'm wrong and that it isn't a bug after all. That would be nice!

Unless anyone tells me in the next day or so that they desperately want this bug, I'm going to assign it to myself.


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