(Dropping Phabricator, since this isn't really about D27163...) On 28 November 2016 at 14:27, John McCall via Phabricator via cfe-commits < cfe-commits@lists.llvm.org> wrote:
> In https://reviews.llvm.org/D27163#607100, @rsmith wrote: > > C has rather different and much less useful TBAA rules > > [...] I don't think the TBAA rules are as different as you're suggesting, I can offhand think of at least the following differences, some of which seem fairly major: * C does not allow changing the effective type of a declared variable, C++ does. * C does not support struct-path TBAA in any form (its model is that lumps of memory have effective types, and it doesn't care how you got there), C++ does. * C allows any assignment to a union member to change the effective type, no matter whether it's locally determinable that you're storing to a union member, C++ requires the assignment's LHS to be a union member access. * C only permits common initial sequence shenanigans for unions where the union type is visible, C++ permits it anywhere. > except that C++ actually tries to provide specific rules for unions (that > don't match what users actually want). In what way don't C++'s union rules match user expectations where C's rules do? Do you mean type punning via unions? C doesn't allow that either (or maybe it does, if you think a footnote carries more normative weight than rules in the main text).
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