aaron.ballman accepted this revision. aaron.ballman added a comment. LGTM!
================ Comment at: clang/test/Sema/struct-packed-align.c:170 +#elif defined(_AIX) +// On AIX, [bool, char, short] bitfields have the same alignment as unsigned +// int. ---------------- hubert.reinterpretcast wrote: > Xiangling_L wrote: > > aaron.ballman wrote: > > > Xiangling_L wrote: > > > > aaron.ballman wrote: > > > > > We're not really testing the behavior of `bool` or `short` anywhere > > > > > and it'd be nice to verify that. Perhaps instead of modifying an > > > > > existing test to add more fields, it'd make sense to make a new test > > > > > structure? > > > > > > > > > > While thinking of other potentially smaller-than-int types, I > > > > > wondered whether `wchar_t` has special behavior here as well (I have > > > > > no idea how that type is defined for AIX, so it's entirely possible > > > > > it's size and alignment already match `int`). > > > > > We're not really testing the behavior of bool or short anywhere and > > > > > it'd be nice to verify that. > > > > > > > > The comment is to explain why char has 4-byte alignment mainly. And the > > > > testcase here is, as comments mentioned, to test `Packed attribute > > > > shouldn't be ignored for bit-field of char types`. Perhaps I should > > > > remove `bool` and `short` so that people wouldn't be confused. > > > > > > > > And the special alignment regarding bool, short etc. has been tested > > > > when the special rule introduced on aix here: > > > > https://reviews.llvm.org/D87029. > > > > > > > > > > > > > > > > > Perhaps instead of modifying an existing test to add more fields, > > > > > it'd make sense to make a new test structure? > > > > > > > > I don't think it's necessary to make a new test structure. The modified > > > > testcase test the same property as the original one. And it is more > > > > capable as it can also test the property for AIX target. > > > > > > > > > > > > > > > > > > > > > I wondered whether wchar_t has special behavior here as well > > > > > > > > I think `wchar_t` has the same special behavior. Basically any type > > > > smaller than 4 bytes will be aligned to 4 when it comes to bitfield. > > > > Please correct me if I am wrong @hubert.reinterpretcast > > > > > > > > > > > > Perhaps I should remove bool and short so that people wouldn't be > > > > confused. > > > > > > That might not be a bad idea. I saw the comment and went to look for the > > > declarations of `bool` and `short` type to verify they were behaving the > > > same way, hence the confusion. > > > > > > > The modified testcase test the same property as the original one. > > > > > > The part that worries me is that it shifts the offset for `e`. Before, > > > the packed field could be packed into the previous allocation unit (4 > > > bits + 8 bits fit comfortably within a 32-bit allocation unit), but now > > > the packed field is in an awkward spot (28 bits + 8 bits no longer fits > > > into a 32-bit allocation unit). So I think it could be subtly changing > > > the behavior of the test, but perhaps not in an observable way that > > > matters (I admit that I don't know all the ins and outs of our packing > > > strategies). > > > but now the packed field is in an awkward spot (28 bits + 8 bits no > > > longer fits into a 32-bit allocation unit) > > > > > > I think this is exactly the purpose of the test. We'd like to tell if the > > `packed` attribute has effect or not. > > > > Before the modification, on AIX, no matter the packed works or not, you > > will see the size = 4, align = 4 since char has 4-byte alignment. > My understanding is that the "awkward spot" was always the intention of the > test. It's just that the assumption around "allocation unit" size being 1 for > `char` was encoded into the test. Ah, thank you both for pointing that out to me. This just shifts the awkward spot to make the test scenario more obvious. That makes a lot more sense to me. CHANGES SINCE LAST ACTION https://reviews.llvm.org/D102715/new/ https://reviews.llvm.org/D102715 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
