hubert.reinterpretcast added inline comments.
================ 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. ---------------- 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. 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
