[PATCH] D102715: Fix LIT failure on native aix

[Aaron Ballman via Phabricator via cfe-commits]

aaron.ballman 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:
> > 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).


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D102715/new/

https://reviews.llvm.org/D102715