https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53037
--- Comment #23 from H.J. Lu <hjl.tools at gmail dot com> --- (In reply to Martin Sebor from comment #22) > The warning on the test case in comment #21 looks good to me. Thanks! > > From reading comment #1 I'm not sure your patch does quite what you > described there. It doesn't warn on the declaration of the global object x > below: > > typedef unsigned long long __u64 __attribute__((aligned(4), > warn_if_not_aligned(8))); > > static char c; > static __u64 x; > > even though x is 4-byte aligned. (It does warn when a __u64 member is > declared in a packed struct.) This is by design. The current description is at: https://gcc.gnu.org/ml/gcc-patches/2017-06/msg00180.html __attribute__((warn_if_not_aligned(N))) issues a warning if the field in a struct or union is not aligned to N: typedef unsigned long long __u64 __attribute__((aligned(4),warn_if_not_aligned(8))); struct foo { int i1; int i2; __u64 x; }; __u64 is aligned to 4 bytes. But inside struct foo, __u64 should be aligned at 8 bytes. It is used to define struct foo in such a way that struct foo has the same layout and x has the same alignment when __u64 is aligned at either 4 or 8 bytes. Since struct foo is normally aligned to 4 bytes, a warning will be issued: warning: alignment 4 of ‘struct foo’ is less than 8 Align struct foo to 8 bytes: struct foo { int i1; int i2; __u64 x; } __attribute__((aligned(8))); silences the warning. It also warns the field with misaligned offset: struct foo { int i1; int i2; int i3; __u64 x; } __attribute__((aligned(8))); warning: ‘x’ offset 12 in ‘struct foo’ isn't aligned to 8 The same warning is also issued without the warn_if_not_aligned attribute for the field with explicitly specified alignment in a packed struct or union: struct __attribute__ ((aligned (8))) S8 { char a[8]; }; struct __attribute__ ((packed)) S { struct S8 s8; }; warning: alignment 1 of ‘struct S’ is less than 8
