Jonathan Lennox wrote:
I have some code which is giving strict-aliasing warnings on gcc 4.4.1, and I don't understand why. When I compile alias-warning2.c (attached), I get $ gcc -O2 -Wall alias-warning2.c -o alias-warning2 alias-warning2.c: In function 'main': alias-warning2.c:31: warning: dereferencing pointer 'd2.16' does break strict-aliasing rules alias-warning2.c:39: note: initialized from here and indeed, when I run, gcc has optimized out the offending code: $ ./alias-warning2 Aliasing is unhappy! d2.base.a == 5 However, as far as I can tell, at line 31 the object d2.base (which has type 'struct base') is indeed being accessed through a pointer of type 'struct base*'. If I change the inline function to take the type 'struct base*' directly, rather than 'union derived_union*', I do not get a warning, or mis-compilaton. Am I missing something, or is gcc mis-compiling correct code?
The development version of gcc doesn't give this warning. I think it's partly a matter of how much inlining gets done, and how far gcc sees through the type casts. However, I think you're misunderstanding the standard. Here is Section 6.3.2.3, Pointers: "A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined. Otherwise, when converted back again, the result shall compare equal to the original pointer." That's all. You can compare the result with the original pointer. If that doesn't work, we have a bug. Andrew.
