https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101578
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There is no bug here:
struct a {
short b;
long c;
};
union {
int d;
struct a c;
} e = {0505355};
In the case of normal ABI, d and c.c don't overlap as c.c is aligned to
alignof(long) but with -fpack-struct they will overlap.
This has nothing to do with -O3 really.
-fpack-struct CHANGES THE ABI as that is documented too:
https://gcc.gnu.org/onlinedocs/gcc-11.1.0/gcc/Code-Gen-Options.html#index-fpack-struct
Without a value specified, pack all structure members together without holes.
When a value is specified (which must be a small power of two), pack structure
members according to this value, representing the maximum alignment (that is,
objects with default alignment requirements larger than this are output
potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not
binary compatible with code generated without that switch. Additionally, it
makes the code suboptimal. Use it to conform to a non-default application
binary interface.
