Please keep in mind that I'm not a GCC internals expert, and this really requires some analysis from an ObjC maintainer (and expert) along with someone who is familiar with the details of how -fstrict-aliasing works.
See also: http://gcc.gnu.org/ml/gcc/2009-04/msg00290.html The short version is this: Currently, it would appear the compiler (and I'm assuming all versions of the compiler that perform -fstrict-aliasing) applies C99 strict-aliasing rules to pointers to Objective-C objects. Although there is no formal language specification for Objective-C, it would seem that the most appropriate way to treat pointers to Objective-C objects is the same as 'char *' in terms of C99 aliasing rules. That is to say, a pointer to an Objective-C object can aliasing anything. As an experiment, I added the following line to c-common.c / c_common_get_alias_set() to determine how often the compiler is applying incorrect aliasing rules while informing the alias analyzer that the pointer can aliasing anything. if(((c_language == clk_objc) || (c_language == clk_objcxx)) && ((TYPE_LANG_SPECIFIC (t) && (TYPE_LANG_SPECIFIC(t)->objc_info)) || objc_is_object_ptr(t))) { warning(OPT_Wstrict_aliasing, "Caught and returning 'can alias anything' for objc type"); return(0); } right before the following line: if (c_language != clk_c || flag_isoc99) This returned a number (actually, a lot) of warnings when compiling Objective-C code at -O2 and -Wstrict-aliasing. In the mailing-list post referenced above, someone mentions that they think that GNUstep uses '-fno-strict-aliasing' when compiling code. This seems like a good test to see how effective a patch like this is. Recommendation: Apply the above 'patch' (or functional equivalent) to the compiler, minus the warning() line, to all versions of the compiler that apply C99 strict-aliasing rules. -- Summary: Objective-C(++) and C99 strict-aliasing interaction bug Product: gcc Version: unknown Status: UNCONFIRMED Severity: major Priority: P3 Component: objc AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: john dot engelhart at gmail dot com http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39753
