> > I believe I understand your general objection. I don't feel strongly > about the current behaviour, except that if it has to change then it > must be a documented extension. > > I don't think we can meaningfully order the space of "undefined > behaviour" and single out some as are "more undefined behaviour than > others". > > -- Gaby >
Let me summarize what has been done in order for someone to decide what should be done. Since gcc 3.3.x inliner ICE'd on some function calls built from incompatible function pointer cast. That was not a "compatible type" as defined in standard, but some implementation-dependent types - for example on x86 it is return type casted from double to int. The ICE was fixed in a way when all calls through function pointer cast incompatible as defined in standard are warned and turned into runtime-trap. That means not only ICE'd examples, but safe 'char *' to 'void *' conversions now aborts at runtime. There was another fix for the same problem, by simply not inlining such calls. But that patch was more intrusive and did not pass frozen 3.3/mainline state for that time. However the fix is incomplete - it has been disabled for objective-c because its runtime depends on ability to call casted fptrs, at least in "safe" variants. Secondly, there still exists ability to build problematic function calls by "double cast to void" - e.g. "(int(*)(int) (void(*)(void) foo" construction somehow slips the check. It ICE's on 3.3.x / 3.4.x on problematic types and (surprise!) not ICE's but generates not-inlined call in 4.1. For gcc-4.2, behavior has been changed again - double cast to void trick is not accepted anymore - warn/trap is generated. But for objective-c, all checks are still bypassed and it is possible to ICE. Now the interesting part, what to do for 4.2: 1) Leave it as is: behavior has been changed again (undocumented), ICE's for objective-c front-end. - has natural drawback of surprise for people switching from previous compiler versions 2) Enable checks for objective-c since its front-end no longer generates casted calls, document behavior changes, suggest people not to cast incompatible function types at all or at least do it via union. - need someone to check real-world effect on objective-c - may become broken in future when inliner will become smart enough to inline casted via assignment or via union calls. 3) Fix ICE in another way - by explicitly disable inlining of casted calls. Remove warn/trap checks, restore natural undefined behavior - than cast really changes types in platform-dependent incompatiple way. - surely most costly to implement - looks right to me since trap for platform-dependent working cast sounds worse than ICE for platform-dependent broken cast :) So, what should be done here?
