http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21855
--- Comment #18 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-01-10 16:35:46 UTC --- (In reply to comment #17) > (In reply to comment #16) > > (In reply to comment #15) > > > (In reply to comment #14) > > > > (In reply to comment #13) > > > > > We can't optimize this because System.out.println can change args[]. > > > > > > > > That's the whole point: System.out.println cannot change args[], which > > > > is a > > > > java array, and the length of a Java array is constant. It is not an > > > > invalid > > > > test case. > > > > > > I suppose > > > > > > public static void main(String[] args) > > > > > > is passing args by value (but the implementation detail uses reference > > > passing for efficiency?). > > > > args is indeed a reference to a Java array. The length field of a Java > > array is immutable. The elements of an array are not immutable. > > You mean that System.out.println could change the elements of the array > (well, it doesn't, but theoretically it could)? > > > > In this case the Java frontend should do > > > like the C++ frontend and tell this to the middle-end by properly > > > marking args as 1) DECL_BY_REFERENCE, 2) use a TYPE_RESTRICT qualified > > > pointer for the reference. Then we would optimize this case. > > > > If we could mark the length field as immutable that would fix it. Is there > > any > > way to do that? > > No. What you can do is, via the method I outlined, tell GCC that > args is to be treated similar to a local automatic variable - thus > it cannot be refered to from other functions (unless you pass them > its address of course). Thus, similar to the C++ case with struct Array { int length; void data[]; } void foo (Array args) { ... } foo cannot change the callers args.length (only its own copy) but it can change the callers args.data[] contents. If the C++ frontend decides to pass args by reference then it sets DECL_BY_REFERENCE and uses a TYPE_RESTRICT qualified pointer. This way the optimization will be the same as if it was passed "really" by value. Not sure if the Java situation is similar enough.