https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110137
--- Comment #15 from Jan Hubicka <hubicka at ucw dot cz> --- > As pointed out in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110035#c13 , > gcc > already assume operator new's retuned pointer cannot alias any existing > pointer. So no change is needed there. Seems you are right, -fsane-operator-new is weaker than I assumed. In addition to the assumption about modifying globals, one missed optimization is removal of unused vectors (we have separate PR for that) #include <vector> void foo (int *); int test() { std::vector <int> a(1); return 0; } int test2(int size) { int *a = new int; *a=0; delete a; return 0; } int test3(int size) { int *a = (int *)::operator new (sizeof (int)); *a=0; ::operator delete (a); return 0; } At -O2 Clang optimizes test and test2 "return 0", while we optimize only test2. libstdc++ uses clang's __builtin_operator_new that enables this optimization based on claim that user's possibly insane new/delete can not rely on internals of std::vector implementation and thus can not reliably use memory being deleted. I expected that -fassume-sane-operator-new should enable clang or GCC to do the optimization for all three variants, but it does not (for clang). I think it would be useful to have such a flag. There are number of transformations we may want to do including - removal of dead stores to memory being deleted - removal of unnecesary reallocations i.e. when user creates empty vector and does sequence of push_backs. - avoiding new/delete for small vectors that fits to stack. I believe those are sane for builtin_operator_new/delete. However since clang does not define it this way, perhaps we will need -fassume-really-sane-operator-new-delete for that? It would be also nice to be able to determine functions that does paired new/delete but no other side effects as pure/const etc. BTW clang also does not optimize out load from global here: int test3(int size) { global = 0; int *a = (int *)::operator new (sizeof (int)); *a=0; return global; } I would expect -fassume-sane-operator-new to make it possible?
