------- Comment #6 from vogel at pi2 dot physik dot uni-erlangen dot de 2007-07-23 10:08 ------- This program demonstrates the problem, it creates different output depending on if compiled with or without optimisation.
Without optimisation, n->next is not cached: n->next = 0xbfb01af0 n->next = 0xbfb01af8 With optimisation, n->next is cached, this is wrong: n->next = 0xbfdb3da0 n->next = 0xbfdb3da0 Note that the pointer c will point exactly one pointer-width above the structure a, so n->next->next->prev=n -- which corresponds to c->prev=n -- will overwrite n->next with n. #include <stdio.h> struct node { struct node *next, *prev; }; void foo(struct node* n) { printf("n->next = %p\n",n->next); n->next->next->prev=n; printf("n->next = %p\n",n->next); }; int main(int argc,char **argv){ struct node a = { },b = { }; struct node *c = NULL; c = ((void*)&(a.next)) - sizeof(void*); b.next = c; a.next = &b; foo(&a); } -- vogel at pi2 dot physik dot uni-erlangen dot de changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |vogel at pi2 dot physik dot | |uni-erlangen dot de http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32856