https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116016
--- Comment #22 from qinzhao at gcc dot gnu.org --- the following is the user documentation I came up based on all the discussion so far, let me know any comment and suggestion. (refer to GCC's __builtin_clear_padding doc on the prototype of the new builtin: https://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#index-_005f_005fbuiltin_005fclear_005fpadding) Builtin-in Function: void __builtin_set_counted_by (ptr, type expr) The built-in function __builtin_set_counted_by checks whether the array object pointed by the pointer PTR has another object associated with it that represents the number of elements in the array object through counted_by attribute (i.e, the counted-by object). If so, sets the corresponding counted-by object to EXPR. If such counted-by object does not exist, do nothing. The first argument must be a pointer to an array. The TYPE of the second argument may be any integral type. This built-in never evaluates its argument for side effects. If there are any side effects in them, the compiler does not set the counted-by object if there is one and issues warnings at the same time. For example: for the following: struct foo1 { int counter1; struct bar1 array[] __attribute__((counted_by(counter))); } *p; struct foo2 { int other; struct bar2 array[]; } *q; __builtin_set_counted_by (p->array, COUNT) behaves like: p->counter1 = COUNT; However, __builtin_set_counted_by (q->array, COUNT) behaves like a no-op since q->array does not have any associated counted-by object through counted-by attribute.
