Re: [PATCH] Use an std::vector for inline_states

[Pedro Alves <palves@redhat.com>]

On 04/07/2018 03:42 PM, Simon Marchi wrote:

>  /* Locate saved inlined frame state for PTID, if it exists
>     and is valid.  */
> @@ -66,43 +70,29 @@ static VEC(inline_state_s) *inline_states;
>  static struct inline_state *
>  find_inline_frame_state (ptid_t ptid)
>  {

> -	  if (current_pc != state->saved_pc)
> -	    {
> -	      /* PC has changed - this context is invalid.  Use the
> -		 default behavior.  */
> -	      VEC_unordered_remove (inline_state_s, inline_states, ix);
> -	      return NULL;

...

> -  state = VEC_safe_push (inline_state_s, inline_states, NULL);
> -  memset (state, 0, sizeof (*state));
> -  state->ptid = ptid;
> +      inline_states.erase (state_it);

The patch looks good, though it made me realize that when we're
replacing VEC_unordered_remove with std::vector::erase, we're introducing
a pessimization, which makes me ponder about having an utility/replacement
for VEC_unordered_remove that works with std::vector and alikes.
I.e., a function that removes an element from a vector simply by moving
the last element to the now-vacant position.  That's more efficient
than erase, because it avoids having to copy/move the remaining
elements, making it O(1).

Something like:

 template<typename Vector>
 void
 unordered_erase (Vector &v, typename Vector::const_iterator pos)
 {
     *pos = std::move (v.back ());
     v.pop_back ();
 }

Thanks,
Pedro Alves