[libgomp, openacc, openmp, PR83046] Prune removed funcs from offload table

[Jakub Jelinek]

On Thu, Dec 28, 2017 at 05:06:57PM +0100, Jakub Jelinek wrote:
> This has O(n^2) complexity for n == vec_safe_length (offload_funcs).
> Can't you instead just have 2 IVs, one for where we read the vector elt and
> one for where we write it if the 2 are different, then truncate the vector
> if needed at the end?
> 
> Another thing, I think you can safely remove elts from the vector (== from
> the host and offloading target arrays) only when !flag_lto, because we rely
> on the two arrays being the same.  So you can't remove elts only on the host
> and not on the device, or vice versa.  The output_offload_tables function
> has:
>   /* In WHOPR mode during the WPA stage the joint offload tables need to be
>      streamed to one partition only.  That's why we free offload_funcs and
>      offload_vars after the first call of output_offload_tables.  */
>   if (flag_wpa)
>     {
>       vec_free (offload_funcs);
>       vec_free (offload_vars);
>     }
> so at least with flag_wpa, if we remove anything in there, it won't be
> reflected by the other tables.  So, can we do something different in case
> we can't easily remove stuff from the vector anymore?  Either store some
> placeholder in the tables (dunno if NULL would work or what), or instead
> ensure corresponding functions can't be removed?

Maybe this removal if (!flag_lto) could be done earlier, e.g. at the
beginning of lto_output, and for nodes we keep around in the table
past that point set DECL_PRESERVE_P to 1 on the fndecl, so that we then
stream that flag.

	Jakub