Hi Richard S./Richi/Jim/Segher,

Thanks a lot for your comments to make this patch more solid.

Based on our discussion, for the vector load/store with length
optab, the length unit would be measured in lanes by default.
For the targets which support length measured in bytes like Power,
they should only define VnQI modes to wrap the other same size
vector modes.  If the length is larger than total lane/byte count
of the given mode, it's taken to load all lanes/bytes implicitly.
For the remaining lanes/bytes which isn't specified by length,
they would be taken as undefined value.  For length in bytes,
it's required that the byte count should be a multiple of the
element size (wrapped vector), otherwise it's undefined.

This patch has been updated as attached.

2/7 for rs6000 optab defintion has been updated to use V16QI.
5/7 for vectorizer change has been updated accordingly.

-----

v6: Updated optab descriptions.

v5:
  - Updated lenload/lenstore optab to len_load/len_store and the docs.
  - Rename expand_mask_{load,store}_optab_fn to expand_partial_{load,store}_optab_fn
  - Added/updated macros for expand_mask_{load,store}_optab_fn
    and expand_len_{load,store}_optab_fn

v4: Update len_load_direct/len_store_direct to align with direct optab.

v3: Get rid of length mode hook.

BR,
Kewen
-----
gcc/ChangeLog:

2020-MM-DD  Kewen Lin  <linkw@gcc.gnu.org>

	* doc/md.texi (len_load_@var{m}): Document.
	(len_store_@var{m}): Likewise.
	* internal-fn.c (len_load_direct): New macro.
	(len_store_direct): Likewise.
	(expand_len_load_optab_fn): Likewise.
	(expand_len_store_optab_fn): Likewise.
	(direct_len_load_optab_supported_p): Likewise.
	(direct_len_store_optab_supported_p): Likewise.
	(expand_mask_load_optab_fn): New macro.  Original renamed to ...
	(expand_partial_load_optab_fn): ... here.  Add handlings for
	len_load_optab.
	(expand_mask_store_optab_fn): New macro.  Original renamed to ...
	(expand_partial_store_optab_fn): ... here. Add handlings for
	len_store_optab.
	(internal_load_fn_p): Handle IFN_LEN_LOAD.
	(internal_store_fn_p): Handle IFN_LEN_STORE.
	(internal_fn_stored_value_index): Handle IFN_LEN_STORE.
	* internal-fn.def (LEN_LOAD): New internal function.
	(LEN_STORE): Likewise.
	* optabs.def (len_load_optab, len_store_optab): New optab.