[Bug tree-optimization/101097] Vectorizer is too eager to use vec_unpack

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101097

--- Comment #13 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Hongtao.liu from comment #12)
> (In reply to rsandifo@gcc.gnu.org from comment #11)
> > FWIW, you could try something similar to how aarch64 handles this
> > for Advanced SIMD, with a combination of:
> > 
> > - TARGET_VECTORIZE_AUTOVECTORIZE_VECTOR_MODES
> > - TARGET_VECTORIZE_RELATED_MODE.
> I added a target_hook to return vector mode with same element number for
> i386 backend.
> It works for this case, but regresses many testcases which are related to
> gather/scatter instructions, because gather/scatter instructions accept same
> vector size but not same element number.
> 
>   /* AVX2 */
>   def_builtin_pure (OPTION_MASK_ISA_AVX2, 0, "__builtin_ia32_gathersiv2df",
> 		    V2DF_FTYPE_V2DF_PCDOUBLE_V4SI_V2DF_INT,
> 		    IX86_BUILTIN_GATHERSIV2DF);

It hits the gcc_assert in tree-vect-stmts.c:vect_build_gather_load_calls

      if (!useless_type_conversion_p (idxtype, TREE_TYPE (op)))
        {
          gcc_assert (known_eq (TYPE_VECTOR_SUBPARTS (TREE_TYPE (op)),
                                TYPE_VECTOR_SUBPARTS (idxtype)));