[Bug target/107432] __builtin_convertvector generates inefficient code

[crazylht at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107432

--- Comment #4 from Hongtao.liu <crazylht at gmail dot com> ---
(In reply to Hongtao.liu from comment #3)
> typedef int v4si __attribute__((vector_size(16)));
> typedef long long v4di __attribute__((vector_size(32)));
> 
> v4si
> foo (v4di a)
> {
>     return __builtin_convertvector (a, v4si);
> }
> 
> hmm, we actually support truncv4div4si2, but some how gcc failed to generate
> .VEC_CONVERT with truncmn2.
> 

/* IFN_VEC_CONVERT is supposed to be expanded at pass_lower_vector.  So this
   dummy function should never be called.  */

static void
expand_VEC_CONVERT (internal_fn, gcall *)
{
  gcc_unreachable ();
}

It's lowered by pass_lower_vector, ideally, can we use truncmn2 in
expand_VEC_CONVERT if src is bigger integer mode than dest.