-                       != CODE_FOR_nothing)
+                         != CODE_FOR_nothing
+                       && can_vec_extract_var_idx_p (
+                         TYPE_MODE (vectype), TYPE_MODE (TREE_TYPE (vectype))))

I think maybe 'can_vec_extract_var_idx_p' check is enough and remove convert_optab_handler (vec_extract_optab,... check.
Looking forward Richi's more comments.

Thanks.


juzhe.zhong@rivai.ai
 
From: Kewen.Lin
Date: 2023-08-14 17:19
To: Robin Dapp
CC: gcc-patches; richard.sandiford; rguenther; juzhe.zhong@rivai.ai
Subject: Re: [PATCH V4] VECT: Support loop len control on EXTRACT_LAST vectorization
Hi Robin,
 
on 2023/8/14 16:58, Robin Dapp wrote:
> Hi Kewen,
> 
>> I did a bootstrapping and regression testing on Power10 (LE) and found a lot of failures.
> 
> I think the problem is that just like for vec_set we're expecting
> the vec_extract expander not to fail.  It is probably passed not a
> const int here anymore and therefore fails to expand?
 
Thanks for the comments!  Yeah, I think the expectation doesn't hold
on Power, as our vec_extract optab only support const index, that
is:
 
(define_expand "vec_extract<mode><VEC_base_l>"
  [(match_operand:<VEC_base> 0 "register_operand")
   (match_operand:VEC_E 1 "vlogical_operand")
   (match_operand 2 "const_int_operand")]
  "VECTOR_MEM_ALTIVEC_OR_VSX_P (<MODE>mode)"
{
  rs6000_expand_vector_extract (operands[0], operands[1], operands[2]);
  DONE;
})
 
> 
> can_vec_extract_var_idx_p is supposed to check if the backend
> supports extracting a variable index.
 
OK, it sounds that this new capability needs to further check with
function can_vec_extract_var_idx_p to ensure the ifn expanding work
as expected.  I re-spined by adding the below as your comments:
 
diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 07f3717ed9d..80ba5cae84a 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -10328,7 +10328,9 @@ vectorizable_live_operation (vec_info *vinfo, stmt_vec_info stmt_info,
               else if (convert_optab_handler (vec_extract_optab,
                                               TYPE_MODE (vectype),
                                               TYPE_MODE (TREE_TYPE (vectype)))
-                       != CODE_FOR_nothing)
+                         != CODE_FOR_nothing
+                       && can_vec_extract_var_idx_p (
+                         TYPE_MODE (vectype), TYPE_MODE (TREE_TYPE (vectype))))
                 vect_record_loop_len (loop_vinfo,
                                       &LOOP_VINFO_LENS (loop_vinfo),
                                       1, vectype, 1);
 
BR,
Kewen