[Bug tree-optimization/98848] [9/10/11 regression] vectorizer failed to reduce max pattern since r9-1590

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98848

--- Comment #6 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
So what about punting if the lhs of the possible over_widen pattern is a PHI on
loop header?

--- gcc/tree-vect-patterns.c.jj 2021-01-04 10:25:38.650235896 +0100
+++ gcc/tree-vect-patterns.c    2021-02-01 10:13:51.755008757 +0100
@@ -1579,6 +1579,20 @@ vect_recog_over_widening_pattern (vec_in
   tree type = TREE_TYPE (lhs);
   tree_code code = gimple_assign_rhs_code (last_stmt);

+  /* Punt if lhs might be used in a reduction.  */
+  if (loop_vec_info loop_vinfo = dyn_cast<loop_vec_info> (vinfo))
+    {
+      use_operand_p use_p;
+      imm_use_iterator iter;
+      FOR_EACH_IMM_USE_FAST (use_p, iter, lhs)
+       {
+         gimple *use_stmt = USE_STMT (use_p);
+         if (gimple_code (use_stmt) == GIMPLE_PHI
+             && gimple_bb (use_stmt) == LOOP_VINFO_LOOP (loop_vinfo)->header)
+           return NULL;
+       }
+    }
+
   /* Keep the first operand of a COND_EXPR as-is: only the other two
      operands are interesting.  */
   unsigned int first_op = (code == COND_EXPR ? 2 : 1);

doesn't regress any vect.exp=*over-widen* tests and let's this testcase be
vectorized.