Vect: use a small step to calculate induction for the unrolled loop (PR tree-optimization/110449)

If a loop is unrolled by n times during vectoriation, two steps are used to
calculate the induction variable:
  - The small step for the unrolled ith-copy: vec_1 = vec_iv + (VF/n * Step)
  - The large step for the whole loop: vec_loop = vec_iv + (VF * Step)

This patch calculates an extra vec_n to replace vec_loop:
  vec_n = vec_prev + (VF/n * S) = vec_iv + (VF/n * S) * n = vec_loop.

So that we can save the large step register and related operations.

gcc/ChangeLog:

	PR tree-optimization/110449
	* tree-vect-loop.cc (vectorizable_induction): use vec_n to replace
	vec_loop for the unrolled loop.

gcc/testsuite/ChangeLog:

	* gcc.target/aarch64/pr110449.c: New testcase.

1 files changed, 18 insertions, 3 deletions

diff --git a/gcc/tree-vect-loop.cc b/gcc/tree-vect-loop.cc
index 36d19a5..7d917bf 100644
--- a/gcc/tree-vect-loop.cc
+++ b/gcc/tree-vect-loop.cc
@@ -10098,7 +10098,7 @@ vectorizable_induction (loop_vec_info loop_vinfo,
 				   new_vec, step_vectype, NULL);
 
       vec_def = induc_def;
-      for (i = 1; i < ncopies; i++)
+      for (i = 1; i < ncopies + 1; i++)
 	{
 	  /* vec_i = vec_prev + vec_step  */
 	  gimple_seq stmts = NULL;
@@ -10108,8 +10108,23 @@ vectorizable_induction (loop_vec_info loop_vinfo,
 	  vec_def = gimple_convert (&stmts, vectype, vec_def);
  
 	  gsi_insert_seq_before (&si, stmts, GSI_SAME_STMT);
-	  new_stmt = SSA_NAME_DEF_STMT (vec_def);
-	  STMT_VINFO_VEC_STMTS (stmt_info).safe_push (new_stmt);
+	  if (i < ncopies)
+	    {
+	      new_stmt = SSA_NAME_DEF_STMT (vec_def);
+	      STMT_VINFO_VEC_STMTS (stmt_info).safe_push (new_stmt);
+	    }
+	  else
+	    {
+	      /* vec_1 = vec_iv + (VF/n * S)
+		 vec_2 = vec_1 + (VF/n * S)
+		 ...
+		 vec_n = vec_prev + (VF/n * S) = vec_iv + VF * S = vec_loop
+
+		 vec_n is used as vec_loop to save the large step register and
+		 related operations.  */
+	      add_phi_arg (induction_phi, vec_def, loop_latch_edge (iv_loop),
+			   UNKNOWN_LOCATION);
+	    }
 	}
     }