VECT: Add SELECT_VL support

This patch address comments from Richard && Richi and rebase to trunk.

This patch is adding SELECT_VL middle-end support
allow target have target dependent optimization in case of
length calculation.

This patch is inspired by RVV ISA and LLVM:
https://reviews.llvm.org/D99750

The SELECT_VL is same behavior as LLVM "get_vector_length" with
these following properties:

1. Only apply on single-rgroup.
2. non SLP.
3. adjust loop control IV.
4. adjust data reference IV.
5. allow non-vf elements processing in non-final iteration

Code
   # void vvaddint32(size_t n, const int*x, const int*y, int*z)
   # { for (size_t i=0; i<n; i++) { z[i]=x[i]+y[i]; } }

Take RVV codegen for example:

Before this patch:
vvaddint32:
        ble     a0,zero,.L6
        csrr    a4,vlenb
        srli    a6,a4,2
.L4:
        mv      a5,a0
        bleu    a0,a6,.L3
        mv      a5,a6
.L3:
        vsetvli zero,a5,e32,m1,ta,ma
        vle32.v v2,0(a1)
        vle32.v v1,0(a2)
        vsetvli a7,zero,e32,m1,ta,ma
        sub     a0,a0,a5
        vadd.vv v1,v1,v2
        vsetvli zero,a5,e32,m1,ta,ma
        vse32.v v1,0(a3)
        add     a2,a2,a4
        add     a3,a3,a4
        add     a1,a1,a4
        bne     a0,zero,.L4
.L6:
        ret

After this patch:

vvaddint32:
    vsetvli t0, a0, e32, ta, ma  # Set vector length based on 32-bit vectors
    vle32.v v0, (a1)         # Get first vector
      sub a0, a0, t0         # Decrement number done
      slli t0, t0, 2         # Multiply number done by 4 bytes
      add a1, a1, t0         # Bump pointer
    vle32.v v1, (a2)         # Get second vector
      add a2, a2, t0         # Bump pointer
    vadd.vv v2, v0, v1       # Sum vectors
    vse32.v v2, (a3)         # Store result
      add a3, a3, t0         # Bump pointer
      bnez a0, vvaddint32    # Loop back
      ret                    # Finished

Co-authored-by: Richard Sandiford<richard.sandiford@arm.com>
Co-authored-by: Richard Biener <rguenther@suse.de>

gcc/ChangeLog:

	* doc/md.texi: Add SELECT_VL support.
	* internal-fn.def (SELECT_VL): Ditto.
	* optabs.def (OPTAB_D): Ditto.
	* tree-vect-loop-manip.cc (vect_set_loop_controls_directly): Ditto.
	* tree-vect-loop.cc (_loop_vec_info::_loop_vec_info): Ditto.
	* tree-vect-stmts.cc (get_select_vl_data_ref_ptr): Ditto.
	(vectorizable_store): Ditto.
	(vectorizable_load): Ditto.
	* tree-vectorizer.h (LOOP_VINFO_USING_SELECT_VL_P): Ditto.

1 files changed, 23 insertions, 9 deletions

diff --git a/gcc/tree-vect-loop-manip.cc b/gcc/tree-vect-loop-manip.cc
index 3f73594..1c8100c 100644
--- a/gcc/tree-vect-loop-manip.cc
+++ b/gcc/tree-vect-loop-manip.cc
@@ -534,7 +534,7 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
 	   _10 = (unsigned long) count_12(D);
 	   ...
 	   # ivtmp_9 = PHI <ivtmp_35(6), _10(5)>
-	   _36 = MIN_EXPR <ivtmp_9, POLY_INT_CST [4, 4]>;
+	   _36 = (MIN_EXPR | SELECT_VL) <ivtmp_9, POLY_INT_CST [4, 4]>;
 	   ...
 	   vect__4.8_28 = .LEN_LOAD (_17, 32B, _36, 0);
 	   ...
@@ -549,15 +549,28 @@ vect_set_loop_controls_directly (class loop *loop, loop_vec_info loop_vinfo,
       tree step = rgc->controls.length () == 1 ? rgc->controls[0]
 					       : make_ssa_name (iv_type);
       /* Create decrement IV.  */
-      create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
-		 &incr_gsi, insert_after, &index_before_incr,
-		 &index_after_incr);
-      gimple_seq_add_stmt (header_seq, gimple_build_assign (step, MIN_EXPR,
-							    index_before_incr,
-							    nitems_step));
+      if (LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
+	{
+	  create_iv (nitems_total, MINUS_EXPR, step, NULL_TREE, loop, &incr_gsi,
+		     insert_after, &index_before_incr, &index_after_incr);
+	  tree len = gimple_build (header_seq, IFN_SELECT_VL, iv_type,
+				   index_before_incr, nitems_step);
+	  gimple_seq_add_stmt (header_seq, gimple_build_assign (step, len));
+	}
+      else
+	{
+	  create_iv (nitems_total, MINUS_EXPR, nitems_step, NULL_TREE, loop,
+		     &incr_gsi, insert_after, &index_before_incr,
+		     &index_after_incr);
+	  gimple_seq_add_stmt (header_seq,
+			       gimple_build_assign (step, MIN_EXPR,
+						    index_before_incr,
+						    nitems_step));
+	}
       *iv_step = step;
       *compare_step = nitems_step;
-      return index_before_incr;
+      return LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo) ? index_after_incr
+						       : index_before_incr;
     }
 
   /* Create increment IV.  */
@@ -888,7 +901,8 @@ vect_set_loop_condition_partial_vectors (class loop *loop,
   /* Get a boolean result that tells us whether to iterate.  */
   edge exit_edge = single_exit (loop);
   gcond *cond_stmt;
-  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo))
+  if (LOOP_VINFO_USING_DECREMENTING_IV_P (loop_vinfo)
+      && !LOOP_VINFO_USING_SELECT_VL_P (loop_vinfo))
     {
       gcc_assert (compare_step);
       tree_code code = (exit_edge->flags & EDGE_TRUE_VALUE) ? LE_EXPR : GT_EXPR;