tree-optimization/104445 - check for vector extraction support

This adds a missing check to epilogue reduction re-use, namely
that we can do hi/lo extracts from the vector when demoting it
to the epilogue vector size.

I've chosen to add a can_vec_extract helper to optabs-query.h,
in the future we might want to simplify the vectorizers life by
handling vector-from-vector extraction via BIT_FIELD_REFs during
RTL expansion via the mode punning when the vec_extract is not
directly supported.

I'm not 100% sure we can always do the punning of the
vec_extract result to a vector mode of the same size, but then
I'm also not sure how to check for that (the vectorizer doesn't
in other places it does that at the moment, but I suppose we
eventually just go through memory there)?

2022-02-09  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104445
	PR tree-optimization/102832
	* optabs-query.h (can_vec_extract): New.
	* optabs-query.cc (can_vec_extract): Likewise.
	* tree-vect-loop.cc (vect_find_reusable_accumulator): Check
	we can extract a hi/lo part from the larger vector, rework
	check iteration from larger to smaller sizes.

	* gcc.dg/vect/pr104445.c: New testcase.

gcc/optabs-query.cc

@@ -763,3 +763,31 @@ supports_vec_scatter_store_p (machine_mode mode)
   return this_fn_optabs->supports_vec_scatter_store[mode] > 0;
 }
 
+/* Whether we can extract part of the vector mode MODE as
+   (scalar or vector) mode EXTR_MODE.  */
+
+bool
+can_vec_extract (machine_mode mode, machine_mode extr_mode)
+{
+  unsigned m;
+  if (!VECTOR_MODE_P (mode)
+      || !constant_multiple_p (GET_MODE_SIZE (mode),
+			       GET_MODE_SIZE (extr_mode), &m))
+    return false;
+
+  if (convert_optab_handler (vec_extract_optab, mode, extr_mode)
+      != CODE_FOR_nothing)
+    return true;
+
+  /* Besides a direct vec_extract we can also use an element extract from
+     an integer vector mode with elements of the size of the extr_mode.  */
+  scalar_int_mode imode;
+  machine_mode vmode;
+  if (!int_mode_for_size (GET_MODE_BITSIZE (extr_mode), 0).exists (&imode)
+      || !related_vector_mode (mode, imode, m).exists (&vmode)
+      || (convert_optab_handler (vec_extract_optab, vmode, imode)
+	  == CODE_FOR_nothing))
+    return false;
+  /* We assume we can pun mode to vmode and imode to extr_mode.  */
+  return true;
+}

gcc/optabs-query.h

@@ -195,6 +195,7 @@ bool can_atomic_load_p (machine_mode);
 bool lshift_cheap_p (bool);
 bool supports_vec_gather_load_p (machine_mode = E_VOIDmode);
 bool supports_vec_scatter_store_p (machine_mode = E_VOIDmode);
+bool can_vec_extract (machine_mode, machine_mode);
 
 /* Version of find_widening_optab_handler_and_mode that operates on
    specific mode types.  */

gcc/testsuite/gcc.dg/vect/pr102832.c

@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O3" } */
+/* { dg-additional-options "-march=armv8.2-a+sve -msve-vector-bits=128" { target aarch64-*-* } } */
+
+int a, b;
+char c;
+signed char d(int e, int f) { return e - f; }
+void g() {
+  a = 0;
+  for (; a >= -17; a = d(a, 1))
+    c ^= b;
+}

gcc/testsuite/gcc.dg/vect/pr104445.c

@@ -0,0 +1,16 @@
+/* { dg-do compile } */
+/* { dg-additional-options "-O3" } */
+/* { dg-additional-options "-mavx -mno-mmx" { target x86_64-*-* i?86-*-* } } */
+
+signed char a;
+signed char f (int i, int j)
+{
+  signed char c;
+  while (i != 0)
+  {
+    a ^= j;
+    ++c;
+    ++i;
+  }
+  return c;
+}

gcc/tree-vect-loop.cc

@@ -4997,17 +4997,21 @@ vect_find_reusable_accumulator (loop_vec_info loop_vinfo,
   if (!constant_multiple_p (TYPE_VECTOR_SUBPARTS (old_vectype),
 			    TYPE_VECTOR_SUBPARTS (vectype), &m))
     return false;
-  /* Check the intermediate vector types are available.  */
-  while (m > 2)
+  /* Check the intermediate vector types and operations are available.  */
+  tree prev_vectype = old_vectype;
+  poly_uint64 intermediate_nunits = TYPE_VECTOR_SUBPARTS (old_vectype);
+  while (known_gt (intermediate_nunits, TYPE_VECTOR_SUBPARTS (vectype)))
     {
-      m /= 2;
+      intermediate_nunits = exact_div (intermediate_nunits, 2);
       tree intermediate_vectype = get_related_vectype_for_scalar_type
-	(TYPE_MODE (vectype), TREE_TYPE (vectype),
-	 exact_div (TYPE_VECTOR_SUBPARTS (old_vectype), m));
+	(TYPE_MODE (vectype), TREE_TYPE (vectype), intermediate_nunits);
       if (!intermediate_vectype
 	  || !directly_supported_p (STMT_VINFO_REDUC_CODE (reduc_info),
-				    intermediate_vectype))
+				    intermediate_vectype)
+	  || !can_vec_extract (TYPE_MODE (prev_vectype),
+			       TYPE_MODE (intermediate_vectype)))
 	return false;
+      prev_vectype = intermediate_vectype;
     }
 
   /* Non-SLP reductions might apply an adjustment after the reduction