RFC/A: Add a targetm.vectorize.related_mode hook

[Richard Sandiford <richard dot sandiford at arm dot com>]

This patch is the first of a series that tries to remove two
assumptions:

(1) that all vectors involved in vectorisation must be the same size

(2) that there is only one vector mode for a given element mode and
    number of elements

Relaxing (1) helps with targets that support multiple vector sizes or
that require the number of elements to stay the same.  E.g. if we're
vectorising code that operates on narrow and wide elements, and the
narrow elements use 64-bit vectors, then on AArch64 it would normally
be better to use 128-bit vectors rather than pairs of 64-bit vectors
for the wide elements.

Relaxing (2) makes it possible for -msve-vector-bits=128 to preoduce
fixed-length code for SVE.  It also allows unpacked/half-size SVE
vectors to work with -msve-vector-bits=256.

The patch adds a new hook that targets can use to control how we
move from one vector mode to another.  The hook takes a starting vector
mode, a new element mode, and (optionally) a new number of elements.
The flexibility needed for (1) comes in when the number of elements
isn't specified.

All callers in this patch specify the number of elements, but a later
vectoriser patch doesn't.  I won't be posting the vectoriser patch
for a few days, hence the RFC/A tag.

Tested individually on aarch64-linux-gnu and as a series on
x86_64-linux-gnu.  OK to install?  Or if not yet, does the idea
look OK?

I'll post some follow-up patches too.

Richard


2019-10-23  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* target.def (related_mode): New hook.
	* doc/tm.texi.in (TARGET_VECTORIZE_RELATED_MODE): New hook.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_vectorize_related_mode): Declare.
	* targhooks.c (default_vectorize_related_mode): New function.
	* machmode.h (related_vector_mode): Declare.
	* stor-layout.c (related_vector_mode): New function.
	* expmed.c (extract_bit_field_1): Use it instead of mode_for_vector.
	* optabs-query.c (qimode_for_vec_perm): Likewise.
	* tree-vect-stmts.c (get_group_load_store_type): Likewise.
	(vectorizable_store, vectorizable_load): Likewise

Index: gcc/target.def
===================================================================
--- gcc/target.def	2019-09-30 17:20:57.370607986 +0100
+++ gcc/target.def	2019-10-23 11:33:01.568510253 +0100
@@ -1909,6 +1909,33 @@ for autovectorization.  The default impl
  (vector_sizes *sizes, bool all),
  default_autovectorize_vector_sizes)
 
+DEFHOOK
+(related_mode,
+ "If a piece of code is using vector mode @var{vector_mode} and also wants\n\
+to operate on elements of mode @var{element_mode}, return the vector mode\n\
+it should use for those elements.  If @var{nunits} is nonzero, ensure that\n\
+the mode has exactly @var{nunits} elements, otherwise pick whichever vector\n\
+size pairs the most naturally with @var{vector_mode}.  Return an empty\n\
+@code{opt_machine_mode} if there is no supported vector mode with the\n\
+required properties.\n\
+\n\
+There is no prescribed way of handling the case in which @var{nunits}\n\
+is zero.  One common choice is to pick a vector mode with the same size\n\
+as @var{vector_mode}; this is the natural choice if the target has a\n\
+fixed vector size.  Another option is to choose a vector mode with the\n\
+same number of elements as @var{vector_mode}; this is the natural choice\n\
+if the target has a fixed number of elements.  Alternatively, the hook\n\
+might choose a middle ground, such as trying to keep the number of\n\
+elements as similar as possible while applying maximum and minimum\n\
+vector sizes.\n\
+\n\
+The default implementation uses @code{mode_for_vector} to find the\n\
+requested mode, returning a mode with the same size as @var{vector_mode}\n\
+when @var{nunits} is zero.  This is the correct behavior for most targets.",
+ opt_machine_mode,
+ (machine_mode vector_mode, scalar_mode element_mode, poly_uint64 nunits),
+ default_vectorize_related_mode)
+
 /* Function to get a target mode for a vector mask.  */
 DEFHOOK
 (get_mask_mode,
Index: gcc/doc/tm.texi.in
===================================================================
--- gcc/doc/tm.texi.in	2019-09-30 17:20:57.350608130 +0100
+++ gcc/doc/tm.texi.in	2019-10-23 11:33:01.564510281 +0100
@@ -4181,6 +4181,8 @@ address;  but often a machine-dependent
 
 @hook TARGET_VECTORIZE_AUTOVECTORIZE_VECTOR_SIZES
 
+@hook TARGET_VECTORIZE_RELATED_MODE
+
 @hook TARGET_VECTORIZE_GET_MASK_MODE
 
 @hook TARGET_VECTORIZE_EMPTY_MASK_IS_EXPENSIVE
Index: gcc/doc/tm.texi
===================================================================
--- gcc/doc/tm.texi	2019-09-30 17:20:57.350608130 +0100
+++ gcc/doc/tm.texi	2019-10-23 11:33:01.560510309 +0100
@@ -6029,6 +6029,30 @@ The hook does not need to do anything if
 for autovectorization.  The default implementation does nothing.
 @end deftypefn
 
+@deftypefn {Target Hook} opt_machine_mode TARGET_VECTORIZE_RELATED_MODE (machine_mode @var{vector_mode}, scalar_mode @var{element_mode}, poly_uint64 @var{nunits})
+If a piece of code is using vector mode @var{vector_mode} and also wants
+to operate on elements of mode @var{element_mode}, return the vector mode
+it should use for those elements.  If @var{nunits} is nonzero, ensure that
+the mode has exactly @var{nunits} elements, otherwise pick whichever vector
+size pairs the most naturally with @var{vector_mode}.  Return an empty
+@code{opt_machine_mode} if there is no supported vector mode with the
+required properties.
+
+There is no prescribed way of handling the case in which @var{nunits}
+is zero.  One common choice is to pick a vector mode with the same size
+as @var{vector_mode}; this is the natural choice if the target has a
+fixed vector size.  Another option is to choose a vector mode with the
+same number of elements as @var{vector_mode}; this is the natural choice
+if the target has a fixed number of elements.  Alternatively, the hook
+might choose a middle ground, such as trying to keep the number of
+elements as similar as possible while applying maximum and minimum
+vector sizes.
+
+The default implementation uses @code{mode_for_vector} to find the
+requested mode, returning a mode with the same size as @var{vector_mode}
+when @var{nunits} is zero.  This is the correct behavior for most targets.
+@end deftypefn
+
 @deftypefn {Target Hook} opt_machine_mode TARGET_VECTORIZE_GET_MASK_MODE (poly_uint64 @var{nunits}, poly_uint64 @var{length})
 A vector mask is a value that holds one boolean result for every element
 in a vector.  This hook returns the machine mode that should be used to
Index: gcc/targhooks.h
===================================================================
--- gcc/targhooks.h	2019-09-30 17:19:45.051128625 +0100
+++ gcc/targhooks.h	2019-10-23 11:33:01.568510253 +0100
@@ -114,6 +114,9 @@ default_builtin_support_vector_misalignm
 extern machine_mode default_preferred_simd_mode (scalar_mode mode);
 extern machine_mode default_split_reduction (machine_mode);
 extern void default_autovectorize_vector_sizes (vector_sizes *, bool);
+extern opt_machine_mode default_vectorize_related_mode (machine_mode,
+							scalar_mode,
+							poly_uint64);
 extern opt_machine_mode default_get_mask_mode (poly_uint64, poly_uint64);
 extern bool default_empty_mask_is_expensive (unsigned);
 extern void *default_init_cost (class loop *);
Index: gcc/targhooks.c
===================================================================
--- gcc/targhooks.c	2019-10-20 13:58:01.283640189 +0100
+++ gcc/targhooks.c	2019-10-23 11:33:01.568510253 +0100
@@ -1307,6 +1307,25 @@ default_autovectorize_vector_sizes (vect
 {
 }
 
+/* The default implementation of TARGET_VECTORIZE_RELATED_MODE.  */
+
+opt_machine_mode
+default_vectorize_related_mode (machine_mode vector_mode,
+				scalar_mode element_mode,
+				poly_uint64 nunits)
+{
+  machine_mode result_mode;
+  if ((maybe_ne (nunits, 0U)
+       || multiple_p (GET_MODE_SIZE (vector_mode),
+		      GET_MODE_SIZE (element_mode), &nunits))
+      && mode_for_vector (element_mode, nunits).exists (&result_mode)
+      && VECTOR_MODE_P (result_mode)
+      && targetm.vector_mode_supported_p (result_mode))
+    return result_mode;
+
+  return opt_machine_mode ();
+}
+
 /* By default a vector of integers is used as a mask.  */
 
 opt_machine_mode
Index: gcc/machmode.h
===================================================================
--- gcc/machmode.h	2019-10-07 09:38:45.627684403 +0100
+++ gcc/machmode.h	2019-10-23 11:33:01.564510281 +0100
@@ -880,6 +880,8 @@ extern opt_scalar_int_mode int_mode_for_
 extern opt_machine_mode bitwise_mode_for_mode (machine_mode);
 extern opt_machine_mode mode_for_vector (scalar_mode, poly_uint64);
 extern opt_machine_mode mode_for_int_vector (unsigned int, poly_uint64);
+extern opt_machine_mode related_vector_mode (machine_mode, scalar_mode,
+					     poly_uint64 = 0);
 
 /* Return the integer vector equivalent of MODE, if one exists.  In other
    words, return the mode for an integer vector that has the same number
Index: gcc/stor-layout.c
===================================================================
--- gcc/stor-layout.c	2019-09-18 10:43:11.562335822 +0100
+++ gcc/stor-layout.c	2019-10-23 11:33:01.564510281 +0100
@@ -530,6 +530,26 @@ mode_for_int_vector (unsigned int int_bi
   return opt_machine_mode ();
 }
 
+/* If a piece of code is using vector mode VECTOR_MODE and also wants
+   to operate on elements of mode ELEMENT_MODE, return the vector mode
+   it should use for those elements.  If NUNITS is nonzero, ensure that
+   the mode has exactly NUNITS elements, otherwise pick whichever vector
+   size pairs the most naturally with VECTOR_MODE; this may mean choosing
+   a mode with a different size and/or number of elements, depending on
+   what the target prefers.  Return an empty opt_machine_mode if there
+   is no supported vector mode with the required properties.
+
+   Unlike mode_for_vector. any returned mode is guaranteed to satisfy
+   both VECTOR_MODE_P and targetm.vector_mode_supported_p.  */
+
+opt_machine_mode
+related_vector_mode (machine_mode vector_mode, scalar_mode element_mode,
+		     poly_uint64 nunits)
+{
+  gcc_assert (VECTOR_MODE_P (vector_mode));
+  return targetm.vectorize.related_mode (vector_mode, element_mode, nunits);
+}
+
 /* Return the alignment of MODE. This will be bounded by 1 and
    BIGGEST_ALIGNMENT.  */
 
Index: gcc/expmed.c
===================================================================
--- gcc/expmed.c	2019-09-10 17:18:39.992121613 +0100
+++ gcc/expmed.c	2019-10-23 11:33:01.564510281 +0100
@@ -1641,12 +1641,10 @@ extract_bit_field_1 (rtx str_rtx, poly_u
 	  poly_uint64 nunits;
 	  if (!multiple_p (GET_MODE_BITSIZE (GET_MODE (op0)),
 			   GET_MODE_UNIT_BITSIZE (tmode), &nunits)
-	      || !mode_for_vector (inner_mode, nunits).exists (&new_mode)
-	      || !VECTOR_MODE_P (new_mode)
+	      || !related_vector_mode (tmode, inner_mode,
+				       nunits).exists (&new_mode)
 	      || maybe_ne (GET_MODE_SIZE (new_mode),
-			   GET_MODE_SIZE (GET_MODE (op0)))
-	      || GET_MODE_INNER (new_mode) != GET_MODE_INNER (tmode)
-	      || !targetm.vector_mode_supported_p (new_mode))
+			   GET_MODE_SIZE (GET_MODE (op0))))
 	    new_mode = VOIDmode;
 	}
       poly_uint64 pos;
Index: gcc/optabs-query.c
===================================================================
--- gcc/optabs-query.c	2019-07-10 19:41:26.387898094 +0100
+++ gcc/optabs-query.c	2019-10-23 11:33:01.564510281 +0100
@@ -354,11 +354,8 @@ can_conditionally_move_p (machine_mode m
 opt_machine_mode
 qimode_for_vec_perm (machine_mode mode)
 {
-  machine_mode qimode;
-  if (GET_MODE_INNER (mode) != QImode
-      && mode_for_vector (QImode, GET_MODE_SIZE (mode)).exists (&qimode)
-      && VECTOR_MODE_P (qimode))
-    return qimode;
+  if (GET_MODE_INNER (mode) != QImode)
+    return related_vector_mode (mode, QImode, GET_MODE_SIZE (mode));
   return opt_machine_mode ();
 }
 
Index: gcc/tree-vect-stmts.c
===================================================================
--- gcc/tree-vect-stmts.c	2019-10-23 11:30:54.953408377 +0100
+++ gcc/tree-vect-stmts.c	2019-10-23 11:33:01.572510226 +0100
@@ -2276,9 +2276,8 @@ get_group_load_store_type (stmt_vec_info
 		  || alignment_support_scheme == dr_unaligned_supported)
 	      && known_eq (nunits, (group_size - gap) * 2)
 	      && known_eq (nunits, group_size)
-	      && mode_for_vector (elmode, (group_size - gap)).exists (&vmode)
-	      && VECTOR_MODE_P (vmode)
-	      && targetm.vector_mode_supported_p (vmode)
+	      && related_vector_mode (TYPE_MODE (vectype), elmode,
+				      group_size - gap).exists (&vmode)
 	      && (convert_optab_handler (vec_init_optab,
 					 TYPE_MODE (vectype), vmode)
 		  != CODE_FOR_nothing))
@@ -7736,9 +7735,8 @@ vectorizable_store (stmt_vec_info stmt_i
 		 of vector elts directly.  */
 	      scalar_mode elmode = SCALAR_TYPE_MODE (elem_type);
 	      machine_mode vmode;
-	      if (!mode_for_vector (elmode, group_size).exists (&vmode)
-		  || !VECTOR_MODE_P (vmode)
-		  || !targetm.vector_mode_supported_p (vmode)
+	      if (!related_vector_mode (TYPE_MODE (vectype), elmode,
+					group_size).exists (&vmode)
 		  || (convert_optab_handler (vec_extract_optab,
 					     TYPE_MODE (vectype), vmode)
 		      == CODE_FOR_nothing))
@@ -7755,9 +7753,8 @@ vectorizable_store (stmt_vec_info stmt_i
 		     element extracts from the original vector type and
 		     element size stores.  */
 		  if (int_mode_for_size (lsize, 0).exists (&elmode)
-		      && mode_for_vector (elmode, lnunits).exists (&vmode)
-		      && VECTOR_MODE_P (vmode)
-		      && targetm.vector_mode_supported_p (vmode)
+		      && related_vector_mode (TYPE_MODE (vectype), elmode,
+					      lnunits).exists (&vmode)
 		      && (convert_optab_handler (vec_extract_optab,
 						 vmode, elmode)
 			  != CODE_FOR_nothing))
@@ -8838,9 +8835,8 @@ vectorizable_load (stmt_vec_info stmt_in
 		 vector elts directly.  */
 	      scalar_mode elmode = SCALAR_TYPE_MODE (TREE_TYPE (vectype));
 	      machine_mode vmode;
-	      if (mode_for_vector (elmode, group_size).exists (&vmode)
-		  && VECTOR_MODE_P (vmode)
-		  && targetm.vector_mode_supported_p (vmode)
+	      if (related_vector_mode (TYPE_MODE (vectype), elmode,
+				       group_size).exists (&vmode)
 		  && (convert_optab_handler (vec_init_optab,
 					     TYPE_MODE (vectype), vmode)
 		      != CODE_FOR_nothing))
@@ -8864,9 +8860,8 @@ vectorizable_load (stmt_vec_info stmt_in
 		  /* If we can't construct such a vector fall back to
 		     element loads of the original vector type.  */
 		  if (int_mode_for_size (lsize, 0).exists (&elmode)
-		      && mode_for_vector (elmode, lnunits).exists (&vmode)
-		      && VECTOR_MODE_P (vmode)
-		      && targetm.vector_mode_supported_p (vmode)
+		      && related_vector_mode (TYPE_MODE (vectype), elmode,
+					      lnunits).exists (&vmode)
 		      && (convert_optab_handler (vec_init_optab, vmode, elmode)
 			  != CODE_FOR_nothing))
 		    {