[01/13] Add a qimode_for_vec_perm helper function

[Richard Sandiford <richard dot sandiford at linaro dot org>]

The vec_perm code falls back to doing byte-level permutes if
element-level permutes aren't supported.  There were two copies
of the code to calculate the mode, and later patches add another,
so this patch splits it out into a helper function.


2017-12-09  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* optabs-query.h (qimode_for_vec_perm): Declare.
	* optabs-query.c (can_vec_perm_p): Split out qimode search to...
	(qimode_for_vec_perm): ...this new function.
	* optabs.c (expand_vec_perm): Use qimode_for_vec_perm.

Index: gcc/optabs-query.h
===================================================================
--- gcc/optabs-query.h	2017-12-09 22:47:12.476364764 +0000
+++ gcc/optabs-query.h	2017-12-09 22:47:14.730310076 +0000
@@ -174,6 +174,7 @@ enum insn_code can_extend_p (machine_mod
 enum insn_code can_float_p (machine_mode, machine_mode, int);
 enum insn_code can_fix_p (machine_mode, machine_mode, int, bool *);
 bool can_conditionally_move_p (machine_mode mode);
+opt_machine_mode qimode_for_vec_perm (machine_mode);
 bool can_vec_perm_p (machine_mode, bool, vec_perm_indices *);
 /* Find a widening optab even if it doesn't widen as much as we want.  */
 #define find_widening_optab_handler(A, B, C) \
Index: gcc/optabs-query.c
===================================================================
--- gcc/optabs-query.c	2017-12-09 22:47:12.476364764 +0000
+++ gcc/optabs-query.c	2017-12-09 22:47:14.729310075 +0000
@@ -345,6 +345,22 @@ can_conditionally_move_p (machine_mode m
   return direct_optab_handler (movcc_optab, mode) != CODE_FOR_nothing;
 }
 
+/* If a target doesn't implement a permute on a vector with multibyte
+   elements, we can try to do the same permute on byte elements.
+   If this makes sense for vector mode MODE then return the appropriate
+   byte vector mode.  */
+
+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;
+  return opt_machine_mode ();
+}
+
 /* Return true if VEC_PERM_EXPR of arbitrary input vectors can be
    expanded using SIMD extensions of the CPU.  SEL may be NULL, which
    stands for an unknown constant.  Note that additional permutations
@@ -375,9 +391,7 @@ can_vec_perm_p (machine_mode mode, bool
     return true;
 
   /* We allow fallback to a QI vector mode, and adjust the mask.  */
-  if (GET_MODE_INNER (mode) == QImode
-      || !mode_for_vector (QImode, GET_MODE_SIZE (mode)).exists (&qimode)
-      || !VECTOR_MODE_P (qimode))
+  if (!qimode_for_vec_perm (mode).exists (&qimode))
     return false;
 
   /* ??? For completeness, we ought to check the QImode version of
Index: gcc/optabs.c
===================================================================
--- gcc/optabs.c	2017-12-09 22:47:12.476364764 +0000
+++ gcc/optabs.c	2017-12-09 22:47:14.731310077 +0000
@@ -5452,9 +5452,7 @@ expand_vec_perm (machine_mode mode, rtx
 
   /* Set QIMODE to a different vector mode with byte elements.
      If no such mode, or if MODE already has byte elements, use VOIDmode.  */
-  if (GET_MODE_INNER (mode) == QImode
-      || !mode_for_vector (QImode, w).exists (&qimode)
-      || !VECTOR_MODE_P (qimode))
+  if (!qimode_for_vec_perm (mode).exists (&qimode))
     qimode = VOIDmode;
 
   /* If the input is a constant, expand it specially.  */