This provides a type-safe way to ask for a float mode and get it as a
scalar_float_mode.

2017-07-13  Richard Sandiford  <richard.sandif...@linaro.org>
            Alan Hayward  <alan.hayw...@arm.com>
            David Sherwood  <david.sherw...@arm.com>

gcc/
        * coretypes.h (opt_scalar_float_mode): New typedef.
        * machmode.h (float_mode_for_size): New function.
        * emit-rtl.c (double_mode): Delete.
        (init_emit_once): Use float_mode_for_size.
        * stor-layout.c (layout_type): Likewise.
        * gdbhooks.py (build_pretty_printer): Handle opt_scalar_float_mode.

Index: gcc/coretypes.h
===================================================================
--- gcc/coretypes.h     2017-07-13 09:18:23.795186280 +0100
+++ gcc/coretypes.h     2017-07-13 09:18:25.915974717 +0100
@@ -57,6 +57,7 @@ typedef struct rtx_def *rtx;
 typedef const struct rtx_def *const_rtx;
 class scalar_float_mode;
 template<typename> class opt_mode;
+typedef opt_mode<scalar_float_mode> opt_scalar_float_mode;
 
 /* Subclasses of rtx_def, using indentation to show the class
    hierarchy, along with the relevant invariant.
Index: gcc/machmode.h
===================================================================
--- gcc/machmode.h      2017-07-13 09:18:23.798185961 +0100
+++ gcc/machmode.h      2017-07-13 09:18:25.916974620 +0100
@@ -521,7 +521,16 @@ #define GET_MODE_COMPLEX_MODE(MODE) ((ma
 
 extern machine_mode mode_for_size (unsigned int, enum mode_class, int);
 
-/* Similar, but find the smallest mode for a given width.  */
+/* Return the machine mode to use for a MODE_FLOAT of SIZE bits, if one
+   exists.  */
+
+inline opt_scalar_float_mode
+float_mode_for_size (unsigned int size)
+{
+  return dyn_cast <scalar_float_mode> (mode_for_size (size, MODE_FLOAT, 0));
+}
+
+/* Similar to mode_for_size, but find the smallest mode for a given width.  */
 
 extern machine_mode smallest_mode_for_size (unsigned int,
                                                 enum mode_class);
Index: gcc/emit-rtl.c
===================================================================
--- gcc/emit-rtl.c      2017-07-13 09:18:21.530429366 +0100
+++ gcc/emit-rtl.c      2017-07-13 09:18:25.916974620 +0100
@@ -71,7 +71,6 @@ #define initial_regno_reg_rtx (this_targ
 
 machine_mode byte_mode;        /* Mode whose width is BITS_PER_UNIT.  */
 machine_mode word_mode;        /* Mode whose width is BITS_PER_WORD.  */
-machine_mode double_mode;      /* Mode whose width is DOUBLE_TYPE_SIZE.  */
 machine_mode ptr_mode; /* Mode whose width is POINTER_SIZE.  */
 
 /* Datastructures maintained for currently processed function in RTL form.  */
@@ -5895,7 +5894,7 @@ init_emit_once (void)
 {
   int i;
   machine_mode mode;
-  machine_mode double_mode;
+  scalar_float_mode double_mode;
 
   /* Initialize the CONST_INT, CONST_WIDE_INT, CONST_DOUBLE,
      CONST_FIXED, and memory attribute hash tables.  */
@@ -5939,7 +5938,7 @@ init_emit_once (void)
   else
     const_true_rtx = gen_rtx_CONST_INT (VOIDmode, STORE_FLAG_VALUE);
 
-  double_mode = mode_for_size (DOUBLE_TYPE_SIZE, MODE_FLOAT, 0);
+  double_mode = *float_mode_for_size (DOUBLE_TYPE_SIZE);
 
   real_from_integer (&dconst0, double_mode, 0, SIGNED);
   real_from_integer (&dconst1, double_mode, 1, SIGNED);
Index: gcc/stor-layout.c
===================================================================
--- gcc/stor-layout.c   2017-07-13 09:18:22.938277517 +0100
+++ gcc/stor-layout.c   2017-07-13 09:18:25.916974620 +0100
@@ -2142,14 +2142,16 @@ layout_type (tree type)
       break;
 
     case REAL_TYPE:
-      /* Allow the caller to choose the type mode, which is how decimal
-        floats are distinguished from binary ones.  */
-      if (TYPE_MODE (type) == VOIDmode)
-       SET_TYPE_MODE (type,
-                      mode_for_size (TYPE_PRECISION (type), MODE_FLOAT, 0));
-      TYPE_SIZE (type) = bitsize_int (GET_MODE_BITSIZE (TYPE_MODE (type)));
-      TYPE_SIZE_UNIT (type) = size_int (GET_MODE_SIZE (TYPE_MODE (type)));
-      break;
+      {
+       /* Allow the caller to choose the type mode, which is how decimal
+          floats are distinguished from binary ones.  */
+       if (TYPE_MODE (type) == VOIDmode)
+         SET_TYPE_MODE (type, *float_mode_for_size (TYPE_PRECISION (type)));
+       scalar_float_mode mode = as_a <scalar_float_mode> (TYPE_MODE (type));
+       TYPE_SIZE (type) = bitsize_int (GET_MODE_BITSIZE (mode));
+       TYPE_SIZE_UNIT (type) = size_int (GET_MODE_SIZE (mode));
+       break;
+      }
 
    case FIXED_POINT_TYPE:
      /* TYPE_MODE (type) has been set already.  */
Index: gcc/gdbhooks.py
===================================================================
--- gcc/gdbhooks.py     2017-07-13 09:18:23.798185961 +0100
+++ gcc/gdbhooks.py     2017-07-13 09:18:25.916974620 +0100
@@ -542,6 +542,8 @@ def build_pretty_printer():
 
     pp.add_printer_for_regex(r'opt_mode<(\S+)>',
                              'opt_mode', OptMachineModePrinter)
+    pp.add_printer_for_types(['opt_scalar_float_mode'],
+                             'opt_mode', OptMachineModePrinter)
     pp.add_printer_for_types(['scalar_float_mode'],
                              'scalar_float_mode', MachineModePrinter)
 

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