Index: gcc/lower-subreg.c
===================================================================
--- gcc/lower-subreg.c	2017-10-23 11:47:11.274393237 +0100
+++ gcc/lower-subreg.c	2017-10-23 11:47:23.555013148 +0100
@@ -103,6 +103,18 @@ #define twice_word_mode \
 #define choices \
   this_target_lower_subreg->x_choices
 
+/* Return true if MODE is a mode we know how to lower.  When returning true,
+   store its byte size in *BYTES and its word size in *WORDS.  */
+
+static inline bool
+interesting_mode_p (machine_mode mode, unsigned int *bytes,
+		    unsigned int *words)
+{
+  *bytes = GET_MODE_SIZE (mode);
+  *words = CEIL (*bytes, UNITS_PER_WORD);
+  return true;
+}
+
 /* RTXes used while computing costs.  */
 struct cost_rtxes {
   /* Source and target registers.  */
@@ -199,10 +211,10 @@ compute_costs (bool speed_p, struct cost
   for (i = 0; i < MAX_MACHINE_MODE; i++)
     {
       machine_mode mode = (machine_mode) i;
-      int factor = GET_MODE_SIZE (mode) / UNITS_PER_WORD;
-      if (factor > 1)
+      unsigned int size, factor;
+      if (interesting_mode_p (mode, &size, &factor) && factor > 1)
 	{
-	  int mode_move_cost;
+	  unsigned int mode_move_cost;
 
 	  PUT_MODE (rtxes->target, mode);
 	  PUT_MODE (rtxes->source, mode);
@@ -469,10 +481,10 @@ find_decomposable_subregs (rtx *loc, enu
 	      continue;
 	    }
 
-	  outer_size = GET_MODE_SIZE (GET_MODE (x));
-	  inner_size = GET_MODE_SIZE (GET_MODE (inner));
-	  outer_words = (outer_size + UNITS_PER_WORD - 1) / UNITS_PER_WORD;
-	  inner_words = (inner_size + UNITS_PER_WORD - 1) / UNITS_PER_WORD;
+	  if (!interesting_mode_p (GET_MODE (x), &outer_size, &outer_words)
+	      || !interesting_mode_p (GET_MODE (inner), &inner_size,
+				      &inner_words))
+	    continue;
 
 	  /* We only try to decompose single word subregs of multi-word
 	     registers.  When we find one, we return -1 to avoid iterating
@@ -507,7 +519,7 @@ find_decomposable_subregs (rtx *loc, enu
 	}
       else if (REG_P (x))
 	{
-	  unsigned int regno;
+	  unsigned int regno, size, words;
 
 	  /* We will see an outer SUBREG before we see the inner REG, so
 	     when we see a plain REG here it means a direct reference to
@@ -527,7 +539,8 @@ find_decomposable_subregs (rtx *loc, enu
 
 	  regno = REGNO (x);
 	  if (!HARD_REGISTER_NUM_P (regno)
-	      && GET_MODE_SIZE (GET_MODE (x)) > UNITS_PER_WORD)
+	      && interesting_mode_p (GET_MODE (x), &size, &words)
+	      && words > 1)
 	    {
 	      switch (*pcmi)
 		{
@@ -567,15 +580,15 @@ find_decomposable_subregs (rtx *loc, enu
 decompose_register (unsigned int regno)
 {
   rtx reg;
-  unsigned int words, i;
+  unsigned int size, words, i;
   rtvec v;
 
   reg = regno_reg_rtx[regno];
 
   regno_reg_rtx[regno] = NULL_RTX;
 
-  words = GET_MODE_SIZE (GET_MODE (reg));
-  words = (words + UNITS_PER_WORD - 1) / UNITS_PER_WORD;
+  if (!interesting_mode_p (GET_MODE (reg), &size, &words))
+    gcc_unreachable ();
 
   v = rtvec_alloc (words);
   for (i = 0; i < words; ++i)
@@ -599,25 +612,29 @@ decompose_register (unsigned int regno)
 simplify_subreg_concatn (machine_mode outermode, rtx op,
 			 unsigned int byte)
 {
-  unsigned int inner_size;
+  unsigned int outer_size, outer_words, inner_size, inner_words;
   machine_mode innermode, partmode;
   rtx part;
   unsigned int final_offset;
 
+  innermode = GET_MODE (op);
+  if (!interesting_mode_p (outermode, &outer_size, &outer_words)
+      || !interesting_mode_p (innermode, &inner_size, &inner_words))
+    gcc_unreachable ();
+
   gcc_assert (GET_CODE (op) == CONCATN);
-  gcc_assert (byte % GET_MODE_SIZE (outermode) == 0);
+  gcc_assert (byte % outer_size == 0);
 
-  innermode = GET_MODE (op);
-  gcc_assert (byte < GET_MODE_SIZE (innermode));
-  if (GET_MODE_SIZE (outermode) > GET_MODE_SIZE (innermode))
+  gcc_assert (byte < inner_size);
+  if (outer_size > inner_size)
     return NULL_RTX;
 
-  inner_size = GET_MODE_SIZE (innermode) / XVECLEN (op, 0);
+  inner_size /= XVECLEN (op, 0);
   part = XVECEXP (op, 0, byte / inner_size);
   partmode = GET_MODE (part);
 
   final_offset = byte % inner_size;
-  if (final_offset + GET_MODE_SIZE (outermode) > inner_size)
+  if (final_offset + outer_size > inner_size)
     return NULL_RTX;
 
   /* VECTOR_CSTs in debug expressions are expanded into CONCATN instead of
@@ -801,9 +818,10 @@ can_decompose_p (rtx x)
 
       if (HARD_REGISTER_NUM_P (regno))
 	{
-	  unsigned int byte, num_bytes;
+	  unsigned int byte, num_bytes, num_words;
 
-	  num_bytes = GET_MODE_SIZE (GET_MODE (x));
+	  if (!interesting_mode_p (GET_MODE (x), &num_bytes, &num_words))
+	    return false;
 	  for (byte = 0; byte < num_bytes; byte += UNITS_PER_WORD)
 	    if (simplify_subreg_regno (regno, GET_MODE (x), byte, word_mode) < 0)
 	      return false;
@@ -826,14 +844,15 @@ resolve_simple_move (rtx set, rtx_insn *
   rtx src, dest, real_dest;
   rtx_insn *insns;
   machine_mode orig_mode, dest_mode;
-  unsigned int words;
+  unsigned int orig_size, words;
   bool pushing;
 
   src = SET_SRC (set);
   dest = SET_DEST (set);
   orig_mode = GET_MODE (dest);
 
-  words = (GET_MODE_SIZE (orig_mode) + UNITS_PER_WORD - 1) / UNITS_PER_WORD;
+  if (!interesting_mode_p (orig_mode, &orig_size, &words))
+    gcc_unreachable ();
   gcc_assert (words > 1);
 
   start_sequence ();
@@ -964,7 +983,7 @@ resolve_simple_move (rtx set, rtx_insn *
     {
       unsigned int i, j, jinc;
 
-      gcc_assert (GET_MODE_SIZE (orig_mode) % UNITS_PER_WORD == 0);
+      gcc_assert (orig_size % UNITS_PER_WORD == 0);
       gcc_assert (GET_CODE (XEXP (dest, 0)) != PRE_MODIFY);
       gcc_assert (GET_CODE (XEXP (dest, 0)) != POST_MODIFY);
 
@@ -1059,7 +1078,7 @@ resolve_clobber (rtx pat, rtx_insn *insn
 {
   rtx reg;
   machine_mode orig_mode;
-  unsigned int words, i;
+  unsigned int orig_size, words, i;
   int ret;
 
   reg = XEXP (pat, 0);
@@ -1067,8 +1086,8 @@ resolve_clobber (rtx pat, rtx_insn *insn
     return false;
 
   orig_mode = GET_MODE (reg);
-  words = GET_MODE_SIZE (orig_mode);
-  words = (words + UNITS_PER_WORD - 1) / UNITS_PER_WORD;
+  if (!interesting_mode_p (orig_mode, &orig_size, &words))
+    gcc_unreachable ();
 
   ret = validate_change (NULL_RTX, &XEXP (pat, 0),
 			 simplify_gen_subreg_concatn (word_mode, reg,
@@ -1332,12 +1351,13 @@ dump_shift_choices (enum rtx_code code,
 static void
 dump_choices (bool speed_p, const char *description)
 {
-  unsigned int i;
+  unsigned int size, factor, i;
 
   fprintf (dump_file, "Choices when optimizing for %s:\n", description);
 
   for (i = 0; i < MAX_MACHINE_MODE; i++)
-    if (GET_MODE_SIZE ((machine_mode) i) > UNITS_PER_WORD)
+    if (interesting_mode_p ((machine_mode) i, &size, &factor)
+	&& factor > 1)
       fprintf (dump_file, "  %s mode %s for copy lowering.\n",
 	       choices[speed_p].move_modes_to_split[i]
 	       ? "Splitting"
