diff --git a/gcc/doc/rtl.texi b/gcc/doc/rtl.texiindex de45a22..0c6dc45 100644
--- a/gcc/doc/rtl.texi
+++ b/gcc/doc/rtl.texi
@@ -1530,7 +1530,9 @@ Represents either a floating-point constant of mode @var{m} or an
 integer constant too large to fit into @code{HOST_BITS_PER_WIDE_INT}
 bits but small enough to fit within twice that number of bits (GCC
 does not provide a mechanism to represent even larger constants).  In
-the latter case, @var{m} will be @code{VOIDmode}.
+the latter case, @var{m} will be @code{VOIDmode}.  For integral values
+the value is a signed value, meaning the top bit of
+@code{CONST_DOUBLE_HIGH} is a sign bit.
 
 @findex CONST_DOUBLE_LOW
 If @var{m} is @code{VOIDmode}, the bits of the value are stored in
diff --git a/gcc/emit-rtl.c b/gcc/emit-rtl.c
index 78ddfc3..c0b24e4 100644
--- a/gcc/emit-rtl.c
+++ b/gcc/emit-rtl.c
@@ -531,10 +531,9 @@ immed_double_const (HOST_WIDE_INT i0, HOST_WIDE_INT i1, enum machine_mode mode)
 
      1) If GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_WIDE_INT, then we use
 	gen_int_mode.
-     2) GET_MODE_BITSIZE (mode) == 2 * HOST_BITS_PER_WIDE_INT, but the value of
-	the integer fits into HOST_WIDE_INT anyway (i.e., i1 consists only
-	from copies of the sign bit, and sign of i0 and i1 are the same),  then
-	we return a CONST_INT for i0.
+     2) If the value of the integer fits into HOST_WIDE_INT anyway
+        (i.e., i1 consists only from copies of the sign bit, and sign
+	of i0 and i1 are the same), then we return a CONST_INT for i0.
      3) Otherwise, we create a CONST_DOUBLE for i0 and i1.  */
   if (mode != VOIDmode)
     {
diff --git a/gcc/explow.c b/gcc/explow.c
index 2fae1a1..c94ad25 100644
--- a/gcc/explow.c
+++ b/gcc/explow.c
@@ -73,14 +73,20 @@ trunc_int_for_mode (HOST_WIDE_INT c, enum machine_mode mode)
   return c;
 }
 
-/* Return an rtx for the sum of X and the integer C.  */
+/* Return an rtx for the sum of X and the integer C, given that X has
+   mode MODE.  This routine should be used instead of plus_constant
+   when they want to ensure that addition happens in a particular
+   mode, which is necessary when x can be a VOIDmode CONST_INT or
+   CONST_DOUBLE and the width of the constant is smaller than the
+   width of the expression.  */
+/* TODO: All callers of plus_constant should migrate to this routine,
+   and once they do, we can assert that mode is not VOIDmode.  */
 
 rtx
-plus_constant (rtx x, HOST_WIDE_INT c)
+plus_constant_mode (enum machine_mode mode, rtx x, HOST_WIDE_INT c)
 {
   RTX_CODE code;
   rtx y;
-  enum machine_mode mode;
   rtx tem;
   int all_constant = 0;
 
@@ -90,12 +96,26 @@ plus_constant (rtx x, HOST_WIDE_INT c)
  restart:
 
   code = GET_CODE (x);
-  mode = GET_MODE (x);
   y = x;
 
   switch (code)
     {
     case CONST_INT:
+      if (GET_MODE_BITSIZE (mode) > HOST_BITS_PER_WIDE_INT)
+	{
+	  unsigned HOST_WIDE_INT l1 = INTVAL (x);
+	  HOST_WIDE_INT h1 = (l1 >> (HOST_BITS_PER_WIDE_INT-1)) ? -1 : 0;
+	  unsigned HOST_WIDE_INT l2 = c;
+	  HOST_WIDE_INT h2 = c < 0 ? -1 : 0;
+	  unsigned HOST_WIDE_INT lv;
+	  HOST_WIDE_INT hv;
+
+	  if (add_double_with_sign (l1, h1, l2, h2, &lv, &hv, false))
+	    gcc_unreachable ();
+
+	  return immed_double_const (lv, hv, VOIDmode);
+	}
+
       return GEN_INT (INTVAL (x) + c);
 
     case CONST_DOUBLE:
@@ -103,11 +123,16 @@ plus_constant (rtx x, HOST_WIDE_INT c)
 	unsigned HOST_WIDE_INT l1 = CONST_DOUBLE_LOW (x);
 	HOST_WIDE_INT h1 = CONST_DOUBLE_HIGH (x);
 	unsigned HOST_WIDE_INT l2 = c;
-	HOST_WIDE_INT h2 = c < 0 ? ~0 : 0;
+	HOST_WIDE_INT h2 = c < 0 ? -1 : 0;
 	unsigned HOST_WIDE_INT lv;
 	HOST_WIDE_INT hv;
 
-	add_double (l1, h1, l2, h2, &lv, &hv);
+	if (add_double_with_sign (l1, h1, l2, h2, &lv, &hv, false))
+	  if (GET_MODE_BITSIZE (mode) > 2*HOST_BITS_PER_WIDE_INT)
+	    /* Sorry, we have no way to represent overflows this
+	       wide.  To fix, add constant support wider than
+	       CONST_DOUBLE.  */
+	    gcc_assert (0);
 
 	return immed_double_const (lv, hv, VOIDmode);
       }
@@ -121,8 +146,8 @@ plus_constant (rtx x, HOST_WIDE_INT c)
 	{
 	  tem
 	    = force_const_mem (GET_MODE (x),
-			       plus_constant (get_pool_constant (XEXP (x, 0)),
-					      c));
+			       plus_constant_mode (mode, get_pool_constant (XEXP (x, 0)),
+						   c));
 	  if (memory_address_p (GET_MODE (tem), XEXP (tem, 0)))
 	    return tem;
 	}
@@ -141,31 +166,17 @@ plus_constant (rtx x, HOST_WIDE_INT c)
       break;
 
     case PLUS:
-      /* The interesting case is adding the integer to a sum.
-	 Look for constant term in the sum and combine
-	 with C.  For an integer constant term, we make a combined
-	 integer.  For a constant term that is not an explicit integer,
-	 we cannot really combine, but group them together anyway.
-
-	 Restart or use a recursive call in case the remaining operand is
-	 something that we handle specially, such as a SYMBOL_REF.
+      /* The interesting case is adding the integer to a sum.  Look
+	 for constant term in the sum and combine with C.  For an
+	 integer constant term or a constant term that is not an
+	 explicit integer, we combine or group them together anyway.
 
 	 We may not immediately return from the recursive call here, lest
 	 all_constant gets lost.  */
 
-      if (CONST_INT_P (XEXP (x, 1)))
+      if (CONSTANT_P (XEXP (x, 1)))
 	{
-	  c += INTVAL (XEXP (x, 1));
-
-	  if (GET_MODE (x) != VOIDmode)
-	    c = trunc_int_for_mode (c, GET_MODE (x));
-
-	  x = XEXP (x, 0);
-	  goto restart;
-	}
-      else if (CONSTANT_P (XEXP (x, 1)))
-	{
-	  x = gen_rtx_PLUS (mode, XEXP (x, 0), plus_constant (XEXP (x, 1), c));
+	  x = gen_rtx_PLUS (mode, XEXP (x, 0), plus_constant_mode (mode, XEXP (x, 1), c));
 	  c = 0;
 	}
       else if (find_constant_term_loc (&y))
@@ -175,7 +186,7 @@ plus_constant (rtx x, HOST_WIDE_INT c)
 	  rtx copy = copy_rtx (x);
 	  rtx *const_loc = find_constant_term_loc (&copy);
 
-	  *const_loc = plus_constant (*const_loc, c);
+	  *const_loc = plus_constant_mode (mode, *const_loc, c);
 	  x = copy;
 	  c = 0;
 	}
@@ -195,6 +206,14 @@ plus_constant (rtx x, HOST_WIDE_INT c)
   else
     return x;
 }
+
+/* Return an rtx for the sum of X and the integer C.  */
+
+rtx
+plus_constant (rtx x, HOST_WIDE_INT c)
+{
+  return plus_constant_mode (GET_MODE (x), x, c);
+}
 

 /* If X is a sum, return a new sum like X but lacking any constant terms.
    Add all the removed constant terms into *CONSTPTR.
diff --git a/gcc/expmed.c b/gcc/expmed.c
index 3507dad..2361b7e 100644
--- a/gcc/expmed.c
+++ b/gcc/expmed.c
@@ -3122,9 +3122,11 @@ expand_mult (enum machine_mode mode, rtx op0, rtx op1, rtx target,
 	    {
 	      int shift = floor_log2 (CONST_DOUBLE_HIGH (op1))
 			  + HOST_BITS_PER_WIDE_INT;
-	      return expand_shift (LSHIFT_EXPR, mode, op0,
-				   build_int_cst (NULL_TREE, shift),
-				   target, unsignedp);
+	      if (shift < 2*HOST_BITS_PER_WIDE_INT-1
+		  || GET_MODE_BITSIZE (mode) <= 2*HOST_BITS_PER_WIDE_INT)
+		return expand_shift (LSHIFT_EXPR, mode, op0,
+				     build_int_cst (NULL_TREE, shift),
+				     target, unsignedp);
 	    }
 	}
 
diff --git a/gcc/rtl.h b/gcc/rtl.h
index 66f2755..9d1a28e 100644
--- a/gcc/rtl.h
+++ b/gcc/rtl.h
@@ -1586,6 +1586,7 @@ extern int ceil_log2 (unsigned HOST_WIDE_INT);
 /* In explow.c */
 extern HOST_WIDE_INT trunc_int_for_mode	(HOST_WIDE_INT, enum machine_mode);
 extern rtx plus_constant (rtx, HOST_WIDE_INT);
+extern rtx plus_constant_mode (enum machine_mode, rtx, HOST_WIDE_INT);
 
 /* In rtl.c */
 extern rtx rtx_alloc_stat (RTX_CODE MEM_STAT_DECL);
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index ce4eab4..ff838a8 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -97,6 +97,7 @@ mode_signbit_p (enum machine_mode mode, const_rtx x)
       width -= HOST_BITS_PER_WIDE_INT;
     }
   else
+    /* FIXME: We don't yet have a representation for wider modes.  */
     return false;
 
   if (width < HOST_BITS_PER_WIDE_INT)
@@ -1219,15 +1220,10 @@ simplify_const_unary_operation (enum rtx_code code, enum machine_mode mode,
       else
 	lv = CONST_DOUBLE_LOW (op),  hv = CONST_DOUBLE_HIGH (op);
 
-      if (op_mode == VOIDmode)
-	{
-	  /* We don't know how to interpret negative-looking numbers in
-	     this case, so don't try to fold those.  */
-	  if (hv < 0)
-	    return 0;
-	}
-      else if (GET_MODE_BITSIZE (op_mode) >= HOST_BITS_PER_WIDE_INT * 2)
-	;
+      if (op_mode == VOIDmode
+	  || GET_MODE_BITSIZE (op_mode) >= HOST_BITS_PER_WIDE_INT * 2)
+	/* We should never get a negative number.  */
+	gcc_assert (hv >= 0);
       else
 	hv = 0, lv &= GET_MODE_MASK (op_mode);
 
@@ -2214,7 +2210,9 @@ simplify_binary_operation_1 (enum rtx_code code, enum machine_mode mode,
 	      || GET_MODE_CLASS (GET_MODE (trueop1)) == MODE_INT)
 	  && GET_MODE (op0) == mode
 	  && CONST_DOUBLE_LOW (trueop1) == 0
-	  && (val = exact_log2 (CONST_DOUBLE_HIGH (trueop1))) >= 0)
+	  && (val = exact_log2 (CONST_DOUBLE_HIGH (trueop1))) >= 0
+	  && (val < 2*HOST_BITS_PER_WIDE_INT-1
+	      || GET_MODE_BITSIZE (mode) <= 2*HOST_BITS_PER_WIDE_INT))
 	return simplify_gen_binary (ASHIFT, mode, op0,
 				    GEN_INT (val + HOST_BITS_PER_WIDE_INT));
 
@@ -4987,6 +4985,7 @@ simplify_immed_subreg (enum machine_mode outermode, rtx op,
 	case CONST_DOUBLE:
 	  if (GET_MODE (el) == VOIDmode)
 	    {
+	      unsigned char extend = 0;
 	      /* If this triggers, someone should have generated a
 		 CONST_INT instead.  */
 	      gcc_assert (elem_bitsize > HOST_BITS_PER_WIDE_INT);
@@ -4999,13 +4998,15 @@ simplify_immed_subreg (enum machine_mode outermode, rtx op,
 		    = CONST_DOUBLE_HIGH (el) >> (i - HOST_BITS_PER_WIDE_INT);
 		  i += value_bit;
 		}
-	      /* It shouldn't matter what's done here, so fill it with
-		 zero.  */
+
+	      if (CONST_DOUBLE_HIGH (el) >> (HOST_BITS_PER_WIDE_INT-1))
+		extend = -1;
 	      for (; i < elem_bitsize; i += value_bit)
-		*vp++ = 0;
+		*vp++ = extend;
 	    }
 	  else
 	    {
+	      unsigned char extend = 0;
 	      long tmp[max_bitsize / 32];
 	      int bitsize = GET_MODE_BITSIZE (GET_MODE (el));
 
@@ -5030,10 +5031,10 @@ simplify_immed_subreg (enum machine_mode outermode, rtx op,
 		  *vp++ = tmp[ibase / 32] >> i % 32;
 		}
 
-	      /* It shouldn't matter what's done here, so fill it with
-		 zero.  */
+	      if (CONST_DOUBLE_HIGH (el) >> (HOST_BITS_PER_WIDE_INT-1))
+		extend = -1;
 	      for (; i < elem_bitsize; i += value_bit)
-		*vp++ = 0;
+		*vp++ = extend;
 	    }
 	  break;
 
