diff --git a/gcc/doc/rtl.texi b/gcc/doc/rtl.texi
index 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..6284d61 100644
--- a/gcc/explow.c
+++ b/gcc/explow.c
@@ -96,6 +96,9 @@ plus_constant (rtx x, HOST_WIDE_INT c)
   switch (code)
     {
     case CONST_INT:
+      if (GET_MODE_BITSIZE (mode) > HOST_WIDE_INT)
+	/* Punt for now.  */
+	goto overflow;
       return GEN_INT (INTVAL (x) + c);
 
     case CONST_DOUBLE:
@@ -103,10 +106,14 @@ 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 ? ~(HOST_WIDE_INT)0 : 0;
 	unsigned HOST_WIDE_INT lv;
 	HOST_WIDE_INT hv;
 
+	if (GET_MODE_BITSIZE (mode) > 2*HOST_WIDE_INT)
+	  /* Punt for now.  */
+	  goto overflow;
+
 	add_double (l1, h1, l2, h2, &lv, &hv);
 
 	return immed_double_const (lv, hv, VOIDmode);
@@ -141,6 +148,9 @@ plus_constant (rtx x, HOST_WIDE_INT c)
       break;
 
     case PLUS:
+      if (GET_MODE_BITSIZE (mode) > HOST_WIDE_INT)
+	/* Punt for now.  */
+	goto overflow;
       /* 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
@@ -185,6 +195,7 @@ plus_constant (rtx x, HOST_WIDE_INT c)
       break;
     }
 
+ overflow:
   if (c != 0)
     x = gen_rtx_PLUS (mode, x, GEN_INT (c));
 
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index ce4eab4..37e46b1 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -101,6 +101,7 @@ mode_signbit_p (enum machine_mode mode, const_rtx x)
 
   if (width < HOST_BITS_PER_WIDE_INT)
     val &= ((unsigned HOST_WIDE_INT) 1 << width) - 1;
+  /* FIXME: audit for TImode and wider correctness.  */
   return val == ((unsigned HOST_WIDE_INT) 1 << (width - 1));
 }