[partial backport of gcc-4.8/trunk r195885, adjusted for double_int being
 a normal ADT rather than a C++ object in 4.7 <mikpe@it.uu.se>]

gcc/

2013-02-08  Georg-Johann Lay  <avr@gjlay.de>

	PR tree-optimization/56064
	* fixed-value.c (fixed_from_double_int): Sign/zero extend payload
	bits according to mode.
	* fixed-value.h (fixed_from_double_int): Adjust comment.

--- gcc-4.7.3/gcc/fixed-value.c.~1~	2013-08-10 18:16:33.284220675 +0200
+++ gcc-4.7.3/gcc/fixed-value.c	2013-08-10 18:22:20.503633679 +0200
@@ -83,7 +83,7 @@ check_real_for_fixed_mode (REAL_VALUE_TY
 
 
 /* Construct a CONST_FIXED from a bit payload and machine mode MODE.
-   The bits in PAYLOAD are used verbatim.  */
+   The bits in PAYLOAD are sign-extended/zero-extended according to MODE.  */
 
 FIXED_VALUE_TYPE
 fixed_from_double_int (double_int payload, enum machine_mode mode)
@@ -92,7 +92,13 @@ fixed_from_double_int (double_int payloa
 
   gcc_assert (GET_MODE_BITSIZE (mode) <= HOST_BITS_PER_DOUBLE_INT);
 
-  value.data = payload;
+  if (SIGNED_SCALAR_FIXED_POINT_MODE_P (mode))
+    value.data = double_int_sext (payload, 1 + GET_MODE_IBIT (mode) + GET_MODE_FBIT (mode));
+  else if (UNSIGNED_SCALAR_FIXED_POINT_MODE_P (mode))
+    value.data = double_int_zext (payload, GET_MODE_IBIT (mode) + GET_MODE_FBIT (mode));
+  else
+    gcc_unreachable();
+
   value.mode = mode;
 
   return value;
--- gcc-4.7.3/gcc/fixed-value.h.~1~	2013-08-10 18:16:56.564181277 +0200
+++ gcc-4.7.3/gcc/fixed-value.h	2013-08-10 18:21:05.293760716 +0200
@@ -50,7 +50,7 @@ extern FIXED_VALUE_TYPE fconst1[MAX_FCON
 extern rtx const_fixed_from_fixed_value (FIXED_VALUE_TYPE, enum machine_mode);
 
 /* Construct a FIXED_VALUE from a bit payload and machine mode MODE.
-   The bits in PAYLOAD are used verbatim.  */
+   The bits in PAYLOAD are sign-extended/zero-extended according to MODE.  */
 extern FIXED_VALUE_TYPE fixed_from_double_int (double_int,
 						     enum machine_mode);