diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index d4f4dee27141..cc5435d54a60 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2017-06-12  Richard Sandiford  <richard.sandiford@linaro.org>
+
+	* combine.c (make_field_assignment): Check len rather than the mode
+	precision when calling force_to_mode.
+
 2017-06-12  Georg-Johann Lay  <avr@gjlay.de>
 
 	Support multilibs and devices that see flash in RAM address range.
diff --git a/gcc/combine.c b/gcc/combine.c
index 39ef3c6ecb82..2d49bc2233c9 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -9634,7 +9634,7 @@ make_field_assignment (rtx x)
 						     other, pos),
 			       dest);
   src = force_to_mode (src, mode,
-		       GET_MODE_PRECISION (mode) >= HOST_BITS_PER_WIDE_INT
+		       len >= HOST_BITS_PER_WIDE_INT
 		       ? HOST_WIDE_INT_M1U
 		       : (HOST_WIDE_INT_1U << len) - 1,
 		       0);