Index: gas/read.c
===================================================================
--- gas/read.c	(revision 394269)
+++ gas/read.c	(working copy)
@@ -1312,10 +1312,10 @@ read_a_source_file (char *name)
 }
 
 /* Convert O_constant expression EXP into the equivalent O_big representation.
-   Take the sign of the number from X_unsigned rather than X_add_number.  */
+   Take the sign of the number from SIGN rather than X_add_number.  */
 
 static void
-convert_to_bignum (expressionS *exp)
+convert_to_bignum (expressionS *exp, int sign)
 {
   valueT value;
   unsigned int i;
@@ -1328,8 +1328,8 @@ convert_to_bignum (expressionS *exp)
     }
   /* Add a sequence of sign bits if the top bit of X_add_number is not
      the sign of the original value.  */
-  if ((exp->X_add_number < 0) != !exp->X_unsigned)
-    generic_bignum[i++] = exp->X_unsigned ? 0 : LITTLENUM_MASK;
+  if ((exp->X_add_number < 0) == !sign)
+    generic_bignum[i++] = sign ? LITTLENUM_MASK : 0;
   exp->X_op = O_big;
   exp->X_add_number = i;
 }
@@ -4390,7 +4390,7 @@ emit_expr (expressionS *exp, unsigned in
   if (op == O_constant && nbytes > sizeof (valueT))
     {
       extra_digit = exp->X_unsigned ? 0 : -1;
-      convert_to_bignum (exp);
+      convert_to_bignum (exp, !exp->X_unsigned);
       op = O_big;
     }
 
@@ -4684,6 +4684,7 @@ parse_bitfield_cons (exp, nbytes)
       exp->X_add_number = value;
       exp->X_op = O_constant;
       exp->X_unsigned = 1;
+      exp->X_extrabit = 0;
     }
 }
 
@@ -5243,12 +5244,12 @@ emit_leb128_expr (expressionS *exp, int 
     }
   else if (op == O_constant
 	   && sign
-	   && (exp->X_add_number < 0) != !exp->X_unsigned)
+	   && (exp->X_add_number < 0) == !exp->X_extrabit)
     {
       /* We're outputting a signed leb128 and the sign of X_add_number
 	 doesn't reflect the sign of the original value.  Convert EXP
 	 to a correctly-extended bignum instead.  */
-      convert_to_bignum (exp);
+      convert_to_bignum (exp, exp->X_extrabit);
       op = O_big;
     }
 
Index: gas/testsuite/gas/all/sleb128-3.d
===================================================================
--- gas/testsuite/gas/all/sleb128-3.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-3.d	(revision 0)
@@ -0,0 +1,7 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (3)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 9c7f2a00 00000000 00000000 00000000  .*
Index: gas/testsuite/gas/all/sleb128-5.d
===================================================================
--- gas/testsuite/gas/all/sleb128-5.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-5.d	(revision 0)
@@ -0,0 +1,7 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (5)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 012a0000 00000000 00000000 00000000  .*
Index: gas/testsuite/gas/all/gas.exp
===================================================================
--- gas/testsuite/gas/all/gas.exp	(revision 394269)
+++ gas/testsuite/gas/all/gas.exp	(working copy)
@@ -352,6 +352,12 @@ if { ![istarget "bfin-*-*"] } then {
     run_dump_test assign
 }
 run_dump_test sleb128
+run_dump_test sleb128-2
+run_dump_test sleb128-3
+run_dump_test sleb128-4
+run_dump_test sleb128-5
+run_dump_test sleb128-6
+run_dump_test sleb128-7
 
 # .byte is 32 bits on tic4x, and .p2align isn't supported on tic54x
 # .space is different on hppa*-hpux.
Index: gas/testsuite/gas/all/sleb128-7.d
===================================================================
--- gas/testsuite/gas/all/sleb128-7.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-7.d	(revision 0)
@@ -0,0 +1,7 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (7)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 cb012ac5 012acb01 2ac5012a 00000000  .*
Index: gas/testsuite/gas/all/sleb128-3.s
===================================================================
--- gas/testsuite/gas/all/sleb128-3.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-3.s	(revision 0)
@@ -0,0 +1,4 @@
+.data
+bar:
+.sleb128 100 - 200
+.byte 42
Index: gas/testsuite/gas/all/sleb128-5.s
===================================================================
--- gas/testsuite/gas/all/sleb128-5.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-5.s	(revision 0)
@@ -0,0 +1,13 @@
+.text
+.globl foo
+foo:
+.L1:
+.byte 0
+.byte 0
+.byte 0
+.L2:
+
+.data
+bar:
+.sleb128 .L1 - .L2 + 4
+.byte 42
Index: gas/testsuite/gas/all/sleb128-7.s
===================================================================
--- gas/testsuite/gas/all/sleb128-7.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-7.s	(revision 0)
@@ -0,0 +1,19 @@
+.text
+.globl foo
+foo:
+.L1:
+.byte 0
+.byte 0
+.byte 0
+.L2:
+
+.data
+bar:
+.sleb128 200+(.L2 - .L1)
+.byte 42
+.sleb128 200+(.L1 - .L2)
+.byte 42
+.sleb128 (.L2 - .L1)+200
+.byte 42
+.sleb128 (.L1 - .L2)+200
+.byte 42
Index: gas/testsuite/gas/all/sleb128-2.d
===================================================================
--- gas/testsuite/gas/all/sleb128-2.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-2.d	(revision 0)
@@ -0,0 +1,7 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (2)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 7d2a0000 00000000 00000000 00000000  .*
Index: gas/testsuite/gas/all/sleb128-4.d
===================================================================
--- gas/testsuite/gas/all/sleb128-4.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-4.d	(revision 0)
@@ -0,0 +1,7 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (4)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 83808080 082a0000 00000000 00000000  .*
Index: gas/testsuite/gas/all/sleb128-6.d
===================================================================
--- gas/testsuite/gas/all/sleb128-6.d	(revision 0)
+++ gas/testsuite/gas/all/sleb128-6.d	(revision 0)
@@ -0,0 +1,9 @@
+#objdump : -s -j .data
+#name : .sleb128 tests (6)
+
+.*: .*
+
+Contents of section \.data:
+ 0000 83808080 80808080 80012afd ffffffff  .*
+ 0010 ffffffff 002a8380 80808080 80808001  .*
+ 0020 2afdffff ffffffff ffff002a 00000000  .*
Index: gas/testsuite/gas/all/sleb128-2.s
===================================================================
--- gas/testsuite/gas/all/sleb128-2.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-2.s	(revision 0)
@@ -0,0 +1,13 @@
+.text
+.globl foo
+foo:
+.L1:
+.byte 0
+.byte 0
+.byte 0
+.L2:
+
+.data
+bar:
+.sleb128 .L1 - .L2
+.byte 42
Index: gas/testsuite/gas/all/sleb128-4.s
===================================================================
--- gas/testsuite/gas/all/sleb128-4.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-4.s	(revision 0)
@@ -0,0 +1,13 @@
+.text
+.globl foo
+foo:
+.L1:
+.byte 0
+.byte 0
+.byte 0
+.L2:
+
+.data
+bar:
+.sleb128 .L2 - .L1 + (1 << 31)
+.byte 42
Index: gas/testsuite/gas/all/sleb128-6.s
===================================================================
--- gas/testsuite/gas/all/sleb128-6.s	(revision 0)
+++ gas/testsuite/gas/all/sleb128-6.s	(revision 0)
@@ -0,0 +1,19 @@
+.text
+.globl foo
+foo:
+.L1:
+.byte 0
+.byte 0
+.byte 0
+.L2:
+
+.data
+bar:
+.sleb128 1<<63+(.L2 - .L1)
+.byte 42
+.sleb128 1<<63+(.L1 - .L2)
+.byte 42
+.sleb128 (.L2 - .L1)+1<<63
+.byte 42
+.sleb128 (.L1 - .L2)+1<<63
+.byte 42
Index: gas/expr.c
===================================================================
--- gas/expr.c	(revision 394269)
+++ gas/expr.c	(working copy)
@@ -90,6 +90,7 @@ make_expr_symbol (expressionS *expressio
       zero.X_op = O_constant;
       zero.X_add_number = 0;
       zero.X_unsigned = 0;
+      zero.X_extrabit = 0;
       clean_up_expression (&zero);
       expressionP = &zero;
     }
@@ -161,6 +162,7 @@ expr_build_uconstant (offsetT value)
   e.X_op = O_constant;
   e.X_add_number = value;
   e.X_unsigned = 1;
+  e.X_extrabit = 0;
   return make_expr_symbol (&e);
 }
 
@@ -732,6 +734,7 @@ operand (expressionS *expressionP, enum 
      something like ``.quad 0x80000000'' is not sign extended even
      though it appears negative if valueT is 32 bits.  */
   expressionP->X_unsigned = 1;
+  expressionP->X_extrabit = 0;
 
   /* Digits, assume it is a bignum.  */
 
@@ -1026,6 +1029,8 @@ operand (expressionS *expressionP, enum 
 		   This is compatible with other people's
 		   assemblers.  Sigh.  */
 		expressionP->X_unsigned = 0;
+		if (expressionP->X_add_number)
+		  expressionP->X_extrabit ^= 1;
 	      }
 	    else if (c == '~' || c == '"')
 	      expressionP->X_add_number = ~ expressionP->X_add_number;
@@ -1078,6 +1083,7 @@ operand (expressionS *expressionP, enum 
 		expressionP->X_add_number = i >= expressionP->X_add_number;
 		expressionP->X_op = O_constant;
 		expressionP->X_unsigned = 1;
+		expressionP->X_extrabit = 0;
 	      }
 	  }
 	else if (expressionP->X_op != O_illegal
@@ -1717,6 +1723,42 @@ operatorf (int *num_chars)
   /* NOTREACHED  */
 }
 
+/* Implement "word-size + 1 bit" addition for
+   {resultP->X_extrabit:resultP->X_add_number} + {rhs_highbit:amount}.  This
+   is used so that the full range of unsigned word values and the full range of
+   signed word values can be represented in an O_constant expression, which is
+   useful e.g. for .sleb128 directives.  */
+
+static void
+add_to_result (expressionS *resultP, offsetT amount, int rhs_highbit)
+{
+  valueT ures = resultP->X_add_number;
+  valueT uamount = amount;
+
+  resultP->X_add_number += amount;
+
+  resultP->X_extrabit ^= rhs_highbit;
+
+  if (ures + uamount < ures)
+    resultP->X_extrabit ^= 1;
+}
+
+/* Similarly, for subtraction.  */
+
+static void
+subtract_from_result (expressionS *resultP, offsetT amount, int rhs_highbit)
+{
+  valueT ures = resultP->X_add_number;
+  valueT uamount = amount;
+
+  resultP->X_add_number -= amount;
+
+  resultP->X_extrabit ^= rhs_highbit;
+
+  if (ures < uamount)
+    resultP->X_extrabit ^= 1;
+}
+
 /* Parse an expression.  */
 
 segT
@@ -1829,7 +1871,7 @@ expr (int rankarg,		/* Larger # is highe
 	  && (md_register_arithmetic || resultP->X_op != O_register))
 	{
 	  /* X + constant.  */
-	  resultP->X_add_number += right.X_add_number;
+	  add_to_result (resultP, right.X_add_number, right.X_extrabit);
 	}
       /* This case comes up in PIC code.  */
       else if (op_left == O_subtract
@@ -1847,10 +1889,11 @@ expr (int rankarg,		/* Larger # is highe
 				       symbol_get_frag (right.X_add_symbol),
 				       &frag_off))
 	{
-	  resultP->X_add_number -= right.X_add_number;
-	  resultP->X_add_number -= frag_off / OCTETS_PER_BYTE;
-	  resultP->X_add_number += (S_GET_VALUE (resultP->X_add_symbol)
-				    - S_GET_VALUE (right.X_add_symbol));
+	  offsetT symval_diff = S_GET_VALUE (resultP->X_add_symbol)
+				- S_GET_VALUE (right.X_add_symbol);
+	  subtract_from_result (resultP, right.X_add_number, right.X_extrabit);
+	  subtract_from_result (resultP, frag_off / OCTETS_PER_BYTE, 0);
+	  add_to_result (resultP, symval_diff, symval_diff < 0);
 	  resultP->X_op = O_constant;
 	  resultP->X_add_symbol = 0;
 	}
@@ -1858,7 +1901,7 @@ expr (int rankarg,		/* Larger # is highe
 	       && (md_register_arithmetic || resultP->X_op != O_register))
 	{
 	  /* X - constant.  */
-	  resultP->X_add_number -= right.X_add_number;
+	  subtract_from_result (resultP, right.X_add_number, right.X_extrabit);
 	}
       else if (op_left == O_add && resultP->X_op == O_constant
 	       && (md_register_arithmetic || right.X_op != O_register))
@@ -1867,7 +1910,7 @@ expr (int rankarg,		/* Larger # is highe
 	  resultP->X_op = right.X_op;
 	  resultP->X_add_symbol = right.X_add_symbol;
 	  resultP->X_op_symbol = right.X_op_symbol;
-	  resultP->X_add_number += right.X_add_number;
+	  add_to_result (resultP, right.X_add_number, right.X_extrabit);
 	  retval = rightseg;
 	}
       else if (resultP->X_op == O_constant && right.X_op == O_constant)
@@ -1907,7 +1950,9 @@ expr (int rankarg,		/* Larger # is highe
 	      /* Constant + constant (O_add) is handled by the
 		 previous if statement for constant + X, so is omitted
 		 here.  */
-	    case O_subtract:		resultP->X_add_number -= v; break;
+	    case O_subtract:
+	      subtract_from_result (resultP, v, 0);
+	      break;
 	    case O_eq:
 	      resultP->X_add_number =
 		resultP->X_add_number == v ? ~ (offsetT) 0 : 0;
@@ -1951,10 +1996,11 @@ expr (int rankarg,		/* Larger # is highe
 	  resultP->X_op = op_left;
 	  resultP->X_op_symbol = right.X_add_symbol;
 	  if (op_left == O_add)
-	    resultP->X_add_number += right.X_add_number;
+	    add_to_result (resultP, right.X_add_number, right.X_extrabit);
 	  else if (op_left == O_subtract)
 	    {
-	      resultP->X_add_number -= right.X_add_number;
+	      subtract_from_result (resultP, right.X_add_number,
+				    right.X_extrabit);
 	      if (retval == rightseg
 		  && SEG_NORMAL (retval)
 		  && !S_FORCE_RELOC (resultP->X_add_symbol, 0)
@@ -1974,6 +2020,7 @@ expr (int rankarg,		/* Larger # is highe
 	  resultP->X_op = op_left;
 	  resultP->X_add_number = 0;
 	  resultP->X_unsigned = 1;
+	  resultP->X_extrabit = 0;
 	}
 
       if (retval != rightseg)
Index: gas/expr.h
===================================================================
--- gas/expr.h	(revision 394269)
+++ gas/expr.h	(working copy)
@@ -136,6 +136,11 @@ typedef struct expressionS {
      when performing arithmetic on these values).
      FIXME: This field is not set very reliably.  */
   unsigned int X_unsigned : 1;
+  /* This is used to implement "word size + 1 bit" arithmetic, so that e.g.
+     expressions used with .sleb128 directives can use the full range available
+     for an unsigned word, but can also properly represent all values of a
+     signed word.  */
+  unsigned int X_extrabit : 1;
 
   /* 7 additional bits can be defined if needed.  */