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[committed 4/5] MIPS16/GAS: Improve non-constant operand error diagnostics

Improve operand error diagnostics for non-constant expressions used for 
a 16-bit immediate, making the message more descriptive and indicating 
the offending operand, e.g.:

foo.s:1: Error: invalid operands `lui $2,foo-bar'

will show as:

foo.s:1: Error: operand 2 must be constant `lui $2,foo-bar'

This case does not currently trigger however, for two reasons.

First, for regular MIPS and microMIPS assembly in the case of no match 
caused by `match_int_operand' here, the function is always called again 
from `mips_ip' via `match_insns', `match_insn' and then `match_operand' 
for the same opcode table's entry with `lax_match' set to TRUE, in which 
case the attempt to match succeeds and no error is issued.

Second, in the case of MIPS16 assembly no call to `match_int_operand' is 
made at all for 16-bit immediates, because such immediates are currently 
only matched with extensible instructions, and these are handled in 
`match_mips16_insn' via `match_expression' directly rather than via 

This will change for MIPS16 code with MIPS16e2 support introduced, where 
non-extensible instructions accepting 16-bit immediates will be added, 
so make the case work well right from the start.

	* config/tc-mips.c (match_int_operand): Call 
	`match_not_constant' before returning failure for a non-constant
	16-bit immediate conditionally allowed.
Index: binutils/gas/config/tc-mips.c
--- binutils.orig/gas/config/tc-mips.c	2017-05-12 13:49:05.778390286 +0100
+++ binutils/gas/config/tc-mips.c	2017-05-12 17:01:44.141609779 +0100
@@ -5082,7 +5082,10 @@ match_int_operand (struct mips_arg_info 
 	  /* Accept non-constant operands if no later alternative matches,
 	     leaving it for the caller to process.  */
 	  if (!arg->lax_match)
-	    return FALSE;
+	    {
+	      match_not_constant (arg);
+	      return FALSE;
+	    }
 	  offset_reloc[0] = BFD_RELOC_LO16;
 	  return TRUE;

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