[committed] MIPS16: Remove "extended" BREAK/SDBBP handling

["Maciej W. Rozycki" <macro at imgtec dot com>]

Remove special casing for the `6' operand code used for the embedded 
trap code of the BREAK and the SDBBP instructions to support supposedly 
extended forms of these instructions.

According to all versions of the MIPS16 ASE specifications these 
instructions are not extensible [1][2][3][4][5][7][8][10][11], and as 
from revision 2.50 of the MIPS16e ASE specifications it has been further 
clarified what was previously implied, that non-extesiable instructions 
when preceded with an EXTEND prefix must cause a Reserved Instruction 
exception [5][6][9][10].

Therefore supposedly extended BREAK and SDBBP instructions do not serve 
their purpose anymore as they do not cause a Bp and a Debug exception 
respectively and supporting these forms in disassembly only causes 
confusion.

References:

[1] "Product Description, MIPS16 Application-Specific Extension", 
    Version 1.3, MIPS Technologies, Inc., 970130, Table 3. "MIPS16 
    Instruction Set Summary", p. 5

[2] same, Table 5 "RR Minor Opcodes (RR-type instructions)", p.10

[3] same, Table 18. "Extendable MIPS16 Instructions", p. 24

[4] "MIPS32 Architecture for Programmers, Volume IV-a: The MIPS16e
    Application-Specific Extension to the MIPS32 Architecture", MIPS 
    Technologies, Inc., Document Number: MD00076, Revision 2.63, July 
    16, 2013, Table 3.8 "MIPS16e Special Instructions", p. 38

[5] same, Section 3.11 "MIPS16e Extensible Instructions, p. 41

[6] same, Table 3.15 "MIPS16e Extensible Instructions", p. 41

[7] same, Table 3.24 "MIPS16e RR Encoding of the Funct Field", p. 49

[8] "MIPS64 Architecture for Programmers, Volume IV-a: The MIPS16e 
    Application-Specific Extension to the MIPS64 Architecture", MIPS 
    Technologies, Inc., Document Number: MD00077, Revision 2.60, June 
    25, 2008, Table 1.8 "MIPS16e Special Instructions", p. 39

[9] same, Section 1.11 "MIPS16e Extensible Instructions", p. 42

[10] same, Table 1.15 "MIPS16e Extensible Instructions", pp. 42-43

[11] same, Table 1.24 "MIPS16e RR Encoding of the Funct Field", p. 50

	gas/
	* config/tc-mips.c (match_mips16_insn): Remove the `6' operand 
	code special case and its associated comment.

	opcodes/
	* mips16-opc.c (decode_mips16_operand) <'6'>: Remove extended 
	encoding support.
---
binutils-mips16-opcode-format-6.diff
Index: binutils/gas/config/tc-mips.c
===================================================================
--- binutils.orig/gas/config/tc-mips.c	2016-12-16 01:00:34.865811996 +0000
+++ binutils/gas/config/tc-mips.c	2016-12-16 03:31:53.423098944 +0000
@@ -8188,14 +8188,7 @@ match_mips16_insn (struct mips_cl_insn *
       if (!operand)
 	abort ();
 
-      /* '6' is a special case.  It is used for BREAK and SDBBP,
-	 whose operands are only meaningful to the software that decodes
-	 them.  This means that there is no architectural reason why
-	 they cannot be prefixed by EXTEND, but in practice,
-	 exception handlers will only look at the instruction
-	 itself.  We therefore allow '6' to be extended when
-	 disassembling but not when assembling.  */
-      if (operand->type != OP_PCREL && c != '6')
+      if (operand->type != OP_PCREL)
 	{
 	  ext_operand = decode_mips16_operand (c, TRUE);
 	  if (operand != ext_operand)
Index: binutils/opcodes/mips16-opc.c
===================================================================
--- binutils.orig/opcodes/mips16-opc.c	2016-12-16 01:00:34.964660479 +0000
+++ binutils/opcodes/mips16-opc.c	2016-12-16 03:31:53.509611972 +0000
@@ -50,6 +50,7 @@ decode_mips16_operand (char type, bfd_bo
   switch (type)
     {
     case '0': MAPPED_REG (0, 0, GP, reg_0_map);
+    case '6': UINT (6, 5);
 
     case 'L': SPECIAL (6, 5, ENTRY_EXIT_LIST);
     case 'M': SPECIAL (7, 0, SAVE_RESTORE_LIST);
@@ -81,7 +82,6 @@ decode_mips16_operand (char type, bfd_bo
 
       case '4': SINT (15, 0);
       case '5': SINT (16, 0);
-      case '6': SINT (16, 0);
       case '8': SINT (16, 0);
 
       case 'A': PCREL (16, 0, TRUE, 0, 2, FALSE, FALSE);
@@ -109,7 +109,6 @@ decode_mips16_operand (char type, bfd_bo
 
       case '4': SINT (4, 0);
       case '5': UINT (5, 0);
-      case '6': UINT (6, 5);
       case '8': UINT (8, 0);
 
       case 'A': PCREL (8, 0, FALSE, 2, 2, FALSE, FALSE);