RISC-V: Non-standard encodings of `fence.i' (disasm)

This commit adds disassembler support for non-standard encodings of the
`fence.i' instruction.  Those fields are supposed to be zero but base
implementation shall ignore those fields.

gas/ChangeLog:

	* config/tc-riscv.c (validate_riscv_insn): Add ignored bit fields.
	(riscv_ip): Add support for ignored bit fields.
	* testsuite/gas/riscv/zifencei-dis-nonstd.s: New disasm test.
	* testsuite/gas/riscv/zifencei-dis-nonstd.d: Likewise.

opcodes/ChangeLog:

	* riscv-dis.c (print_insn_args): Add support for ignored bit fields.
	* riscv-opc.c (riscv_opcodes): Ignore RD/RS1/immediate (I-type)
	fields on the `fence.i' instruction.

gas/config/tc-riscv.c

@@ -1253,6 +1253,20 @@ validate_riscv_insn (const struct riscv_opcode *opc, int length)
 		goto unknown_validate_operand;
 	    }
 	  break;
+	case '~': /* Ignored fields (w/o defaults).  */
+	  while (*++oparg)
+	    {
+	      switch (*oparg)
+		{
+		case 'd': USE_BITS (OP_MASK_RD, OP_SH_RD); break;
+		case 's': USE_BITS (OP_MASK_RS1, OP_SH_RS1); break;
+		case 'j': used_bits |= ENCODE_ITYPE_IMM (-1U); break;
+		default:
+		  goto unknown_validate_operand;
+		}
+	    }
+	  --oparg;
+	  break;
 	case '!': /* Operand with special handlings.  */
 	  switch (*++oparg)
 	    {
@@ -3274,6 +3288,22 @@ riscv_ip (char *str, struct riscv_cl_insn *ip, expressionS *imm_expr,
 	      asarg = expr_end;
 	      continue;
 
+	    case '~': /* Ignored fields (w/o defaults).  */
+	      while (*++oparg)
+		{
+		  switch (*oparg)
+		    {
+		    case 'd': /* Ignored RD.  */
+		    case 's': /* Ignored RS1.  */
+		    case 'j': /* Ignored immediate (I-type).  */
+		      break;
+		    default:
+		      goto unknown_riscv_ip_operand;
+		    }
+		}
+	      --oparg;
+	      continue;
+
 	    case '!': /* Operand with special handlings.  */
 	      switch (*++oparg)
 		{

gas/testsuite/gas/riscv/zifencei-dis-nonstd.d

@@ -0,0 +1,15 @@
+#as: -march=rv32i_zifencei
+#source: zifencei-dis-nonstd.s
+#objdump: -dr
+
+.*:[ 	]+file format .*
+
+
+Disassembly of section .text:
+
+0+000 <target>:
+[ 	]+[0-9a-f]+:[ 	]+0000100f[ 	]+fence\.i
+[ 	]+[0-9a-f]+:[ 	]+0000100f[ 	]+fence\.i
+[ 	]+[0-9a-f]+:[ 	]+00001f8f[ 	]+fence\.i
+[ 	]+[0-9a-f]+:[ 	]+000f900f[ 	]+fence\.i
+[ 	]+[0-9a-f]+:[ 	]+fff0100f[ 	]+fence\.i

gas/testsuite/gas/riscv/zifencei-dis-nonstd.s

@@ -0,0 +1,8 @@
+target:
+	fence.i
+	# Standard encoding of fence.i
+	.insn i	MISC_MEM, 1,  x0,  x0,  0
+	# Following are non-standard encodings
+	.insn i	MISC_MEM, 1, x31,  x0,  0
+	.insn i	MISC_MEM, 1,  x0, x31,  0
+	.insn i	MISC_MEM, 1,  x0,  x0, -1

opcodes/riscv-dis.c

@@ -198,7 +198,7 @@ print_insn_args (const char *oparg, insn_t l, bfd_vma pc, disassemble_info *info
   fprintf_styled_ftype print = info->fprintf_styled_func;
   const char *opargStart;
 
-  if (*oparg != '\0')
+  if (*oparg != '\0' && *oparg != '~')
     print (info->stream, dis_style_text, "\t");
 
   for (; *oparg != '\0'; oparg++)
@@ -554,6 +554,9 @@ print_insn_args (const char *oparg, insn_t l, bfd_vma pc, disassemble_info *info
 	  print (info->stream, dis_style_text, "%d", rs1);
 	  break;
 
+	case '~': /* Ignored fields.  */
+	  return;
+
 	case '!': /* Operand with special handlings.  */
 	  switch (*++oparg)
 	    {

opcodes/riscv-opc.c

@@ -420,7 +420,7 @@ const struct riscv_opcode riscv_opcodes[] =
 {"pause",       0, INSN_CLASS_ZIHINTPAUSE, "",MATCH_PAUSE, MASK_PAUSE, match_opcode, 0 },
 {"fence",       0, INSN_CLASS_I, "",          MATCH_FENCE|MASK_PRED|MASK_SUCC, MASK_FENCE|MASK_RD|MASK_RS1|MASK_IMM, match_opcode, INSN_ALIAS },
 {"fence",       0, INSN_CLASS_I, "P,Q",       MATCH_FENCE, MASK_FENCE|MASK_RD|MASK_RS1|(MASK_IMM & ~MASK_PRED & ~MASK_SUCC), match_opcode, 0 },
-{"fence.i",     0, INSN_CLASS_ZIFENCEI, "",   MATCH_FENCE_I, MASK_FENCE|MASK_RD|MASK_RS1|MASK_IMM, match_opcode, 0 },
+{"fence.i",     0, INSN_CLASS_ZIFENCEI, "~dsj", MATCH_FENCE_I, MASK_FENCE_I, match_opcode, 0 },
 {"fence.tso",   0, INSN_CLASS_I, "",          MATCH_FENCE_TSO, MASK_FENCE_TSO|MASK_RD|MASK_RS1, match_opcode, INSN_ALIAS },
 {"rdcycle",     0, INSN_CLASS_I, "d",         MATCH_RDCYCLE, MASK_RDCYCLE, match_opcode, INSN_ALIAS },
 {"rdinstret",   0, INSN_CLASS_I, "d",         MATCH_RDINSTRET, MASK_RDINSTRET, match_opcode, INSN_ALIAS },