RISC-V: Cache instruction support

Calling riscv_subset_supports repeatedly harms the performance in a
measurable way (about 3-13% in total on the most cases).

As a simple solution, this commit now caches instruction class support
(whether specific instruction class is supported) as a signed char array.

It is expected to have 5-7% performance improvements when disassembling
linked RISC-V ELF programs using objdump but this is particularly effective
with programs with many CSR instructions (up to ~42% on the author's PC).

include/ChangeLog:

	* opcode/riscv.h (enum riscv_insn_class): Add NUM_INSN_CLASSES.

opcodes/ChangeLog:

	* riscv-dis.c (riscv_insn_support_cache) New.
	(init_riscv_dis_state_for_arch): Clear the instruction support
	cache.  (riscv_disassemble_insn): Cache the instruction support.

include/opcode/riscv.h

@@ -456,6 +456,8 @@ enum riscv_insn_class
   INSN_CLASS_XTHEADMEMPAIR,
   INSN_CLASS_XTHEADSYNC,
   INSN_CLASS_XVENTANACONDOPS,
+
+  NUM_INSN_CLASSES,
 };
 
 /* This structure holds information for a particular instruction.  */

opcodes/riscv-dis.c

@@ -110,6 +110,9 @@ static bool no_aliases = false;
 
 /* If set, disassemble with numeric register names.  */
 static bool is_numeric = false;
+
+/* Instruction support cache.  */
+static signed char riscv_insn_support_cache[NUM_INSN_CLASSES];
 
 
 /* Set current disassembler context (dis_arch_context_current).
@@ -214,6 +217,9 @@ static void
 init_riscv_dis_state_for_arch (void)
 {
   is_arch_changed = true;
+  /* Clear instruction support cache.  */
+  for (size_t i = 0; i < NUM_INSN_CLASSES; i++)
+    riscv_insn_support_cache[i] = 0;
 }
 
 /* Initialization (for arch and options).  */
@@ -1023,7 +1029,14 @@ riscv_disassemble_insn (bfd_vma memaddr,
       if ((op->xlen_requirement != 0) && (op->xlen_requirement != xlen))
 	continue;
       /* Is this instruction supported by the current architecture?  */
-      if (!riscv_multi_subset_supports (&riscv_rps_dis, op->insn_class))
+      if (riscv_insn_support_cache[op->insn_class] == 0)
+	{
+	  riscv_insn_support_cache[op->insn_class]
+	      = riscv_multi_subset_supports (&riscv_rps_dis, op->insn_class)
+		    ? +1
+		    : -1;
+	}
+      if (riscv_insn_support_cache[op->insn_class] < 0)
 	continue;
 
       matched_op = op;