[PATCH v2 2/3] RISC-V: Fallback on faster hash table

[Tsukasa OI <research_trasio@irq.a4lg.com>]

From: Tsukasa OI <research_trasio@irq.a4lg.com>

Although it does not have a problem on current GNU Binutils implementation,
if the custom vendor implements an instruction which spans across multiple
major opcodes (e.g. uses both CUSTOM_0 and CUSTOM_1 in a *single* custom
instruction), the original assumption of the sorted hash table breaks.

In this case, this commit enables the fallback mode to disable all
optimizations except filtering macros out.

Note that, if a such instruction (that disables this disassembler
optimization) is upstreamed to Binutils, a separate solution will be
required to avoid major performance degradation when such instruction is
not used.  The intent of this commit is to make a room for custom vendors
to implement such instructions in *their* tree without causing
disassembler problems.

opcodes/ChangeLog:

	* riscv-dis.c (is_riscv_hash_fallback) New.
	(build_riscv_opcodes_hash_table): If an instruction spans across
	multiple major opcodes, enable fallback mode and disable sorting.
	(riscv_disassemble_insn): If the fallback mode is enabled, scan
	through all instructions instead of scanning only instruction
	entries matching the hash value.
---
 opcodes/riscv-dis.c | 31 ++++++++++++++++++++++++++-----
 1 file changed, 26 insertions(+), 5 deletions(-)

diff --git a/opcodes/riscv-dis.c b/opcodes/riscv-dis.c
index 4267b3ccf88c..3f6cbf5a3680 100644
--- a/opcodes/riscv-dis.c
+++ b/opcodes/riscv-dis.c
@@ -838,6 +838,9 @@ print_insn_args (const char *oparg, insn_t l, bfd_vma pc, disassemble_info *info
 static const struct riscv_opcode **riscv_hash[OP_HASH_LEN + 1];
 static const struct riscv_opcode **riscv_opcodes_sorted;
 
+/* Whether the fallback should be used.  */
+static bool is_riscv_hash_fallback = false;
+
 /* Compare two riscv_opcode* objects to sort by hash index.  */
 
 static int
@@ -868,15 +871,25 @@ build_riscv_opcodes_hash_table (void)
 
   /* Sort riscv_opcodes entry pointers (except macros).  */
   for (op = riscv_opcodes; op->name; op++)
-    if (op->pinfo != INSN_MACRO)
+    {
+      if (op->pinfo == INSN_MACRO)
+	continue;
       len++;
+      if (is_riscv_hash_fallback)
+	continue;
+      if (OP_HASH_IDX (op->match) < OP_MASK_OP2
+	      ? (op->mask & OP_MASK_OP2) != OP_MASK_OP2
+	      : (op->mask & OP_MASK_OP)  != OP_MASK_OP)
+	is_riscv_hash_fallback = true;
+    }
   riscv_opcodes_sorted = xcalloc (len, sizeof (struct riscv_opcode *));
   pop_end = riscv_opcodes_sorted;
   for (op = riscv_opcodes; op->name; op++)
     if (op->pinfo != INSN_MACRO)
       *pop_end++ = op;
-  qsort (riscv_opcodes_sorted, len, sizeof (struct riscv_opcode *),
-	 compare_opcodes);
+  if (!is_riscv_hash_fallback)
+    qsort (riscv_opcodes_sorted, len, sizeof (struct riscv_opcode *),
+	   compare_opcodes);
 
   /* Initialize faster hash table.  */
   pop = riscv_opcodes_sorted;
@@ -922,8 +935,16 @@ riscv_disassemble_insn (bfd_vma memaddr,
   info->target2 = 0;
 
   matched_op = NULL;
-  pop     = riscv_hash[OP_HASH_IDX (word)];
-  pop_end = riscv_hash[OP_HASH_IDX (word) + 1];
+  if (!is_riscv_hash_fallback)
+    {
+      pop     = riscv_hash[OP_HASH_IDX (word)];
+      pop_end = riscv_hash[OP_HASH_IDX (word) + 1];
+    }
+  else
+    {
+      pop     = riscv_hash[0];
+      pop_end = riscv_hash[OP_HASH_LEN];
+    }
   for (; pop != pop_end; pop++)
     {
       op = *pop;
-- 
2.38.1