[RFC PATCH 1/1] gdb/riscv: Cache per-BFD disassembler

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

On RISC-V, calling the disassembler function (libopcodes) is not a small
cost.  This is because riscv_get_disassembler function sets the default
architecture from given BFD's .riscv.attributes section.  However, by
default, GDB calls this function for every instruction.

This commit replaces RISC-V's disassembler function and stop calling
riscv_get_disassembler function if current BFD has not changed.

It expects around 30-80% improvements on disassembling relatively large
chunk of RISC-V code but most of them will be obscured by a RISC-V
disassembler optimization the author is currently working on.  Still, 3-5%
of performance improvements will remain (due to reduced BFD section reads).
---
 gdb/riscv-tdep.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index feca17d9141..ed68a52df4c 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -55,6 +55,7 @@
 #include "cli/cli-decode.h"
 #include "observable.h"
 #include "prologue-value.h"
+#include "progspace.h"
 #include "arch/riscv.h"
 #include "riscv-ravenscar-thread.h"
 
@@ -1308,6 +1309,27 @@ riscv_print_one_register_info (struct gdbarch *gdbarch,
   gdb_printf (file, "\n");
 }
 
+/* Calling disassembler function on RISC-V is not fast as others.
+   We cache the disassembler function as long as current BFD is the same.  */
+
+static int
+riscv_print_insn (bfd_vma memaddr, disassemble_info *info)
+{
+  static disassembler_ftype disassemble_fn = NULL;
+  static bfd *abfd = NULL;
+  bfd *ebfd = current_program_space->exec_bfd ();
+
+  if (disassemble_fn == NULL || abfd != ebfd)
+    {
+      disassemble_fn = disassembler (
+	  info->arch, info->endian == BFD_ENDIAN_BIG, info->mach, ebfd);
+      abfd = ebfd;
+    }
+
+  gdb_assert (disassemble_fn != NULL);
+  return (*disassemble_fn) (memaddr, info);
+}
+
 /* Return true if REGNUM is a valid CSR register.  The CSR register space
    is sparsely populated, so not every number is a named CSR.  */
 
@@ -3926,6 +3948,7 @@ riscv_gdbarch_init (struct gdbarch_info info,
   set_gdbarch_pc_regnum (gdbarch, RISCV_PC_REGNUM);
 
   set_gdbarch_print_registers_info (gdbarch, riscv_print_registers_info);
+  set_gdbarch_print_insn (gdbarch, riscv_print_insn);
 
   set_tdesc_pseudo_register_name (gdbarch, riscv_pseudo_register_name);
   set_tdesc_pseudo_register_type (gdbarch, riscv_pseudo_register_type);
-- 
2.34.1