gdb: add extension language print_insn hook

This commit is setup for the next commit.

In the next commit I will add a Python API to intercept the print_insn
calls within GDB, each print_insn call is responsible for
disassembling, and printing one instruction.  After the next commit it
will be possible for a user to write Python code that either wraps
around the existing disassembler, or even, in extreme situations,
entirely replaces the existing disassembler.

This commit does not add any new Python API.

What this commit does is put the extension language framework in place
for a print_insn hook.  There's a new callback added to 'struct
extension_language_ops', which is then filled in with nullptr for Python
and Guile.

Finally, in the disassembler, the code is restructured so that the new
extension language function ext_lang_print_insn is called before we
delegate to gdbarch_print_insn.

After this, the next commit can focus entirely on providing a Python
implementation of the new print_insn callback.

There should be no user visible change after this commit.

1 files changed, 26 insertions, 3 deletions

diff --git a/gdb/disasm.c b/gdb/disasm.c
index 6ac8438..4af40c9 100644
--- a/gdb/disasm.c
+++ b/gdb/disasm.c
@@ -851,6 +851,29 @@ gdb_disassemble_info::~gdb_disassemble_info ()
   disassemble_free_target (&m_di);
 }
 
+/* Wrapper around calling gdbarch_print_insn.  This function takes care of
+   first calling the extension language hooks for print_insn, and, if none
+   of the extension languages can print this instruction, calls
+   gdbarch_print_insn to do the work.
+
+   GDBARCH is the architecture to disassemble in, VMA is the address of the
+   instruction being disassembled, and INFO is the libopcodes disassembler
+   related information.  */
+
+static int
+gdb_print_insn_1 (struct gdbarch *gdbarch, CORE_ADDR vma,
+		  struct disassemble_info *info)
+{
+  /* Call into the extension languages to do the disassembly.  */
+  gdb::optional<int> length = ext_lang_print_insn (gdbarch, vma, info);
+  if (length.has_value ())
+    return *length;
+
+  /* No extension language wanted to do the disassembly, so do it
+     manually.  */
+  return gdbarch_print_insn (gdbarch, vma, info);
+}
+
 /* See disasm.h.  */
 
 bool gdb_disassembler::use_ext_lang_colorization_p = true;
@@ -864,7 +887,7 @@ gdb_disassembler::print_insn (CORE_ADDR memaddr,
   m_err_memaddr.reset ();
   m_buffer.clear ();
 
-  int length = gdbarch_print_insn (arch (), memaddr, &m_di);
+  int length = gdb_print_insn_1 (arch (), memaddr, &m_di);
 
   /* If we have successfully disassembled an instruction, styling is on, we
      think that the extension language might be able to perform styling for
@@ -899,7 +922,7 @@ gdb_disassembler::print_insn (CORE_ADDR memaddr,
 	  gdb_assert (!m_buffer.term_out ());
 	  m_buffer.~string_file ();
 	  new (&m_buffer) string_file (true);
-	  length = gdbarch_print_insn (arch (), memaddr, &m_di);
+	  length = gdb_print_insn_1 (arch (), memaddr, &m_di);
 	  gdb_assert (length > 0);
 	}
     }
@@ -1054,7 +1077,7 @@ gdb_buffered_insn_length (struct gdbarch *gdbarch,
   gdb_buffered_insn_length_init_dis (gdbarch, &di, insn, max_len, addr,
 				     &disassembler_options_holder);
 
-  int result = gdbarch_print_insn (gdbarch, addr, &di);
+  int result = gdb_print_insn_1 (gdbarch, addr, &di);
   disassemble_free_target (&di);
   return result;
 }