Re: [PATCH 2/8] Add new gdbarch method, unconditional_branch_address

[Kevin Buettner <kevinb at redhat dot com>]

On Tue, 25 Aug 2015 13:13:49 +0100
Pedro Alves <palves@redhat.com> wrote:

> > +# Examine instruction at PC.  If instruction at PC is an unconditional
> > +# branch, return the address to which control is transferred when the
> > +# branch is taken.  Return 0 when this method is not implemented by
> > +# architecture, PC refers to an invalid address, or instruction at PC
> > +# is not an unconditional branch.
> > +m:CORE_ADDR:unconditional_branch_address:CORE_ADDR pc:pc::default_unconditional_branch_address::0
> > +
> 
> In addition to the comments in the other patch, I wonder if we would
> better name this in terms of a higher level concept, around
> "breakpoint address" instead of a "low level branch destination"?

I gave this some thought, but could not come up with a name that was
meaningful yet not overly long.  If you have a name that you like, I'm
quite willing to revise the patches again.

In the midst of adding a parameter for holding the branch destination,
I noticed that gdbarch.sh has a method called insn_is_jump, which is
used in btrace.c.  (There is also insn_is_call and insn_is_ret.)  I
thought about using insn_is_jump, but I would need to modify it
to return the branch destination.

I decided that the work I'm doing here should have it's own method.
If GCC should someday emit DWARF which sets is_stmt to 0 for the
branch instruction at the beginning of a while loop, this work might
not be needed any longer.  It'll be easier to rip it out and clean things
up again if I don't modify gdbarch methods that were added for
other purposes.

In the end, I decided to go with the name that you suggested in your
review of patch #3, unconditional_branch_destination.

Here's the revised patch:


    Add new gdbarch method, unconditional_branch_destination.
    
    gdb/ChangeLog:
    
    	* gdbarch.sh (unconditional_branch_destination): New gdbarch method.
    	* gdbarch.h, gdbarch.c: Regenerate.
    	* arch-utils.h (default_unconditional_branch_destination): Declare.
    	* arch-utils.c (default_unconditional_branch_destination): New
    	function.
---
 gdb/arch-utils.c |  7 +++++++
 gdb/arch-utils.h |  4 ++++
 gdb/gdbarch.c    | 23 +++++++++++++++++++++++
 gdb/gdbarch.h    |  9 +++++++++
 gdb/gdbarch.sh   |  6 ++++++
 5 files changed, 49 insertions(+)

diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 7df5570..23f9e69 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -821,6 +821,13 @@ default_gen_return_address (struct gdbarch *gdbarch,
 }
 
 int
+default_unconditional_branch_destination (struct gdbarch *gdbarch, CORE_ADDR pc,
+                                          CORE_ADDR *dest)
+{
+  return 0;
+}
+
+int
 default_return_in_first_hidden_param_p (struct gdbarch *gdbarch,
 					struct type *type)
 {
diff --git a/gdb/arch-utils.h b/gdb/arch-utils.h
index 18e2290..e3a9a7d 100644
--- a/gdb/arch-utils.h
+++ b/gdb/arch-utils.h
@@ -204,4 +204,8 @@ extern char *default_gcc_target_options (struct gdbarch *gdbarch);
 extern const char *default_gnu_triplet_regexp (struct gdbarch *gdbarch);
 extern int default_addressable_memory_unit_size (struct gdbarch *gdbarch);
 
+extern int default_unconditional_branch_destination (struct gdbarch *gdbarch,
+                                                     CORE_ADDR pc,
+					             CORE_ADDR *dest);
+
 #endif
diff --git a/gdb/gdbarch.c b/gdb/gdbarch.c
index f04eef9..a2e7dfb 100644
--- a/gdb/gdbarch.c
+++ b/gdb/gdbarch.c
@@ -330,6 +330,7 @@ struct gdbarch
   gdbarch_gcc_target_options_ftype *gcc_target_options;
   gdbarch_gnu_triplet_regexp_ftype *gnu_triplet_regexp;
   gdbarch_addressable_memory_unit_size_ftype *addressable_memory_unit_size;
+  gdbarch_unconditional_branch_destination_ftype *unconditional_branch_destination;
 };
 
 /* Create a new ``struct gdbarch'' based on information provided by
@@ -432,6 +433,7 @@ gdbarch_alloc (const struct gdbarch_info *info,
   gdbarch->gcc_target_options = default_gcc_target_options;
   gdbarch->gnu_triplet_regexp = default_gnu_triplet_regexp;
   gdbarch->addressable_memory_unit_size = default_addressable_memory_unit_size;
+  gdbarch->unconditional_branch_destination = default_unconditional_branch_destination;
   /* gdbarch_alloc() */
 
   return gdbarch;
@@ -674,6 +676,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of gcc_target_options, invalid_p == 0 */
   /* Skip verify of gnu_triplet_regexp, invalid_p == 0 */
   /* Skip verify of addressable_memory_unit_size, invalid_p == 0 */
+  /* Skip verify of unconditional_branch_destination, invalid_p == 0 */
   buf = ui_file_xstrdup (log, &length);
   make_cleanup (xfree, buf);
   if (length > 0)
@@ -1364,6 +1367,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
                       "gdbarch_dump: target_desc = %s\n",
                       host_address_to_string (gdbarch->target_desc));
   fprintf_unfiltered (file,
+                      "gdbarch_dump: unconditional_branch_destination = <%s>\n",
+                      host_address_to_string (gdbarch->unconditional_branch_destination));
+  fprintf_unfiltered (file,
                       "gdbarch_dump: gdbarch_unwind_pc_p() = %d\n",
                       gdbarch_unwind_pc_p (gdbarch));
   fprintf_unfiltered (file,
@@ -4761,6 +4767,23 @@ set_gdbarch_addressable_memory_unit_size (struct gdbarch *gdbarch,
   gdbarch->addressable_memory_unit_size = addressable_memory_unit_size;
 }
 
+int
+gdbarch_unconditional_branch_destination (struct gdbarch *gdbarch, CORE_ADDR pc, CORE_ADDR *dest)
+{
+  gdb_assert (gdbarch != NULL);
+  gdb_assert (gdbarch->unconditional_branch_destination != NULL);
+  if (gdbarch_debug >= 2)
+    fprintf_unfiltered (gdb_stdlog, "gdbarch_unconditional_branch_destination called\n");
+  return gdbarch->unconditional_branch_destination (gdbarch, pc, dest);
+}
+
+void
+set_gdbarch_unconditional_branch_destination (struct gdbarch *gdbarch,
+                                              gdbarch_unconditional_branch_destination_ftype unconditional_branch_destination)
+{
+  gdbarch->unconditional_branch_destination = unconditional_branch_destination;
+}
+
 
 /* Keep a registry of per-architecture data-pointers required by GDB
    modules.  */
diff --git a/gdb/gdbarch.h b/gdb/gdbarch.h
index 82e0259..2d0cf8d 100644
--- a/gdb/gdbarch.h
+++ b/gdb/gdbarch.h
@@ -1478,6 +1478,15 @@ typedef int (gdbarch_addressable_memory_unit_size_ftype) (struct gdbarch *gdbarc
 extern int gdbarch_addressable_memory_unit_size (struct gdbarch *gdbarch);
 extern void set_gdbarch_addressable_memory_unit_size (struct gdbarch *gdbarch, gdbarch_addressable_memory_unit_size_ftype *addressable_memory_unit_size);
 
+/* Examine instruction at PC.  If instruction at PC is an unconditional
+   branch, return 1 and set *DEST to the branch destination address.
+   Return 0 when this method is not implemented by architecture, PC refers
+   to an invalid address, or instruction at PC is not an unconditional branch. */
+
+typedef int (gdbarch_unconditional_branch_destination_ftype) (struct gdbarch *gdbarch, CORE_ADDR pc, CORE_ADDR *dest);
+extern int gdbarch_unconditional_branch_destination (struct gdbarch *gdbarch, CORE_ADDR pc, CORE_ADDR *dest);
+extern void set_gdbarch_unconditional_branch_destination (struct gdbarch *gdbarch, gdbarch_unconditional_branch_destination_ftype *unconditional_branch_destination);
+
 /* Definition for an unknown syscall, used basically in error-cases.  */
 #define UNKNOWN_SYSCALL (-1)
 
diff --git a/gdb/gdbarch.sh b/gdb/gdbarch.sh
index 388920f..b87fd19 100755
--- a/gdb/gdbarch.sh
+++ b/gdb/gdbarch.sh
@@ -1123,6 +1123,12 @@ m:const char *:gnu_triplet_regexp:void:::default_gnu_triplet_regexp::0
 # each address in memory.
 m:int:addressable_memory_unit_size:void:::default_addressable_memory_unit_size::0
 
+# Examine instruction at PC.  If instruction at PC is an unconditional
+# branch, return 1 and set *DEST to the branch destination address.
+# Return 0 when this method is not implemented by architecture, PC refers
+# to an invalid address, or instruction at PC is not an unconditional branch.
+m:int:unconditional_branch_destination:CORE_ADDR pc, CORE_ADDR *dest:pc, dest::default_unconditional_branch_destination::0
+
 EOF
 }