[tools] Add a simple gdb frame unwinder

Add a simple unwinder for gdb which, on x64,  walks frame pointers
whenever there is no source information available. Ideally we would only
do this for V8 PCs but this appears hard to do in an Unwinder without
messing with gdb's internal assumptions.

Change-Id: Iba1e62a3768340ee912e81d691237c1920a8ae91
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3608628
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80204}

tools/gdbinit

@@ -259,3 +259,55 @@ gdb.events.new_objfile.connect(newobj_handler)
 gdb.execute("set environment V8_GDBINIT_SOURCED=1")
 
 end
+
+# Add a simple unwinder which, on x64, walks frame pointers when there
+# is no source information available.
+python
+
+from gdb.unwinder import Unwinder
+
+class V8UnwinderFrameId(object):
+  def __init__(self, sp, pc):
+    self.sp = sp
+    self.pc = pc
+
+class V8Unwinder(Unwinder):
+  def __init__(self):
+    super(V8Unwinder, self).__init__("V8Unwinder")
+    self.enabled = True
+
+  def __call__(self, pending_frame):
+    try:
+      # Only supported on x64.
+      if gdb.selected_inferior().architecture().name() != "i386:x86-64":
+        return None
+
+      pc = pending_frame.read_register("rip")
+      sym_and_line = gdb.current_progspace().find_pc_line(int(pc))
+
+      if sym_and_line.symtab is not None:
+        return None
+      fp = pending_frame.read_register("rbp").reinterpret_cast(
+          gdb.lookup_type("void").pointer().pointer())
+
+      next_sp = fp
+      next_fp = fp.dereference()
+      next_pc = (fp+1).dereference()
+
+      frame_info = V8UnwinderFrameId(next_sp, next_pc)
+
+      # create_unwind_info seems to sometimes have issues accessing
+      # the frame_info if it's not first accessed in Python.
+      _lol_gdb_workaround = frame_info.pc + 1
+
+      unwind_info = pending_frame.create_unwind_info(frame_info)
+      unwind_info.add_saved_register("rsp", next_sp)
+      unwind_info.add_saved_register("rip", next_pc)
+      unwind_info.add_saved_register("rbp", next_fp)
+      return unwind_info
+    except Exception as e:
+      return None
+
+gdb.unwinder.register_unwinder(None, V8Unwinder(), replace=True)
+
+end