debugging: Add GDB pretty printers for common types (#1957)

By default, GDB will not autoload this for security reasons. See https://sourceware.org/gdb/current/onlinedocs/gdb.html/Auto_002dloading-safe-path.html to see how to enable it.

.gdbinit

@@ -0,0 +1,47 @@
+python
+
+import gdb
+import re
+
+
+class BetypePrinter:
+    PATTERN = re.compile("^betype<.*>$")
+
+    def __init__(self, obj):
+        self._obj = obj
+
+    def to_string(self):
+        underlying = self._obj["m_value"]
+        reversed_bytes = bytes(reversed(underlying.bytes))
+        return gdb.Value(reversed_bytes, underlying.type)
+
+class MemptrPrinter:
+    PATTERN = re.compile("^MEMPTR<.*>$")
+
+    def __init__(self, obj):
+        self._ptr_type = obj.type.strip_typedefs().unqualified().template_argument(0).pointer()
+        self._guest_address = obj["m_value"].cast(gdb.lookup_type("uint32"))
+
+    def ptr(self):
+        if self._guest_address == 0:
+            return gdb.Value(0, self._ptr_type)
+        base_addr = gdb.parse_and_eval('memory_base', global_context=True)
+        return (self._guest_address + base_addr).reinterpret_cast(self._ptr_type)
+
+    def children(self):
+        return [("raw", self.ptr())]
+
+    def to_string(self):
+        return self._guest_address.format_string(format="x")
+
+def lookup(val):
+    tag = val.type.strip_typedefs().unqualified().tag
+    if tag is None:
+        return None
+    printers = [BetypePrinter, MemptrPrinter]
+    for printer in printers:
+        if printer.PATTERN.match(tag):
+            return printer(val)
+    return None
+
+gdb.pretty_printers.append(lookup)