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diff --git a/js/src/gdb/mozilla/prettyprinters.py b/js/src/gdb/mozilla/prettyprinters.py
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+++ b/js/src/gdb/mozilla/prettyprinters.py
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+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this file,
+# You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# mozilla/prettyprinters.py --- infrastructure for SpiderMonkey's auto-loaded pretty-printers.
+
+import re
+
+import gdb
+
+# Decorators for declaring pretty-printers.
+#
+# In each case, the decoratee should be a SpiderMonkey-style pretty-printer
+# factory, taking both a gdb.Value instance and a TypeCache instance as
+# arguments; see TypeCache, below.
+
+
+def check_for_reused_pretty_printer(fn):
+    # Check that |fn| hasn't been registered as a pretty-printer under some
+    # other name already. (The 'enabled' flags used by GDB's
+    # 'enable/disable/info pretty-printer' commands are simply stored as
+    # properties of the function objects themselves, so a single function
+    # object can't carry the 'enabled' flags for two different printers.)
+    if hasattr(fn, "enabled"):
+        raise RuntimeError("pretty-printer function %r registered more than once" % fn)
+
+
+# a dictionary mapping gdb.Type tags to pretty-printer functions.
+printers_by_tag = {}
+
+# A decorator: add the decoratee as a pretty-printer lookup function for types
+# named |type_name|.
+
+
+def pretty_printer(type_name):
+    def add(fn):
+        check_for_reused_pretty_printer(fn)
+        add_to_subprinter_list(fn, type_name)
+        printers_by_tag[type_name] = fn
+        return fn
+
+    return add
+
+
+# a dictionary mapping gdb.Type tags to pretty-printer functions for pointers to
+# that type.
+ptr_printers_by_tag = {}
+
+# A decorator: add the decoratee as a pretty-printer lookup function for
+# pointers to types named |type_name|.
+
+
+def ptr_pretty_printer(type_name):
+    def add(fn):
+        check_for_reused_pretty_printer(fn)
+        add_to_subprinter_list(fn, "ptr-to-" + type_name)
+        ptr_printers_by_tag[type_name] = fn
+        return fn
+
+    return add
+
+
+# a dictionary mapping gdb.Type tags to pretty-printer functions for
+# references to that type.
+ref_printers_by_tag = {}
+
+# A decorator: add the decoratee as a pretty-printer lookup function for
+# references to instances of types named |type_name|.
+
+
+def ref_pretty_printer(type_name):
+    def add(fn):
+        check_for_reused_pretty_printer(fn)
+        add_to_subprinter_list(fn, "ref-to-" + type_name)
+        ref_printers_by_tag[type_name] = fn
+        return fn
+
+    return add
+
+
+# a dictionary mapping the template name portion of gdb.Type tags to
+# pretty-printer functions for instantiations of that template.
+template_printers_by_tag = {}
+
+# A decorator: add the decoratee as a pretty-printer lookup function for
+# instantiations of templates named |template_name|.
+
+
+def template_pretty_printer(template_name):
+    def add(fn):
+        check_for_reused_pretty_printer(fn)
+        add_to_subprinter_list(fn, "instantiations-of-" + template_name)
+        template_printers_by_tag[template_name] = fn
+        return fn
+
+    return add
+
+
+# A list of (REGEXP, PRINTER) pairs, such that if REGEXP (a RegexObject)
+# matches the result of converting a gdb.Value's type to a string, then
+# PRINTER is a pretty-printer lookup function that will probably like that
+# value.
+printers_by_regexp = []
+
+# A decorator: add the decoratee as a pretty-printer factory for types
+# that, when converted to a string, match |pattern|. Use |name| as the
+# pretty-printer's name, when listing, enabling and disabling.
+
+
+def pretty_printer_for_regexp(pattern, name):
+    compiled = re.compile(pattern)
+
+    def add(fn):
+        check_for_reused_pretty_printer(fn)
+        add_to_subprinter_list(fn, name)
+        printers_by_regexp.append((compiled, fn))
+        return fn
+
+    return add
+
+
+# Forget all pretty-printer lookup functions defined in the module name
+# |module_name|, if any exist. Use this at the top of each pretty-printer
+# module like this:
+#
+#   clear_module_printers(__name__)
+
+
+def clear_module_printers(module_name):
+    global printers_by_tag, ptr_printers_by_tag, ref_printers_by_tag
+    global template_printers_by_tag, printers_by_regexp
+
+    # Remove all pretty-printers defined in the module named |module_name|
+    # from d.
+    def clear_dictionary(d):
+        # Walk the dictionary, building a list of keys whose entries we
+        # should remove. (It's not safe to delete entries from a dictionary
+        # while we're iterating over it.)
+        to_delete = []
+        for (k, v) in d.items():
+            if v.__module__ == module_name:
+                to_delete.append(k)
+                remove_from_subprinter_list(v)
+        for k in to_delete:
+            del d[k]
+
+    clear_dictionary(printers_by_tag)
+    clear_dictionary(ptr_printers_by_tag)
+    clear_dictionary(ref_printers_by_tag)
+    clear_dictionary(template_printers_by_tag)
+
+    # Iterate over printers_by_regexp, deleting entries from the given module.
+    new_list = []
+    for p in printers_by_regexp:
+        if p.__module__ == module_name:
+            remove_from_subprinter_list(p)
+        else:
+            new_list.append(p)
+    printers_by_regexp = new_list
+
+
+# Our subprinters array. The 'subprinters' attributes of all lookup
+# functions returned by lookup_for_objfile point to this array instance,
+# which we mutate as subprinters are added and removed.
+subprinters = []
+
+# Set up the 'name' and 'enabled' attributes on |subprinter|, and add it to our
+# list of all SpiderMonkey subprinters.
+
+
+def add_to_subprinter_list(subprinter, name):
+    subprinter.name = name
+    subprinter.enabled = True
+    subprinters.append(subprinter)
+
+
+# Remove |subprinter| from our list of all SpiderMonkey subprinters.
+
+
+def remove_from_subprinter_list(subprinter):
+    subprinters.remove(subprinter)
+
+
+# An exception class meaning, "This objfile has no SpiderMonkey in it."
+
+
+class NotSpiderMonkeyObjfileError(TypeError):
+    pass
+
+
+# TypeCache: a cache for frequently used information about an objfile.
+#
+# When a new SpiderMonkey objfile is loaded, we construct an instance of
+# this class for it. Then, whenever we construct a pretty-printer for some
+# gdb.Value, we also pass, as a second argument, the TypeCache for the
+# objfile to which that value's type belongs.
+#
+# if objfile doesn't seem to have SpiderMonkey code in it, the constructor
+# raises NotSpiderMonkeyObjfileError.
+#
+# Pretty-printer modules may add attributes to this to hold their own
+# cached values. Such attributes should be named mod_NAME, where the module
+# is named mozilla.NAME; for example, mozilla.JSString should store its
+# metadata in the TypeCache's mod_JSString attribute.
+
+
+class TypeCache(object):
+    def __init__(self, objfile):
+        self.objfile = objfile
+
+        # Unfortunately, the Python interface doesn't allow us to specify
+        # the objfile in whose scope lookups should occur. But simply
+        # knowing that we need to lookup the types afresh is probably
+        # enough.
+        self.void_t = gdb.lookup_type("void")
+        self.void_ptr_t = self.void_t.pointer()
+        self.uintptr_t = gdb.lookup_type("uintptr_t")
+        try:
+            self.JSString_ptr_t = gdb.lookup_type("JSString").pointer()
+            self.JSSymbol_ptr_t = gdb.lookup_type("JS::Symbol").pointer()
+            self.JSObject_ptr_t = gdb.lookup_type("JSObject").pointer()
+        except gdb.error:
+            raise NotSpiderMonkeyObjfileError
+
+        self.mod_GCCellPtr = None
+        self.mod_Interpreter = None
+        self.mod_JSObject = None
+        self.mod_JSOp = None
+        self.mod_JSString = None
+        self.mod_JS_Value = None
+        self.mod_ExecutableAllocator = None
+        self.mod_IonGraph = None
+
+
+# Yield a series of all the types that |t| implements, by following typedefs
+# and iterating over base classes. Specifically:
+# - |t| itself is the first value yielded.
+# - If we yield a typedef, we later yield its definition.
+# - If we yield a type with base classes, we later yield those base classes.
+# - If we yield a type with some base classes that are typedefs,
+#   we yield all the type's base classes before following the typedefs.
+#
+# This is a hokey attempt to order the implemented types by meaningfulness when
+# pretty-printed. Perhaps it is entirely misguided, and we should actually
+# collect all applicable pretty-printers, and then use some ordering on the
+# pretty-printers themselves.
+#
+# We may yield a type more than once (say, if it appears more than once in the
+# class hierarchy).
+
+
+def implemented_types(t):
+
+    # Yield all types that follow |t|.
+    def followers(t):
+        if t.code == gdb.TYPE_CODE_TYPEDEF:
+            yield t.target()
+            for t2 in followers(t.target()):
+                yield t2
+        elif is_struct_or_union(t):
+            base_classes = []
+            for f in t.fields():
+                if f.is_base_class:
+                    yield f.type
+                    base_classes.append(f.type)
+            for b in base_classes:
+                for t2 in followers(b):
+                    yield t2
+
+    yield t
+    for t2 in followers(t):
+        yield t2
+
+
+template_regexp = re.compile("([\w_:]+)<")
+
+
+def is_struct_or_union(t):
+    return t.code in (gdb.TYPE_CODE_STRUCT, gdb.TYPE_CODE_UNION)
+
+
+def is_struct_or_union_or_enum(t):
+    return t.code in (gdb.TYPE_CODE_STRUCT, gdb.TYPE_CODE_UNION, gdb.TYPE_CODE_ENUM)
+
+
+# Construct and return a pretty-printer lookup function for objfile, or
+# return None if the objfile doesn't contain SpiderMonkey code
+# (specifically, definitions for SpiderMonkey types).
+
+
+def lookup_for_objfile(objfile):
+    # Create a type cache for this objfile.
+    try:
+        cache = TypeCache(objfile)
+    except NotSpiderMonkeyObjfileError:
+        if gdb.parameter("verbose"):
+            gdb.write(
+                "objfile '%s' has no SpiderMonkey code; not registering pretty-printers\n"
+                % (objfile.filename,)
+            )
+        return None
+
+    # Return a pretty-printer for |value|, if we have one. This is the lookup
+    # function object we place in each gdb.Objfile's pretty-printers list, so it
+    # carries |name|, |enabled|, and |subprinters| attributes.
+    def lookup(value):
+        # If |table| has a pretty-printer for |tag|, apply it to |value|.
+        def check_table(table, tag):
+            if tag in table:
+                f = table[tag]
+                if f.enabled:
+                    return f(value, cache)
+            return None
+
+        def check_table_by_type_name(table, t):
+            if t.code == gdb.TYPE_CODE_TYPEDEF:
+                return check_table(table, str(t))
+            elif is_struct_or_union_or_enum(t) and t.tag:
+                return check_table(table, t.tag)
+            else:
+                return None
+
+        for t in implemented_types(value.type):
+            if t.code == gdb.TYPE_CODE_PTR:
+                for t2 in implemented_types(t.target()):
+                    p = check_table_by_type_name(ptr_printers_by_tag, t2)
+                    if p:
+                        return p
+            elif t.code == gdb.TYPE_CODE_REF:
+                for t2 in implemented_types(t.target()):
+                    p = check_table_by_type_name(ref_printers_by_tag, t2)
+                    if p:
+                        return p
+            else:
+                p = check_table_by_type_name(printers_by_tag, t)
+                if p:
+                    return p
+                if is_struct_or_union(t) and t.tag:
+                    m = template_regexp.match(t.tag)
+                    if m:
+                        p = check_table(template_printers_by_tag, m.group(1))
+                        if p:
+                            return p
+
+        # Failing that, look for a printer in printers_by_regexp. We have
+        # to scan the whole list, so regexp printers should be used
+        # sparingly.
+        s = str(value.type)
+        for (r, f) in printers_by_regexp:
+            if f.enabled:
+                m = r.match(s)
+                if m:
+                    p = f(value, cache)
+                    if p:
+                        return p
+
+        # No luck.
+        return None
+
+    # Give |lookup| the attributes expected of a pretty-printer with
+    # subprinters, for enabling and disabling.
+    lookup.name = "SpiderMonkey"
+    lookup.enabled = True
+    lookup.subprinters = subprinters
+
+    return lookup
+
+
+# A base class for pretty-printers for pointer values that handles null
+# pointers, by declining to construct a pretty-printer for them at all.
+# Derived classes may simply assume that self.value is non-null.
+#
+# To help share code, this class can also be used with reference types.
+#
+# This class provides the following methods, which subclasses are free to
+# override:
+#
+# __init__(self, value, cache): Save value and cache as properties by those names
+#     on the instance.
+#
+# to_string(self): format the type's name and address, as GDB would, and then
+#     call a 'summary' method (which the subclass must define) to produce a
+#     description of the referent.
+#
+#     Note that pretty-printers returning a 'string' display hint must not use
+#     this default 'to_string' method, as GDB will take everything it returns,
+#     including the type name and address, as string contents.
+
+
+class Pointer(object):
+    def __new__(cls, value, cache):
+        # Don't try to provide pretty-printers for NULL pointers.
+        if value.type.strip_typedefs().code == gdb.TYPE_CODE_PTR and value == 0:
+            return None
+        return super(Pointer, cls).__new__(cls)
+
+    def __init__(self, value, cache):
+        self.value = value
+        self.cache = cache
+
+    def to_string(self):
+        # See comment above.
+        assert not hasattr(self, "display_hint") or self.display_hint() != "string"
+        concrete_type = self.value.type.strip_typedefs()
+        if concrete_type.code == gdb.TYPE_CODE_PTR:
+            address = self.value.cast(self.cache.void_ptr_t)
+        elif concrete_type.code == gdb.TYPE_CODE_REF:
+            address = "@" + str(self.value.address.cast(self.cache.void_ptr_t))
+        else:
+            assert not "mozilla.prettyprinters.Pointer applied to bad value type"
+        try:
+            summary = self.summary()
+        except gdb.MemoryError as r:
+            summary = str(r)
+        v = "(%s) %s %s" % (self.value.type, address, summary)
+        return v
+
+    def summary(self):
+        raise NotImplementedError
+
+
+field_enum_value = None
+
+# Given |t|, a gdb.Type instance representing an enum type, return the
+# numeric value of the enum value named |name|.
+#
+# Pre-2012-4-18 versions of GDB store the value of an enum member on the
+# gdb.Field's 'bitpos' attribute; later versions store it on the 'enumval'
+# attribute. This function retrieves the value from either.
+
+
+def enum_value(t, name):
+    global field_enum_value
+    f = t[name]
+    # Monkey-patching is a-okay in polyfills! Just because.
+    if not field_enum_value:
+        if hasattr(f, "enumval"):
+
+            def field_enum_value(f):
+                return f.enumval
+
+        else:
+
+            def field_enum_value(f):
+                return f.bitpos
+
+    return field_enum_value(f)