Re: [PATCH] Fix gdb printers for std::string

[Jonathan Wakely <jwakely@redhat.com>]

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On Sat, 1 Oct 2022 at 08:20, François Dumont via Libstdc++
<libstdc++@gcc.gnu.org> wrote:
>
> I had forgotten to re-run tests after I removed the #define
> _GLIBCXX_USE_CXX11_ABI 0.
>
> The comment was misleading, it could also impact output of std::list.
>
> I am also restoring the correct std::string alias for
> std::__cxx11::basic_string, even if with my workaround it doesn't really
> matter as the one for std::basic_string will be used.
>
> I also restored the printer for std::__cxx11::string typedef. Is it
> intentional to keep this ?

Yes, I kept that intentionally. There can be programs where some
objects still use that typedef, if those objects were compiled with
GCC 8.x or older.

>
>      libstdc++: Fix gdb pretty printers when dealing with std::string
>
>      Since revision 33b43b0d8cd2de722d177ef823930500948a7487 std::string
> and other
>      similar typedef are ambiguous from a gdb point of view because it
> matches both
>      std::basic_string<char> and std::__cxx11::basic_string<char>
> symbols. For those
>      typedef add a workaround to accept the substitution as long as the
> same regardless
>      of __cxx11 namespace.

Thanks for figuring out what was going wrong here, and how to fix it.


>
>      Also avoid to register printers for types in std::__cxx11::__8::
> namespace, there is
>      no such symbols.
>
>      libstdc++-v3/ChangeLog:
>
>              * libstdc++-v3/python/libstdcxx/v6/printers.py
> (Printer.add_version): Do not add
>              version namespace for __cxx11 symbols.
>              (add_one_template_type_printer): Likewise.
>              (add_one_type_printer): Likewise.
>              (FilteringTypePrinter._recognizer.recognize): Add a
> workaround for std::string & al
>              ambiguous typedef matching both std:: and std::__cxx11::
> symbols.
>              (register_type_printers): Refine type registration to limit
> false positive in
>              FilteringTypePrinter._recognize.recognize requiring to look
> for the type in gdb.

I don't really like this part of the change though:

@@ -2105,29 +2120,29 @@ def register_type_printers(obj):
         return

     # Add type printers for typedefs std::string, std::wstring etc.
-    for ch in ('', 'w', 'u8', 'u16', 'u32'):
-        add_one_type_printer(obj, 'basic_string', ch + 'string')
-        add_one_type_printer(obj, '__cxx11::basic_string', ch + 'string')
+    for ch in (('char', ''), ('wchar_t', 'w'), ('char8_t', 'u8'),
('char16_t', 'u16'), ('char32_t', 'u32')):
+        add_one_type_printer(obj, 'basic_string<' + ch[0], ch[1] + 'string')
+        add_one_type_printer(obj, '__cxx11::basic_string<' + ch[0],
ch[1] + 'string')


As the docs for FilteringTypePrinter says, the first argument is
supposed to be the class template name:

class FilteringTypePrinter(object):
    r"""
    A type printer that uses typedef names for common template specializations.

    Args:
        match (str): The class template to recognize.
        name (str): The typedef-name that will be used instead.

    Checks if a specialization of the class template 'match' is the same type
    as the typedef 'name', and prints it as 'name' instead.

    e.g. if an instantiation of std::basic_istream<C, T> is the same type as
    std::istream then print it as std::istream.
    """

With this change, the "class template" is sometimes just a string
prefix of a particular specialization, e.g. "basic_string<char"
The class template is "basic_string", and that's how the
FilteringTypePrinter was intended to work.

How about something like the attached (untested) change instead. which
keeps the 'match' argument as the class template name, but optionally
supports passing the first template argument? Then you can use
match.split('::')[-1] is 'basic_string' to check if we are dealing
with one of the possibly ambiguous typedefs.

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diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
index 245b6e3dbcd..6a0b8a22f1d 100644
--- a/libstdc++-v3/python/libstdcxx/v6/printers.py
+++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
@@ -2045,25 +2045,35 @@ class FilteringTypePrinter(object):
     Args:
         match (str): The class template to recognize.
         name (str): The typedef-name that will be used instead.
+        targ1 (str, optional): The first template argument. Defaults to None.
+            If arg1 is provided, only match specializations with this type
+            as the first template argument, e.g. if match='basic_string'
 
     Checks if a specialization of the class template 'match' is the same type
     as the typedef 'name', and prints it as 'name' instead.
 
-    e.g. if an instantiation of std::basic_istream<C, T> is the same type as
+    e.g. for match='basic_istream', name='istream', if any specialization of
+    std::basic_istream<C, T> is the same type as std::istream then print it as
+    std::istream.
+
+    e.g. for match='basic_istream', name='istream', targ1='char', if any
+    specialization of std::basic_istream<char, T> is the same type as
     std::istream then print it as std::istream.
     """
 
-    def __init__(self, match, name):
+    def __init__(self, match, name, targ1 = None):
         self.match = match
         self.name = name
+        self.targ1 = targ1
         self.enabled = True
 
     class _recognizer(object):
         "The recognizer class for FilteringTypePrinter."
 
-        def __init__(self, match, name):
+        def __init__(self, match, name, targ1):
             self.match = match
             self.name = name
+            self.targ1 = targ1
             self.type_obj = None
 
         def recognize(self, type_obj):
@@ -2075,7 +2085,11 @@ class FilteringTypePrinter(object):
                 return None
 
             if self.type_obj is None:
-                if not type_obj.tag.startswith(self.match):
+                if self.tag1 is not None:
+                    if not type_obj.tag.startswith('{}<{}'.format(self.match, self.targ1):
+                        # Filter didn't match.
+                        return None
+                elif not type_obj.tag.startswith(self.match):
                     # Filter didn't match.
                     return None
                 try:
@@ -2084,18 +2098,23 @@ class FilteringTypePrinter(object):
                     pass
             if self.type_obj == type_obj:
                 return strip_inline_namespaces(self.name)
+
+            if self.match.split('::')[-1] is 'basic_string':
+                if self.type_obj.tag.replace('__cxx11::', '') == type_obj.tag.replace('__cxx11::', ''):
++                    return strip_inline_namespaces(self.name)
+
             return None
 
     def instantiate(self):
         "Return a recognizer object for this type printer."
         return self._recognizer(self.match, self.name)
 
-def add_one_type_printer(obj, match, name):
-    printer = FilteringTypePrinter('std::' + match, 'std::' + name)
+def add_one_type_printer(obj, match, name, targ1 = None):
+    printer = FilteringTypePrinter('std::' + match, 'std::' + name, targ1)
     gdb.types.register_type_printer(obj, printer)
     if _versioned_namespace:
         ns = 'std::' + _versioned_namespace
-        printer = FilteringTypePrinter(ns + match, ns + name)
+        printer = FilteringTypePrinter(ns + match, ns + name, targ1)
         gdb.types.register_type_printer(obj, printer)
 
 def register_type_printers(obj):