[committed] libstdc++: Fix pretty printing of std::unique_ptr [PR103086]

Jonathan Wakely jwakely@redhat.com
Thu Nov 4 23:08:35 GMT 2021


Tested x86_64-linux, committed to trunk. Backport to gcc-11 to follow,
and maybe to other branches too.


Since std::tuple started using [[no_unique_address]] the tuple<T*, D>
member of std::unique_ptr<T, D> has two _M_head_impl subobjects, in
different base classes. That means this printer code is ambiguous:

    tuple_head_type = tuple_impl_type.fields()[1].type   # _Head_base
    head_field = tuple_head_type.fields()[0]
    if head_field.name == '_M_head_impl':
        self.pointer = tuple_member['_M_head_impl']

In older versions of GDB it happened to work by chance, because GDB
returned the last _M_head_impl member and std::tuple's base classes are
stored in reverse order, so the last one was the T* element of the
tuple. Since GDB 11 it returns the first _M_head_impl, which is the
deleter element.

The fix is for the printer to stop using an ambiguous field name and
cast the tuple to the correct base class before accessing the
_M_head_impl member.

Instead of fixing this in both UniquePointerPrinter and StdPathPrinter a
new unique_ptr_get function is defined to do it correctly. That is
defined in terms of new tuple_get and _tuple_impl_get functions.

It would be possible to reuse _tuple_impl_get to access each element in
StdTuplePrinter._iterator.__next__, but that already does the correct
casting, and wouldn't be much simpler anyway.

libstdc++-v3/ChangeLog:

	PR libstdc++/103086
	* python/libstdcxx/v6/printers.py (_tuple_impl_get): New helper
	for accessing the tuple element stored in a _Tuple_impl node.
	(tuple_get): New function for accessing a tuple element.
	(unique_ptr_get): New function for accessing a unique_ptr.
	(UniquePointerPrinter, StdPathPrinter): Use unique_ptr_get.
	* python/libstdcxx/v6/xmethods.py (UniquePtrGetWorker): Cast
	tuple to its base class before accessing _M_head_impl.
---
 libstdc++-v3/python/libstdcxx/v6/printers.py | 71 ++++++++++++++------
 libstdc++-v3/python/libstdcxx/v6/xmethods.py |  2 +-
 2 files changed, 52 insertions(+), 21 deletions(-)

diff --git a/libstdc++-v3/python/libstdcxx/v6/printers.py b/libstdc++-v3/python/libstdcxx/v6/printers.py
index c7da4079a7d..c5072c52281 100644
--- a/libstdc++-v3/python/libstdcxx/v6/printers.py
+++ b/libstdc++-v3/python/libstdcxx/v6/printers.py
@@ -240,32 +240,63 @@ class SharedPointerPrinter:
                 state = 'use count %d, weak count %d' % (usecount, weakcount - 1)
         return '%s<%s> (%s)' % (self.typename, str(self.val.type.template_argument(0)), state)
 
+def _tuple_impl_get(val):
+    "Return the tuple element stored in a _Tuple_impl<N, T> base class."
+    bases = val.type.fields()
+    if not bases[-1].is_base_class:
+        raise ValueError("Unsupported implementation for std::tuple: %s" % str(val.type))
+    # Get the _Head_base<N, T> base class:
+    head_base = val.cast(bases[-1].type)
+    fields = head_base.type.fields()
+    if len(fields) == 0:
+        raise ValueError("Unsupported implementation for std::tuple: %s" % str(val.type))
+    if fields[0].name == '_M_head_impl':
+        # The tuple element is the _Head_base::_M_head_impl data member.
+        return head_base['_M_head_impl']
+    elif fields[0].is_base_class:
+        # The tuple element is an empty base class of _Head_base.
+        # Cast to that empty base class.
+        return head_base.cast(fields[0].type)
+    else:
+        raise ValueError("Unsupported implementation for std::tuple: %s" % str(val.type))
+
+def tuple_get(n, val):
+    "Return the result of std::get<n>(val) on a std::tuple"
+    tuple_size = len(get_template_arg_list(val.type))
+    if n > tuple_size:
+        raise ValueError("Out of range index for std::get<N> on std::tuple")
+    # Get the first _Tuple_impl<0, T...> base class:
+    node = val.cast(val.type.fields()[0].type)
+    while n > 0:
+        # Descend through the base classes until the Nth one.
+        node = node.cast(node.type.fields()[0].type)
+        n -= 1
+    return _tuple_impl_get(node)
+
+def unique_ptr_get(val):
+    "Return the result of val.get() on a std::unique_ptr"
+    # std::unique_ptr<T, D> contains a std::tuple<D::pointer, D>,
+    # either as a direct data member _M_t (the old implementation)
+    # or within a data member of type __uniq_ptr_data.
+    impl_type = val.type.fields()[0].type.strip_typedefs()
+    # Check for new implementations first:
+    if is_specialization_of(impl_type, '__uniq_ptr_data') \
+        or is_specialization_of(impl_type, '__uniq_ptr_impl'):
+        tuple_member = val['_M_t']['_M_t']
+    elif is_specialization_of(impl_type, 'tuple'):
+        tuple_member = val['_M_t']
+    else:
+        raise ValueError("Unsupported implementation for unique_ptr: %s" % str(impl_type))
+    return tuple_get(0, tuple_member)
+
 class UniquePointerPrinter:
     "Print a unique_ptr"
 
     def __init__ (self, typename, val):
         self.val = val
-        impl_type = val.type.fields()[0].type.strip_typedefs()
-        # Check for new implementations first:
-        if is_specialization_of(impl_type, '__uniq_ptr_data') \
-            or is_specialization_of(impl_type, '__uniq_ptr_impl'):
-            tuple_member = val['_M_t']['_M_t']
-        elif is_specialization_of(impl_type, 'tuple'):
-            tuple_member = val['_M_t']
-        else:
-            raise ValueError("Unsupported implementation for unique_ptr: %s" % str(impl_type))
-        tuple_impl_type = tuple_member.type.fields()[0].type # _Tuple_impl
-        tuple_head_type = tuple_impl_type.fields()[1].type   # _Head_base
-        head_field = tuple_head_type.fields()[0]
-        if head_field.name == '_M_head_impl':
-            self.pointer = tuple_member['_M_head_impl']
-        elif head_field.is_base_class:
-            self.pointer = tuple_member.cast(head_field.type)
-        else:
-            raise ValueError("Unsupported implementation for tuple in unique_ptr: %s" % str(impl_type))
 
     def children (self):
-        return SmartPtrIterator(self.pointer)
+        return SmartPtrIterator(unique_ptr_get(self.val))
 
     def to_string (self):
         return ('std::unique_ptr<%s>' % (str(self.val.type.template_argument(0))))
@@ -1370,7 +1401,7 @@ class StdPathPrinter:
     def __init__ (self, typename, val):
         self.val = val
         self.typename = typename
-        impl = self.val['_M_cmpts']['_M_impl']['_M_t']['_M_t']['_M_head_impl']
+        impl = unique_ptr_get(self.val['_M_cmpts']['_M_impl'])
         self.type = impl.cast(gdb.lookup_type('uintptr_t')) & 3
         if self.type == 0:
             self.impl = impl
diff --git a/libstdc++-v3/python/libstdcxx/v6/xmethods.py b/libstdc++-v3/python/libstdcxx/v6/xmethods.py
index 77870e9a264..991d945321a 100644
--- a/libstdc++-v3/python/libstdcxx/v6/xmethods.py
+++ b/libstdc++-v3/python/libstdcxx/v6/xmethods.py
@@ -597,7 +597,7 @@ class UniquePtrGetWorker(gdb.xmethod.XMethodWorker):
         tuple_head_type = tuple_impl_type.fields()[1].type   # _Head_base
         head_field = tuple_head_type.fields()[0]
         if head_field.name == '_M_head_impl':
-            return tuple_member['_M_head_impl']
+            return tuple_member.cast(tuple_head_type)['_M_head_impl']
         elif head_field.is_base_class:
             return tuple_member.cast(head_field.type)
         else:
-- 
2.31.1



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