[gdb] Decay type before passing into printer

[LeszekSwirski]

Summary

The existing gdb printer logic didn't work for const types, references, or typedefs. Now we strip const, references, and typedefs, before doing any of the matching checks or printer lookup.

[LeszekSwirski]

Is there a more correct reviewer to send this to?

[saugustine]

This is puzzling to me, as we use this internally at Google all over the place, and it seems to work just fine. So I'm surprised we haven't noticed anything.

Could you provide a test that fails before this change, and works afterward? Should be easy to add.

[LeszekSwirski]

I'll add a test -- the failure I had (on the V8 codebase) was with a std::vector<Foo> that was typedefed to FooVector, and then passed around by const ref. Since the typedef wasn't stripped, afair it didn't even make it past the "^std::" check.

utils/gdb/libcxx/printers.py

def __init__(self, name):
             # is a reasonable choice.
             if subprinter not in self.subprinters:
                 subprinter.name = name
                 self.subprinters.append(subprinter)
 
     def __call__(self, val):
         """Return the pretty printer for a val, if the type is supported."""
 
+        # Get a non-qualfied, non-reference version of the type, with typedefs
+        # stripped.
+        val_type = val.type
+        while val_type.code in [gdb.TYPE_CODE_REF, gdb.TYPE_CODE_RVALUE_REF]:
+          val_type = val_type.target()
+        val_type = val_type.unqualified()
+        val_type = val_type.strip_typedefs()
+
         # Do not handle any type that is not a struct/class.
-        if val.type.strip_typedefs().code != gdb.TYPE_CODE_STRUCT:
+        if val_type.code != gdb.TYPE_CODE_STRUCT:
             return None
 
         # Don't attempt types known to be inside libstdcxx.
-        typename = val.type.name or val.type.tag or str(val.type)
+        typename = val_type.name or val_type.tag or str(val_type)
         match = re.match("^std::(__.*?)::", typename)
         if match is None or match.group(1) in ["__cxx1998",
                                                "__debug",
                                                "__7",
                                                "__g"]:
             return None
 
         # Handle any using declarations or other typedefs.
-        typename = _prettify_typename(val.type)
+        typename = _prettify_typename(val_type)
         if not typename:
             return None
         without_generics = _remove_generics(typename)
         lookup_name = _remove_std_prefix(without_generics)
         if lookup_name in self.lookup:
             return self.lookup[lookup_name](val)
         return None