Re: Built-in type handling in gdb

[vijay nag <vijunag at gmail dot com>]

On Fri, May 16, 2014 at 10:54 PM, Doug Evans <dje@google.com> wrote:
> On Thu, May 15, 2014 at 1:35 AM, vijay nag <vijunag@gmail.com> wrote:
>> Hello GDB,
>>
>> I have a simple GDB script to walk through the heap given a core file.
>> The data types used in the scripts are all primitive C data types and
>> any non primitive user defined data types have been avoided to speed
>> up the execution. In the older version of GDB(say gdb-7.0) this script
>> finished execution in a jiffy, the new gdb is way too slow in
>> execution. I built gdb-7.0/7.6 from source and observed the difference
>> in execution.
>>
>> As part of this commit "NEWS: Mention OpenCL C language support
>> 2010-11-05  Ken Werner
>> <ken.werner@de.ibm.com>(https://github.com/dov/gdb/commit/100d4cd4f6f42014c07e6acd0d9b6187d1259b2e)
>> * c-exp.y: Lookup the primitive types instead of referring to the
>> builtins.", parse_type macro(get from builtin) has been changed to a
>> function call lookup_signed_typename(). This function seems to be
>> doing an exhaustive global/static symbols search even for a C
>> primitive data type(say int) there by consuming plenty of CPU cycles.
>> Should we be doing this exhaustive search of data types from the
>> binary file even for basic C primitive data types ?
>
> Hi.
>
> I agree the current situation is less then stellar.
>
> There is one catch that needs to be handled that isn't necessarily obvious.
> The size of each primitive type is specific to each .o file (CU in
> DWARF parlance).
> E.g., If I compile foo.c with -fshort-double then sizeof(double) == 4 in foo.o.
> While it's difficult for an app to make this work in general, gdb
> should still support it.
>
> The order in which types should be looked up is:
> - current CU
> - builtin type
> - globally (fallback in the case of base types)
>   [N.B. that's a qualified "globally" as base types live in gdb's STATIC_BLOCK]
>
> I think the fix isn't that hard, but it will require some changes to
> symbol lookup of base types.
> It's on my TODO list, but I'm happy to guide anyone through the
> changes required.

Hello Doug,

Is the below patch plausible ? I have basically changed the look-up
order of user defined data type and primitive data type.

diff --git a/systemsw/tools/src/gdb-7.6/gdb/gdbtypes.c
b/systemsw/tools/src/gdb-7.6/gdb/gdbtypes.c
index 12730d7..8211b35 100644
--- a/systemsw/tools/src/gdb-7.6/gdb/gdbtypes.c
+++ b/systemsw/tools/src/gdb-7.6/gdb/gdbtypes.c
@@ -1201,13 +1201,14 @@ lookup_typename (const struct language_defn *language,
   struct symbol *sym;
   struct type *type;

+  type = language_lookup_primitive_type_by_name (language, gdbarch, name);
+  if (type)
+    return type;
+
   sym = lookup_symbol (name, block, VAR_DOMAIN, 0);
   if (sym != NULL && SYMBOL_CLASS (sym) == LOC_TYPEDEF)
     return SYMBOL_TYPE (sym);

-  type = language_lookup_primitive_type_by_name (language, gdbarch, name);
-  if (type)
-    return type;

   if (noerr)
     return NULL;