Re: libstdc++/4150: catastrophic performance decrease in C++ code

[Jason Merrill <jason at redhat dot com>]

>>>>> "Benjamin" == Benjamin Kosnik <bkoz@redhat.com> writes:

>> A problem with the current implementation of this is that if we do a
>> read on an input/output filebuf, we end up writing the contents of the
>> buffer back out to the file, even if we've never requested a write. 
>> Oops.

> Hmmm.

> Please write a testcase that demonstrates this, and add it to
> 27_io/filebuf_members.cc

I'm not sure how to check for this without strace...thoughts?

>> I feel like I know my way around streambufs a lot better now.

> Great. So, how do you like debugging C++ with the current tools?
> Painful, huh? Does it make you feel psychic when you fix things?

> Please let me know if you have any special kung-fu to pass on.

1) When I started adding -static to my links, the random gdb crashes went
   away.
2) Sometimes 'step' fails to step into a function, but 'stepi' always seems
   to work; once I'm into the function, I can start using 'step' again.
3) The patch below fixes inspection of class contents.

>> Tested i686-pc-linux-gnu, no regressions.  Any objections?

> Mainline and branch have diverged a bit right now. Did you test with
> branch or mainline?

trunk.  I'll test on the branch, too.

> I need to get solaris back in shape on mainline: bsd's, hpux, aix,
> cygwin are all back in shape now, but solaris is still kind of dicy. I'm
> going to ask you to hold off, at least on the branch, till I have the
> libstdc++/4164 patch integrated. Also, you'll need to do the testsuite
> entry before you can check in. Okay?

I can hold off a bit; let me know.

> Does this mean that the FSEEK hacks in config/os/*/bits/os_defines.h can
> be removed, since this define is no longer used?

Yep.

Jason

*** gnu-v3-abi.c.~1~	Sun Mar 17 17:10:01 2002
--- gnu-v3-abi.c	Sun Apr 14 22:59:39 2002
*************** gnuv3_rtti_type (struct value *value,
*** 241,262 ****
    vtable_symbol_name = SYMBOL_DEMANGLED_NAME (vtable_symbol);
    if (vtable_symbol_name == NULL
        || strncmp (vtable_symbol_name, "vtable for ", 11))
!     error ("can't find linker symbol for virtual table for `%s' value",
!            TYPE_NAME (value_type));
    class_name = vtable_symbol_name + 11;
  
    /* Try to look up the class name as a type name.  */
    class_symbol = lookup_symbol (class_name, 0, STRUCT_NAMESPACE, 0, 0);
    if (! class_symbol)
!     error ("can't find class named `%s', as given by C++ RTTI", class_name);
  
    /* Make sure the type symbol is sane.  (An earlier version of this
       code would find constructor functions, who have the same name as
       the class.)  */
    if (SYMBOL_CLASS (class_symbol) != LOC_TYPEDEF
        || TYPE_CODE (SYMBOL_TYPE (class_symbol)) != TYPE_CODE_CLASS)
!     error ("C++ RTTI gives a class name of `%s', but that isn't a type name",
!            class_name);
  
    /* This is the object's run-time type!  */
    run_time_type = SYMBOL_TYPE (class_symbol);
--- 241,273 ----
    vtable_symbol_name = SYMBOL_DEMANGLED_NAME (vtable_symbol);
    if (vtable_symbol_name == NULL
        || strncmp (vtable_symbol_name, "vtable for ", 11))
!     {
!       warning ("can't find linker symbol for virtual table for `%s' value",
! 	       TYPE_NAME (value_type));
!       if (vtable_symbol_name)
! 	warning ("  found `%s' instead", vtable_symbol_name);
!       return NULL;
!     }
    class_name = vtable_symbol_name + 11;
  
    /* Try to look up the class name as a type name.  */
    class_symbol = lookup_symbol (class_name, 0, STRUCT_NAMESPACE, 0, 0);
    if (! class_symbol)
!     {
!       warning ("can't find class named `%s', as given by C++ RTTI", class_name);
!       return NULL;
!     }
  
    /* Make sure the type symbol is sane.  (An earlier version of this
       code would find constructor functions, who have the same name as
       the class.)  */
    if (SYMBOL_CLASS (class_symbol) != LOC_TYPEDEF
        || TYPE_CODE (SYMBOL_TYPE (class_symbol)) != TYPE_CODE_CLASS)
!     {
!       warning ("C++ RTTI gives a class name of `%s', but that isn't a type name",
! 	       class_name);
!       return NULL;
!     }
  
    /* This is the object's run-time type!  */
    run_time_type = SYMBOL_TYPE (class_symbol);