[Bug libstdc++/28765] New: __gnu_cxx::__vstring::clear() is slow

[Bug libstdc++/28765] New: __gnu_cxx::__vstring::clear() is slow

[ian at airs dot com]

Compile this code with -O2:

#include <ext/vstring.h>

void foo(__gnu_cxx::__vstring s)
{
  s.clear();
}

Look at the resulting assembler code.  Do not show it to small children.  It is
too scary.

With this simple patch:

--- include/ext/sso_string_base.h       (revision 116231)
+++ include/ext/sso_string_base.h       (working copy)
@@ -540,7 +540,7 @@ _GLIBCXX_BEGIN_NAMESPACE(__gnu_cxx)
     }

   template<typename _CharT, typename _Traits, typename _Alloc>
-    void
+    inline void
     __sso_string_base<_CharT, _Traits, _Alloc>::
     _M_erase(size_type __pos, size_type __n)
     {

I get this code:

        pushl   %ebp
.LCFI0:
        movl    %esp, %ebp
.LCFI1:
        movl    8(%ebp), %eax
        movl    $0, 4(%eax)
        movl    (%eax), %eax
        movb    $0, (%eax)
        popl    %ebp
        ret

Much nicer.

I think this would be an appropriate fix, as _M_erase is not large.  Another
possible fix would be to change clear to call _M_set_length(0).


-- 
           Summary: __gnu_cxx::__vstring::clear() is slow
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: ian at airs dot com


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[pcarlini at suse dot de]

------- Comment #1 from pcarlini at suse dot de  2006-08-18 02:13 -------
Hi Ian. Please go ahead with the inlining: only, from a stylistical point of
view, please move the entire body inline, do not mark inline the out of line
code. Patch preapproved, thanks a lot!


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at gcc dot gnu   |ian at airs dot com
                   |dot org                     |
             Status|UNCONFIRMED                 |ASSIGNED
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-08-18 02:13:54
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[ian at airs dot com]

------- Comment #2 from ian at airs dot com  2006-08-18 06:32 -------
Unfortunately I do not currently have a valid copyright assignment with the
FSF.  I think my one word change is fine, but I don't feel comfortable moving a
function around (although I agree that is the right approach).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[pcarlini at suse dot de]

------- Comment #3 from pcarlini at suse dot de  2006-08-18 08:31 -------
Ok, I see... Then I will do the change, a little embarassing ;) By the way, if
it's not obvious, the reason we don't simply call _M_set_length is the other
base class, where clear cannot  be trivial (due to refcounting) and the
meaningful thing to do is considering clear a special case of erase and share
code. At some point we should add a new _M_clear to both base classes and
either forward to _M_erase or just call _M_set_length.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|ian at airs dot com         |pcarlini at suse dot de


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[paolo at gcc dot gnu dot org]

------- Comment #5 from paolo at gcc dot gnu dot org  2006-08-18 15:42 -------
Subject: Bug 28765

Author: paolo
Date: Fri Aug 18 15:42:27 2006
New Revision: 116241

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=116241
Log:
2006-08-18  Paolo Carlini  <pcarlini@suse.de>

        PR libstdc++/28765
        * include/ext/rc_string_base.h (_M_clear): New.
        * include/ext/sso_string_base.h (_M_clear): Likewise.
        * include/ext/vstring.h (clear): Use it.

Modified:
    branches/gcc-4_1-branch/libstdc++-v3/ChangeLog
    branches/gcc-4_1-branch/libstdc++-v3/include/ext/rc_string_base.h
    branches/gcc-4_1-branch/libstdc++-v3/include/ext/sso_string_base.h
    branches/gcc-4_1-branch/libstdc++-v3/include/ext/vstring.h


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[paolo at gcc dot gnu dot org]

------- Comment #4 from paolo at gcc dot gnu dot org  2006-08-18 15:42 -------
Subject: Bug 28765

Author: paolo
Date: Fri Aug 18 15:42:05 2006
New Revision: 116240

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=116240
Log:
2006-08-18  Paolo Carlini  <pcarlini@suse.de>

        PR libstdc++/28765
        * include/ext/rc_string_base.h (_M_clear): New.
        * include/ext/sso_string_base.h (_M_clear): Likewise.
        * include/ext/vstring.h (clear): Use it.

Modified:
    trunk/libstdc++-v3/ChangeLog
    trunk/libstdc++-v3/include/ext/rc_string_base.h
    trunk/libstdc++-v3/include/ext/sso_string_base.h
    trunk/libstdc++-v3/include/ext/vstring.h


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[pcarlini at suse dot de]

------- Comment #6 from pcarlini at suse dot de  2006-08-18 15:44 -------
Fixed for 4.1.2.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |RESOLVED
         Resolution|                            |FIXED
   Target Milestone|---                         |4.1.2


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765

[Bug libstdc++/28765] __gnu_cxx::__vstring::clear() is slow

[ian at airs dot com]

------- Comment #7 from ian at airs dot com  2006-08-18 15:58 -------
Thanks!


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28765