[Bug c++/29688] New: resize initializes whole array

[Bug c++/29688] New: resize initializes whole array

[theo dot bosman at net dot HCC dot nl]

in valarray.h I found that resize is always deleting the old array,
reallocating a new array and initialize to the value specified. The text book
(Stroustrup) indicates that only the newly allocated elements should be
initialized. In my case the array size was not changed but the array
reinitialized (bad code will be changed). I assume this has been corrected in
later versions. I solved the problem by putting the "destroy" and "fill" inside
the if block.
Regards,
Theo Bosman

  template <class _Tp>
    inline void
    valarray<_Tp>::resize (size_t __n, _Tp __c)
    {
      // This complication is so to make valarray<valarray<T> > work
      // even though it is not required by the standard.  Nobody should
      // be saying valarray<valarray<T> > anyway.  See the specs.

      __valarray_destroy_elements(_M_data, _M_data + _M_size);

      if (_M_size != __n)
        {
          __valarray_release_memory(_M_data);
          _M_size = __n;
          _M_data = __valarray_get_storage<_Tp>(__n); 
        }
          __valarray_fill_construct(_M_data, _M_data + __n, __c);
    }


-- 
           Summary: resize initializes whole array
           Product: gcc
           Version: 3.3.5
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: theo dot bosman at net dot HCC dot nl


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[pcarlini at suse dot de]

------- Comment #1 from pcarlini at suse dot de  2006-11-02 13:34 -------
I do not have Stroustrup at hand, but certainly the ISO C++ Standard 2003, the
real reference for our work (we are implementing it), says, in 26.3.2.7/9, that
resize first changes the length of *this to sz and then assigns to each element
the value of the second argument. It also says that the operation invalidates
all pointers and references to elements in the array, thus, the meaning is
clear and our implementation is 100% conforming.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
          Component|c++                         |libstdc++
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[pcarlini at suse dot de]

------- Comment #2 from pcarlini at suse dot de  2006-11-02 13:51 -------
The only possible change I can see, as an optimization, is using
__valarray_fill instead of __valarray_destroy_elements and
__valarray_fill_construct, when _M_size == __n. Let's ask Gaby...


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |pcarlini at suse dot de, gdr
                   |                            |at integrable-solutions dot
                   |                            |net
             Status|RESOLVED                    |UNCONFIRMED
         Resolution|INVALID                     |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[theo dot bosman at net dot HCC dot nl]

------- Comment #3 from theo dot bosman at net dot HCC dot nl  2006-11-02 15:56 -------
Subject: Re:  resize initializes whole array

There is no argument against the ISO standard, but to a non C/C++ programmer 
it seems a waist of time to reallocate the array and initialize it when one 
wants to add something to an array. Some other compilers will copy the 
existing data into the newly allocated space which seems more in line with 
the text book.
The application will be changed to avoid the loss of data on a resize of 
arrays, which is a better idea anyway. Thanks for the speedy reply.
Theo Bosman

----- Original Message ----- 
From: "pcarlini at suse dot de" <gcc-bugzilla@gcc.gnu.org>
To: <theo.bosman@net.HCC.nl>
Sent: Thursday, November 02, 2006 2:51 PM
Subject: [Bug libstdc++/29688] resize initializes whole array


>
>
> ------- Comment #2 from pcarlini at suse dot de  2006-11-02 13:51 -------
> The only possible change I can see, as an optimization, is using
> __valarray_fill instead of __valarray_destroy_elements and
> __valarray_fill_construct, when _M_size == __n. Let's ask Gaby...
>
>
> -- 
>
> pcarlini at suse dot de changed:
>
>           What    |Removed                     |Added
> ----------------------------------------------------------------------------
>                 CC|                            |pcarlini at suse dot de, 
> gdr
>                   |                            |at integrable-solutions 
> dot
>                   |                            |net
>             Status|RESOLVED                    |UNCONFIRMED
>         Resolution|INVALID                     |
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688
>
> ------- You are receiving this mail because: -------
> You reported the bug, or are watching the reporter.
>
> 


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[pcarlini at suse dot de]

------- Comment #4 from pcarlini at suse dot de  2006-11-03 00:08 -------
(In reply to comment #3)
> Subject: Re:  resize initializes whole array
> 
> There is no argument against the ISO standard, but to a non C/C++ programmer 
> it seems a waist of time to reallocate the array and initialize it when one 
> wants to add something to an array. Some other compilers will copy the 
> existing data into the newly allocated space which seems more in line with 
> the text book.

I don't know. We are definitely implementing the ISO C++ Standard, and, by the
way, many details in Stroustrup' books, predating the Standard, are know to be
slightly different. I will also add that valarray is *very* special - its
design was motivated by considerations of highest performance on superscalar
architectures, etc. - for example vector::resize behaves in a completely
different way, which probably you like better.

> The application will be changed to avoid the loss of data on a resize of 
> arrays, which is a better idea anyway. Thanks for the speedy reply.

For portability, that is the safe thing to do, yes, I agree.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[fang at csl dot cornell dot edu]

------- Comment #5 from fang at csl dot cornell dot edu  2006-11-03 07:28 -------
> There is no argument against the ISO standard, but to a non C/C++ programmer 
> it seems a waist of time to reallocate the array and initialize it when one 
> wants to add something to an array. Some other compilers will copy the 
> existing data into the newly allocated space which seems more in line with 
> the text book.
> The application will be changed to avoid the loss of data on a resize of 
> arrays, which is a better idea anyway. Thanks for the speedy reply.

If you really want efficient appending to a dynamically allocated array, then
use std::vector, which is capable of pre-allocating more memory than the number
of elements, using the reserve() member function.  From looking in the
implementation, N push_back operations result in lg N reallocations, by
doubling the reserve size each time the capacity is exceeded.  vector achieves
this by tracking three pointers: beginning, end-of-elements (size),
end-of-storage (capacity).  valarray is lighter-weight, having only two
pointers, beginning- and end-of-storage; thus it must reallocate upon resize
(otherwise, how could size() be reported correctly?).  I'd only use valarray
when the sizeof(container) is crucial and/or the size of the container is
unlikely/infrequently to change at run-time.  


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688

[Bug libstdc++/29688] resize initializes whole array

[pcarlini at suse dot de]

------- Comment #6 from pcarlini at suse dot de  2006-11-14 00:53 -------
I have analized in detail the case at issue (resize to the same size of the
current one) and came to the conclusion that trying to optimize for it (at the
cost of increasing the size of the inlined function) isn't really worth the
trouble, taking also into account that for PODs __valarray_destroy_elements is
already optimized out.


-- 

pcarlini at suse dot de changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|                            |INVALID


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29688