Re: libstdc++/40

[anders at broadcom dot com (Anders Johnson)]

The following reply was made to PR libstdc++/40; it has been noted by GNATS.

From: anders@broadcom.com (Anders Johnson)
To: bkoz@cygnus.com
Cc: libstdc++-gnats@sourceware.cygnus.com
Subject: Re: libstdc++/40
Date: Wed, 22 Mar 2000 11:13:14 -0800

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 From anders Fri Mar 10 16:23:54 2000
 To: bkoz@cygnus.com
 Subject: Re: libstdc++/40: bitset::operator<<= and >>= incorrect for multiples
  of 32
 
 > can you please send me a patch and change log entry?
 
 I'm new at this, so please bear with me.
 
 The problem was actually observed on gcc/2.95.2/5.6/include/g++-3, because
 I can't build libstdc++-2.90.7 on my system because "wmemcmp.c" doesn't
 compile because 'wint_t' is not defined in either "string.h" or "stddef.h".
 However, the code in libstdc++-2.90.7 looks the same.
 
 Here is the patch:
 
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 diff -c3pr libstdc++-2.90.7-1/stl/bits/std_bitset.h libstdc++-2.90.7/stl/bits/std_bitset.h
 *** libstdc++-2.90.7/stl/bits/std_bitset.h	Tue Dec 21 21:07:58 1999
 --- NEW/stl/bits/std_bitset.h	Fri Mar 10 14:50:36 2000
 *************** void _Base_bitset<_Nw, _WordT>::_M_do_le
 *** 196,204 ****
       const size_t __offset = __shift % __BITS_PER_WORDT(_WordT);
       const size_t __sub_offset = __BITS_PER_WORDT(_WordT) - __offset;
       size_t __n = _Nw - 1;
 !     for ( ; __n > __wshift; --__n)
 !       _M_w[__n] = (_M_w[__n - __wshift] << __offset) |
 !                 (_M_w[__n - __wshift - 1] >> __sub_offset);
       if (__n == __wshift)
         _M_w[__n] = _M_w[0] << __offset;
       for (size_t __n1 = 0; __n1 < __n; ++__n1)
 --- 196,206 ----
       const size_t __offset = __shift % __BITS_PER_WORDT(_WordT);
       const size_t __sub_offset = __BITS_PER_WORDT(_WordT) - __offset;
       size_t __n = _Nw - 1;
 !     for ( ; __n > __wshift; --__n) {
 !       _M_w[__n] = (_M_w[__n - __wshift] << __offset);
 !       if(__offset)
 !         _M_w[__n] |= _M_w[__n - __wshift - 1] >> __sub_offset;
 !     }
       if (__n == __wshift)
         _M_w[__n] = _M_w[0] << __offset;
       for (size_t __n1 = 0; __n1 < __n; ++__n1)
 *************** void _Base_bitset<_Nw, _WordT>::_M_do_ri
 *** 215,223 ****
       const size_t __sub_offset = __BITS_PER_WORDT(_WordT) - __offset;
       const size_t __limit = _Nw - __wshift - 1;
       size_t __n = 0;
 !     for ( ; __n < __limit; ++__n)
 !       _M_w[__n] = (_M_w[__n + __wshift] >> __offset) |
 !                   (_M_w[__n + __wshift + 1] << __sub_offset);
       _M_w[__limit] = _M_w[_Nw-1] >> __offset;
       for (size_t __n1 = __limit + 1; __n1 < _Nw; ++__n1)
         _M_w[__n1] = static_cast<_WordT>(0);
 --- 217,227 ----
       const size_t __sub_offset = __BITS_PER_WORDT(_WordT) - __offset;
       const size_t __limit = _Nw - __wshift - 1;
       size_t __n = 0;
 !     for ( ; __n < __limit; ++__n) {
 !       _M_w[__n] = (_M_w[__n + __wshift] >> __offset);
 !       if(__offset)
 !         _M_w[__n] |= _M_w[__n + __wshift + 1] << __sub_offset;
 !     }
       _M_w[__limit] = _M_w[_Nw-1] >> __offset;
       for (size_t __n1 = __limit + 1; __n1 < _Nw; ++__n1)
         _M_w[__n1] = static_cast<_WordT>(0);
 
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 Here is a ChangeLog entry (not sure about the format):
 
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 - Fri Mar 10 16:12:00 PST 2000 stl/bits/std_bitset.h: Modified
 _M_do_left_shift and _M_do_right_shift not to rely on the behavior
 of _WordT << __BITS_PER_WORDT(_WordT), which is undefined according
 to the C standard, and does the wrong thing on Sparc.
 
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 Here is a test case:
 
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 #include <climits>
 #include <algobase.h>
 #include <bitset>
 #include <iostream>
 
 int main() {
 	bitset<66> b(1);
 	cout << b << endl;
 	b<<=32;
 	cout << b << endl;
 	b |= bitset<66>(1)<<65;
 	cout << b << endl;
 	b>>=32;
 	cout << b << endl;
 	return 0;
 }
 
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 The expected output is:
 000000000000000000000000000000000000000000000000000000000000000001
 000000000000000000000000000000000100000000000000000000000000000000
 100000000000000000000000000000000100000000000000000000000000000000
 000000000000000000000000000000001000000000000000000000000000000001
 
 The actual output on SunOS 5.5.1 sun4u sparc is:
 000000000000000000000000000000000000000000000000000000000000000001
 010000000000000000000000000000000100000000000000000000000000000000
 110000000000000000000000000000000100000000000000000000000000000000
 000000000000000000000000000000001100000000000000000000000000000011
 
 Please let me know if you need anything else.
 
 Thanks,
 --
  n  _   _ _    Anders Johnson        ``The ultimate test of all
  X /_)|' (_`   Broadcom Corporation    knowledge is experiment.''
 (_X\_,|  ._)   anders(at)ieee(dot)org         -- Richard Feynman
 
 
 
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