Re: c++/4026: bad code generated with optimization

Re: c++/4026: bad code generated with optimization

[mmitchel]

Synopsis: bad code generated with optimization

State-Changed-From-To: analyzed->closed
State-Changed-By: mmitchel
State-Changed-When: Mon Mar 18 14:47:42 2002
State-Changed-Why:
    Fixed in GCC 3.1.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=4026

Re: c++/4026: bad code generated with optimization

[mmitchel]

Synopsis: bad code generated with optimization

Responsible-Changed-From-To: unassigned->mmitchel
Responsible-Changed-By: mmitchel
Responsible-Changed-When: Thu Mar 14 14:58:36 2002
Responsible-Changed-Why:
    I will investigate.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=4026

Re: c++/4026: bad code generated with optimization

[nathan]

Synopsis: bad code generated with optimization

State-Changed-From-To: open->analyzed
State-Changed-By: nathan
State-Changed-When: Tue Feb 26 14:03:26 2002
State-Changed-Why:
    confirmed as a 2.95 regression. with current CVS I get,
    nathan@uha:28>../../../xg++ incomming/4026.ii -O2
    nathan@uha:29>./a.out                            
    aa: 3fffe130 123
    aa: 3fffe110 123
    bb 3fffe130 123

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=4026

c++/4026: bad code generated with optimization

[snyder]

>Number:         4026
>Category:       c++
>Synopsis:       bad code generated with optimization
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Aug 14 20:16:00 PDT 2001
>Closed-Date:
>Last-Modified:
>Originator:     scott snyder
>Release:        3.0.1 20010813 (prerelease)
>Organization:
>Environment:
System: Linux karma 2.4.2-2 #1 Sun Apr 8 20:41:30 EDT 2001 i686 unknown
Architecture: i686

	
host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
configured with: ../egcs/configure --prefix=/usr/local/egcs --enable-threads=posix --enable-long-long
>Description:

The program below crashes when compiled with -O2:

$ g++ -O2 -o x x.cc
$ ./x
aa: bfffca70 123
aa: bfffca50 123
Segmentation fault (core dumped)
$ 

Without -O2, it executes correctly:

$ g++  -o x x.cc
$ ./x
aa: bfffca80 123
aa: bfffca50 123
bb bfffca50 123
bb bfffca80 123
$

The problem also goes away if -fno-strict-aliasing is used.

I see the problem already in the 00.rtl dump, so the problem is in the
front- or middle-end, not the backend.

I do not see this problem with gcc 2.95, so this is a regression
from that version.  It is, however, present in gcc 3.0.

Here's a condensed version of the initial RTL output for main()
(in hopefully obvious notation, omitting exception handling code
that is not normally reached):

  8 $esp = $esp & ~0x0f
 10 $42 = 0
 12 $esp = $esp - $42

    ;; Set up temp1 --- located at offset -32
 14 $43 = $virtual-stack-dynamic
 27 $44 = $virtual-stack-vars - 32
 33 $45 = $44
 42 $46 = $45 + 4
 48 *$46 = 0
 52 *($46 + 12) = 0
 65 $esp = $esp - 12
 67 *(--$esp) = $45
 68 call initialize_map
 70 $esp += 16

121 $53 = &$52

    ;; Make the temporary copy of temp1 to pass to yyy ---
    ;; located at offset -64
125 $54 = $virtual-stack-vars - 64
127 $55 = $virtual-stack-vars - 32
133 $56 = $54
142 $57 = $56 + 4
148 *$57 = 0
152 *($57 + 12) = 0
168 $esp -= 12
170 *(--$esp) = $56
171 call initialize_map
173 $esp += 16

    ;; Call yyy.  Not sure about the value that's getting passed; looks
    ;; like it's uninitialized?
235 $esp -= 12
237 *(--$esp) = &$52
238 call yyy
241 $esp += 16

    ;; Start executing the dtor for the temporary at - 64.
    ;;   ~deque() {
    ;;      _Deque_iterator i = _M_start;
    ;;    }
    ;; Copy _M_start to i, where i is at offset - 48.
    ;; 
    ;; __BUT__ the temporary at -64 is _20_ bytes long ---
    ;; so this overwrites the last word of the temporary!

328 $67 = $virtual-stack-vars - 64
343 $68 = $virtual-stack-vars - 48
345 $69 = $67 + 4
352 $70 = *$69
354 *$68 = $70
358 $71 = *($69 + 12)
360 *($68 + 12) = $71
376 $esp -= 12
378 *(--$esp) = $67
379 call ~Deque_base
381 $esp += 16

    ;; Start executing the dtor for temp1 at - 32.
481 $80 = $virtual-stack-vars - 32
496 $81 = $virtual-stack-vars - 48
498 $82 = $80 + 4
505 $83 = *$82
507 *$81 = $83
511 $84 = *($82 + 12)
513 *($81 + 12) = $84
529 $esp -= 12
531 *(--$esp) = $80
532 call ~Deque_base
534 $esp += 16

    ;; Return.
387 $eax = 0




>How-To-Repeat:

Compile with -O2:

----------------------------------------------------------------
extern "C" int printf (...);



struct _Deque_iterator {
  int _M_cur;
  int x[2];
  int* _M_node;

  _Deque_iterator() : _M_cur(0), _M_node(0) {}
  _Deque_iterator(const _Deque_iterator& __x)
    : _M_cur(__x._M_cur),
      _M_node(__x._M_node) {}
};

class _Deque_base
{
public:
  int yy;

  _Deque_base()
    : _M_start()
    { _M_initialize_map(); }
  ~_Deque_base();    

  void _M_initialize_map();
  _Deque_iterator _M_start;
};


_Deque_base::~_Deque_base() {
  printf ("bb %x %x\n", this, *_M_start._M_node);
}

void
_Deque_base::_M_initialize_map()
{
  yy = 0x123;
  printf ("aa: %x %x\n", this, yy);

  _M_start._M_node = &yy;
  _M_start._M_cur = yy;
}


class deque : protected _Deque_base
{
public:
  deque () {}
  deque(const deque& __x) {}

  ~deque() {
    _Deque_iterator i = _M_start;
  }
};



class GeometryAddress {
public:
  GeometryAddress(deque addressStack) {}
};

void yyy (const GeometryAddress& gb)
{
}

int main()
{
  deque temp1;
  yyy (GeometryAddress (temp1));
  return 0;
}
----------------------------------------------------------------

>Fix:
	
>Release-Note:
>Audit-Trail:
>Unformatted: