[Bug c/35249] gcc miscompiles emacs' src/intervals.c when using optimisation on solaris 8

["simon dot marshall at misys dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #6 from simon dot marshall at misys dot com  2008-02-19 14:40 -------
Created an attachment (id=15184)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=15184&action=view)
intervals2.c and output of -v -save-temps

Maybe it was easier than I thought.  I took out the function update_interval
from the file internals.c, in which Emacs was detecting internal data structure
inconsistency, and compiled it in a new file internals2.c.  With this separate
file, I can reproduce the error if I build with "-g -O1 -fno-unit-at-a-time" or
"-g -O2 -fno-unit-at-a-time", but not with "-g -O0 -fno-unit-at-a-time". 
Attached is the output generated by "-v -save-temps -g -O1 -fno-unit-at-a-time"
for this file, ie, the .c, .i, .s and the output of gcc itself.  All other
files in the Emacs executable were compiled with "-g -O2 -fno-unit-at-a-time".

The offending call of error occurs at internals.c:34:

          else if (NULL_PARENT (i))
            error ("Point before start of properties");

which according to internals.i is:

   else if (((i)->up_obj || (i)->up.interval == 0))
     error ("Point before start of properties");

Note the condition that needs to be true for error to be called.  However, when
error is called:

$ gdb emacs
GNU gdb 6.7.1
Copyright (C) 2007 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "sparc-sun-solaris2.8"...
SIGINT is used by the debugger.
Are you sure you want to change it? (y or n) [answered Y; input not from
terminal]
DISPLAY = dtp020071.misys-risk.com:0.0
TERM = xterm
Breakpoint 1 at 0xe4ee4: file emacs.c, line 432.
Breakpoint 2 at 0x100138: file sysdep.c, line 1384.
(gdb) b intervals2.c:34
Breakpoint 3 at 0x19420c: file intervals2.c, line 34.
(gdb) r -Q
Starting program:
/homedev/marshals/ftp/emacs-22.2-pretests/gcc-4.2.3-g-O2/src/emacs -Q
warning: Temporarily disabling breakpoints for unloaded shared library
"/usr/lib/ld.so.1"
Breakpoint 4 at 0xc552c: file xterm.c, line 7866.

Breakpoint 3, update_interval (i=0x8286f4, pos=1771) at intervals2.c:34
34                  error ("Point before start of properties");
(gdb) p i->up_obj
$1 = 0
(gdb) p i->up.interval
$2 = (struct interval *) 0x828534
(gdb) p *i
$3 = {
  total_length = 36, 
  position = 1782, 
  left = 0x828710, 
  right = 0x828550, 
  up = {
    interval = 0x828534, 
    obj = 8553780
  }, 
  up_obj = 0, 
  gcmarkbit = 0, 
  write_protect = 0, 
  visible = 0, 
  front_sticky = 0, 
  rear_sticky = 0, 
  plist = 8527837
}
(gdb) p i->up_obj || i->up.interval == 0 
$4 = 0
(gdb) 

So, if the data appears to suggest that the condition should be false, why does
the behaviour appear as if the condition is true?

Or did I do something stupid?  Is this really a symptom of some other problem?


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35249