c/10426: -O2 messes with ordering of pointer dereferencing

c/10426: -O2 messes with ordering of pointer dereferencing

[jonathan.sambrook]

>Number:         10426
>Category:       c
>Synopsis:       -O2 messes with ordering of pointer dereferencing
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 17 11:56:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     jonathan.sambrook@dsvr.co.uk
>Release:        3.2.2 (don't know when from, but after 2.96 which works fine)
>Organization:
>Environment:
Linux 2.4.20
gcc 3.2.2
>Description:
Expected o/p : "abcde", but with -O2 get "bcde":

int main( int argc, char ** argv )
{
  char str[17];
  sprintf( str, "ABCDE" );
  char * p = str;
  for( ;*p ; )
    *p++ = tolower(*p);
  printf( "'%s'\n", str );
  return 0;
}

Yes it's nasty coding; no *I* didn't write it in its wild form :)

[.i file redundant]
>How-To-Repeat:
Works:
  gcc     -o test test.c

Doesn't:
  gcc -O2 -o test test.c
>Fix:

>Release-Note:
>Audit-Trail:
>Unformatted:
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Re: c/10426: -O2 messes with ordering of pointer dereferencing

[ebotcazou]

Synopsis: -O2 messes with ordering of pointer dereferencing

State-Changed-From-To: open->closed
State-Changed-By: ebotcazou
State-Changed-When: Thu Apr 17 14:24:48 2003
State-Changed-Why:
    Not a bug. The compiler is allowed to evaluate the side-effect 
    operator in any order in *p++ = tolower(*p).

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=10426