optimization/8613: -O3 optimisation of ++ generates wrong code

optimization/8613: -O3 optimisation of ++ generates wrong code

[mark.odonohue]

>Number:         8613
>Category:       optimization
>Synopsis:       -O3 optimisation of ++ generates wrong code
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Sun Nov 17 05:46:06 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     mark.odonohue@cytopia.com.au
>Release:        gcc 3.2
>Organization:
>Environment:
mandrake linux 9.0  
>Description:

In the provided code, 

*dpb++ = strlen(userName)

dpb is incorrectly incremented by two bytes, not one.

The output buffer should result in:
1, 35, 6, 'sysdba'.

Instead it results in:
1, 35, 6, x, 'sysdba'

where x is random.

Tracing in gdb shows that dpb is incremented by a value of two.
>How-To-Repeat:
g++ -O2 -march=i686 -fPIC test1.cpp
./a.out 1
>Fix:
 -O1 seems to work.
>Release-Note:
>Audit-Trail:
>Unformatted:
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Re: optimization/8613: -O3 optimisation of ++ generates wrong code

[Mark O'Donohue]

The following reply was made to PR optimization/8613; it has been noted by GNATS.

From: "Mark O'Donohue" <mark.odonohue@cytopia.com.au>
To: gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org, mark.odonohue@cytopia.com.au,
        gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org
Cc:  
Subject: Re: optimization/8613: -O3 optimisation of ++ generates wrong code
Date: Mon, 18 Nov 2002 12:28:45 +1100

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8613
 
 
 confirmed on redhat 8.0
 
 
 Also it seem tied up with i686 architecture.
 
 Since on redhat 8.0 gcc 3.2:
 
 g++ -O2 test1.cpp
 
 works:
 
 and on mandrak9.0 gcc 3.2
 
 g++ -O2 -march=i386 test1.cpp
 
 also works.
 
 
 Mark