c/5929: gcc 2.95-3.04 miscompiles anonymous unions inside anonymous structs

c/5929: gcc 2.95-3.04 miscompiles anonymous unions inside anonymous structs

[degnbol]

>Number:         5929
>Category:       c
>Synopsis:       gcc 2.95-3.04 miscompiles anonymous unions inside anonymous structs
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Mar 12 15:06:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator:     Gunnar Degnbol
>Release:        Cygwin gcc 2.95-3, RH gcc 2.96.98, gcc 3.0.3, gcc  3.0.4
>Organization:
>Environment:
Cygwin/Windows NT4 (gcc 2.95-3)
RH Linux 7.2 (gcc 3.0.3, gcc 3.0.4)
>Description:
The members of an anonymous union inside an anonymous struct are aligned with the first member of the anonymous struct. This affects Windows VARIANTs used in C programs. The bug does 
not affect C++ programs. 
>How-To-Repeat:
$ gcc anonbug.c -o anonbug
$ ./anonbug
offset v1 = 0
offset v2 = 4
offset v3 = 8
offset u1 = 4
offset u2 = 4
$ g++ anonbug.c -o anonbug # or MS/Borland/intel C/C++
$ ./anonbug 
offset v1 = 0
offset v2 = 4
offset v3 = 8
offset u1 = 12
offset u2 = 12
>Fix:
Not a fix but a workaround that works for me.

A Windows VARIANT (Cygwin/Mingw32 oaidl.h, WINE oleauto.h) has this structure: 

struct tagVARIANT {
    union {
        struct {
            VARTYPE vt;
            // reserved fields
            union {
                // any kind of variable
            };
        };
        DECIMAL decVal;
    };
} VARIANT;

Since nobody (?) actually uses decVal, a workaround for this specific instance is to #ifdef __cplusplus the outer anonymous union and struct.
>Release-Note:
>Audit-Trail:
>Unformatted:
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Content-Disposition: attachment; filename="anonbug.c"

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Re: c/5929: gcc 2.95-3.04 miscompiles anonymous unions inside anonymous structs

[neil]

Synopsis: gcc 2.95-3.04 miscompiles anonymous unions inside anonymous structs

Responsible-Changed-From-To: unassigned->neil
Responsible-Changed-By: neil
Responsible-Changed-When: Tue Mar 12 15:36:37 2002
Responsible-Changed-Why:
    Mine.
State-Changed-From-To: open->closed
State-Changed-By: neil
State-Changed-When: Tue Mar 12 15:36:37 2002
State-Changed-Why:
    I fixed this for 3.1, which will give:
    
    neil@cat:/tmp$ ./a.out 
    offset v1 = 0
    offset v2 = 4
    offset v3 = 8
    offset u1 = 12
    offset u2 = 12
    neil@cat:/tmp$

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=5929