optimization/6377: Bitfield and Optimizer 2.96 and later, multi-platform

[charles@Provis.com]

>Number:         6377
>Category:       optimization
>Synopsis:       Optimization (>= -O2) with bitfields yields incorrect result
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Fri Apr 19 11:36:07 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Charles Rennolet
>Release:        3.0.3
>Organization:
Provis Corporation
>Environment:
System: SunOS dog 5.8 Generic_108528-12 sun4u sparc SUNW,Sun-Blade-100
Architecture: sun4

	
host: sparc-sun-solaris2.8
build: sparc-sun-solaris2.8
target: sparc-sun-solaris2.8
configured with: ../gcc-3.0.3/configure 
>Description:
   gcc versions 2.96 and later have a problem with bitfields
and optimization.  There appears to be a mismatch between where the
results are left and where they should go, especially when there
is an intermediate cast.  The result is that the bitfield structure
is left with random data in it.  This happens on Solaris (with 3.0.3)
and Linux (with 2.96, 3.0.3, and 3.0.4) using optimization levels -O2
or higher.

>How-To-Repeat:
/* 
To Repeat: save this example program (self-contained
except for the printf at the bottom) and compile with -O3.  The
result should contain nonzero bits, but doesn't.  (The problem is
that the result in the getFoo is being put in the stack frame, while
the calling program expects the result in edx:eax.)

Charles Rennolet (charles@provis.com, clr@thurse.mn.org)

Example: (A very short one.)
*/
typedef struct
{
    unsigned int   device   : 28;
    unsigned int   stuckat  : 2;
    unsigned int   direction: 2;
    unsigned int   pin      : 12;
    unsigned int   port     : 12;
    unsigned int   status   : 4;
    unsigned int   type     : 4;
} *pFoo;

unsigned long long getFoo(int type,
                     int device,
                     int port,
                     int pin,
                     int InOut,
                     int stuckat)
{
    unsigned long long retValue = 0;

    ((pFoo)&retValue)->status    = 0;
    ((pFoo)&retValue)->type      = type;
    ((pFoo)&retValue)->device    = device;
    ((pFoo)&retValue)->port      = port;
    ((pFoo)&retValue)->pin       = pin;
    ((pFoo)&retValue)->direction = InOut;
    ((pFoo)&retValue)->stuckat   = stuckat;
    return(retValue);
}

int main( int argc, char **argv)
{
    unsigned long long foo;
    unsigned int one = 1;
    unsigned int two = 2;
    unsigned int three = 3;
    unsigned int four = 4;
    unsigned int five = 5;
    unsigned int six = 6;
    unsigned int seven = 7;
    unsigned int tmp;

    foo = getFoo( one, two, three, four, five, six);

    printf("foo = %016llx\n", foo);
}

/* ============================Example Ends============================= */

Preprocessed input file:

# 22 "foo.c"
typedef struct
{
    unsigned int device : 28;
    unsigned int stuckat : 2;
    unsigned int direction: 2;
    unsigned int pin : 12;
    unsigned int port : 12;
    unsigned int status : 4;
    unsigned int type : 4;
} *pFoo;

unsigned long long getFoo(int type,
                     int device,
                     int port,
                     int pin,
                     int InOut,
                     int stuckat)
{
    unsigned long long retValue = 0;

    ((pFoo)&retValue)->status = 0;
    ((pFoo)&retValue)->type = type;
    ((pFoo)&retValue)->device = device;
    ((pFoo)&retValue)->port = port;
    ((pFoo)&retValue)->pin = pin;
    ((pFoo)&retValue)->direction = InOut;
    ((pFoo)&retValue)->stuckat = stuckat;
    return(retValue);
}

int main( int argc, char **argv)
{
    unsigned long long foo;
    unsigned int one = 1;
    unsigned int two = 2;
    unsigned int three = 3;
    unsigned int four = 4;
    unsigned int five = 5;
    unsigned int six = 6;
    unsigned int seven = 7;
    unsigned int tmp;

    foo = getFoo( one, two, three, four, five, six);

    printf("foo = %016llx\n", foo);
}
>Fix:
	Don't compile with -O2 or greater?  Yeah, I know it's lame.


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