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* c/8996: Unreliable bitfields
@ 2002-12-18 14:26 hakonrk
0 siblings, 0 replies; 2+ messages in thread
From: hakonrk @ 2002-12-18 14:26 UTC (permalink / raw)
To: gcc-gnats
>Number: 8996
>Category: c
>Synopsis: Unreliable bitfields
>Confidential: no
>Severity: serious
>Priority: medium
>Responsible: unassigned
>State: open
>Class: wrong-code
>Submitter-Id: net
>Arrival-Date: Wed Dec 18 14:26:00 PST 2002
>Closed-Date:
>Last-Modified:
>Originator: hakonrk@fys.uio.no
>Release: gcc 3.2.1
>Organization:
>Environment:
System: Linux 2.4.20 i686
configured with: /local/build/gcc/gcc-3.2.1/configure --prefix=/local/gcc/3.2.1 --enable-languages=c --disable-nls
>Description:
Hard to explain, easy to show. In some cases, bit-field operations are very unreliable. The following program illustrates the problem:
# 1 "foo.c"
int main(void)
{
struct {
unsigned a : 8;
unsigned b : 23;
unsigned c : 1;
} x;
*(unsigned *) &x = -1;
x.c = 1;
x.b = 1;
x.a = 0;
printf("%08X\n", *(unsigned *) &x);
return 0;
}
The correct output from this program is 80000100. When compiling with -O2 (or higher) using GCC 3.2.1, the final assignment is not done (x.a = 0), and the output is 800001FF. A slight change in the program will make the bug disappear. Here are some examples of changes that will make the bug disappear:
- commenting out the assignments to either x.b or x.c
- changing the bit widths of x.a/x.b/x.c
- inserting a "nop" between the printf and the assignments
- changing the order of the assignments
>How-To-Repeat:
See description.
>Fix:
>Release-Note:
>Audit-Trail:
>Unformatted:
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: c/8996: Unreliable bitfields
@ 2002-12-18 15:17 reichelt
0 siblings, 0 replies; 2+ messages in thread
From: reichelt @ 2002-12-18 15:17 UTC (permalink / raw)
To: gcc-bugs, gcc-prs, hakonrk, nobody
Synopsis: Unreliable bitfields
State-Changed-From-To: open->closed
State-Changed-By: reichelt
State-Changed-When: Wed Dec 18 15:17:02 2002
State-Changed-Why:
Not a bug.
Your program does not obey aliasing rules.
You can use the option "-fno-strict-aliasing" as a workaround
or you can correct your code:
int main(void)
{
union {
struct {
unsigned a : 8;
unsigned b : 23;
unsigned c : 1;
} x;
unsigned i;
} u;
u.i = -1;
u.x.c = 1;
u.x.b = 1;
u.x.a = 0;
printf("%08X\n", u.i);
return 0;
}
http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8996
^ permalink raw reply [flat|nested] 2+ messages in thread
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