>Submitter-Id: net
>Originator: Johannes Stezenbach
>Organization:
>Confidential: no
>Synopsis: gcc-3.1.1 (debian/i386): wrong code with -O2 / bitfields /
>pointer aliasing
>Severity: serious
>Priority: medium
>Category: optimization
>Class: wrong-code
>Release: 3.1.1 (Debian testing/unstable)
>Environment:
System: Linux hell 2.4.19pre1 #3 Sun May 12 19:15:17 CEST 2002 i686 unknown
unknown GNU/Linux
Architecture: i686
host: i386-pc-linux-gnu
build: i386-pc-linux-gnu
target: i386-pc-linux-gnu
configured with: /mnt/data/gcc-3.1/gcc-3.1-3.1.1ds3/src/configure -v
--enable-languages=c,c++,java,f77,proto,objc,ada --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.1 --enable-shared --with-system-zlib
--enable-long-long --enable-nls --without-included-gettext --enable-clocale=gnu
--enable-__cxa_atexit --enable-threads=posix --enable-java-gc=boehm
--enable-objc-gc i386-linux
>Description:
The attached sample program produces wrong output when
compiled with -O2. Output produced when compiled with-O is
correct.
I'm attaching the C source as well as compressed preprcessor output.
$ gcc-3.1 -Wall -O2 bug.c -o bug
$ ./bug
8 8
0xbeaddeef 0x34127856
0xa307401c 0x12345678
$ gcc-3.1 -Wall -O bug.c -o bug
$ ./bug
8 8
0xbeaddeef 0x34127856
0xdeadbeef 0x12345678
$ gcc-3.1 -v -Wall -O2 bug.c -o bug
Reading specs from /usr/lib/gcc-lib/i386-linux/3.1.1/specs
Configured with: /mnt/data/gcc-3.1/gcc-3.1-3.1.1ds3/src/configure -v
--enable-languages=c,c++,java,f77,proto,objc,ada --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-gxx-include-dir=/usr/include/c++/3.1 --enable-shared --with-system-zlib
--enable-long-long --enable-nls --without-included-gettext --enable-clocale=gnu
--enable-__cxa_atexit --enable-threads=posix --enable-java-gc=boehm
--enable-objc-gc i386-linux
Thread model: posix
gcc version 3.1.1
/usr/lib/gcc-lib/i386-linux/3.1.1/cc1 -lang-c -v -D__GNUC__=3
-D__GNUC_MINOR__=1 -D__GNUC_PATCHLEVEL__=1 -D__ELF__ -Dunix -D__gnu_linux__
-Dlinux -D__ELF__ -D__unix__ -D__gnu_linux__ -D__linux__ -D__unix -D__linux
-Asystem=posix -D__OPTIMIZE__ -D__STDC_HOSTED__=1 -Acpu=i386 -Amachine=i386
-Di386 -D__i386 -D__i386__ -D__tune_i386__ bug.c -quiet -dumpbase bug.c -O2
-Wall -version -o /home/js/tmp/ccA9puym.s
GNU CPP version 3.1.1 (cpplib) (i386 Linux/ELF)
GNU C version 3.1.1 (i386-linux)
compiled by GNU C version 3.1.1.
ignoring nonexistent directory "/usr/i386-linux/include"
#include "..." search starts here:
#include <...> search starts here:
/usr/local/include
/usr/lib/gcc-lib/i386-linux/3.1.1/include
/usr/include
End of search list.
as -V -Qy -o /home/js/tmp/ccA3HBCF.o /home/js/tmp/ccA9puym.s
GNU assembler version 2.12.90.0.15 (i386-linux) using BFD version 2.12.90.0.15
20020717 Debian GNU/Linux
/usr/lib/gcc-lib/i386-linux/3.1.1/collect2 --eh-frame-hdr -m elf_i386
-dynamic-linker /lib/ld-linux.so.2 -o bug
/usr/lib/gcc-lib/i386-linux/3.1.1/../../../crt1.o
/usr/lib/gcc-lib/i386-linux/3.1.1/../../../crti.o
/usr/lib/gcc-lib/i386-linux/3.1.1/crtbegin.o
-L/usr/lib/gcc-lib/i386-linux/3.1.1
-L/usr/lib/gcc-lib/i386-linux/3.1.1/../../.. /home/js/tmp/ccA3HBCF.o -lgcc
-lgcc_eh -lc -lgcc -lgcc_eh /usr/lib/gcc-lib/i386-linux/3.1.1/crtend.o
/usr/lib/gcc-lib/i386-linux/3.1.1/../../../crtn.o
>How-To-Repeat:
/* gcc-3.1.1 bug: wrong results with -O2 */
#include <stdio.h>
#include <asm/byteorder.h>
struct foo {
unsigned int a: 8,
b:24;
unsigned int c:16,
d:16;
};
struct foo_swabbed {
unsigned int b:24,
a: 8;
unsigned int d:16,
c:16;
};
static void swab_foo(struct foo *f)
{
struct foo_swabbed fs = *(struct foo_swabbed *) f;
unsigned int *p = (unsigned int *)&fs;
p[0] = __swab32(p[0]);
p[1] = __swab32(p[1]);
f->a = fs.a;
f->b = fs.b;
f->c = fs.c;
f->d = fs.d;
}
int main(void)
{
struct foo f = { 0xef, 0xbeadde, 0x7856, 0x3412 };
unsigned int *p = (unsigned int *)&f;
printf("%u %u\n", sizeof(struct foo), sizeof(struct foo_swabbed));
printf("%#010x %#010x\n", p[0], p[1]);
swab_foo(&f);
printf("%#010x %#010x\n", p[0], p[1]);
return 0;
}
>Fix:
