Re: set_bit() is broken on i386?



On Fri, 20 Jan 2006, Valdis.Kletnieks@xxxxxx wrote:

On Fri, 20 Jan 2006 19:53:14 EST, Chuck Ebbert said:
/*
 * setbit.c -- test the Linux set_bit() function
 *
 * Compare the output of this program with and without the
 * -finline-functions option to GCC.
 *
 * If they are not the same, set_bit is broken.
 *
 * Result on i386 with gcc 3.3.2 (Fedora Core 2):
 *
 * [me@d2 t]$ gcc -O2 -o setbit.ex setbit.c ; ./setbit.ex
 * 00010001
 * [me@d2 t]$ gcc -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
 * 00000001

It certainly seems to be gcc version dependent (and I'd not be surprised if the exact combo of -O2, -Os, and -mfoo and -fwhatever flags as well). Trond is probably right that a memory clobber will force gcc to DTIT (Do The Intended Thing, which may be different from a DTRT) regardless of what gcc's code generator decides to do....

% gcc -v
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man --infodir=/usr/share/info --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-libgcj-multifile --enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk --disable-dssi --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --host=i386-redhat-linux
Thread model: posix
gcc version 4.1.0 20060117 (Red Hat 4.1.0-0.15)
% gcc -O2 -o setbit.ex setbit.c ; ./setbit.ex
00000001
% gcc -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
00000001

Fedora Core -devel tree as of this morning (so sort-of FC5 test2).

This is what I am getting under Gentoo with different gcc's:

$ export CC=gcc-3.4.5; $CC --version; rm ./setbit.ex; $CC -O2 -o setbit.ex setbit.c ; ./setbit.ex; rm ./setbit.ex; $CC -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
gcc-3.4.5 (GCC) 3.4.5 (Gentoo 3.4.5, ssp-3.4.5-1.0, pie-8.7.9)
Copyright (C) 2004 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


00000001
00000001


$ export CC=gcc-4.0.2; $CC --version; rm ./setbit.ex; $CC -O2 -o setbit.ex setbit.c ; ./setbit.ex; rm ./setbit.ex; $CC -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
gcc-4.0.2 (GCC) 4.0.2 (Gentoo 4.0.2-r3, pie-8.7.8)
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


00000001
00000001


$ export CC=gcc-4.1.0-beta20060113; $CC --version; rm ./setbit.ex; $CC -O2 -o setbit.ex setbit.c ; ./setbit.ex; rm ./setbit.ex; $CC -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
gcc-4.1.0-beta20060113 (GCC) 4.1.0-beta20060113 (prerelease)
Copyright (C) 2005 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


00000001
00000001


And on some really old and not upgraded server (with Gentoo too of course):


$ export CC=gcc32; $CC --version; rm ./setbit.ex; $CC -O2 -o setbit.ex setbit.c ; ./setbit.ex; rm ./setbit.ex; $CC -O2 -o setbit.ex -finline-functions setbit.c ; ./setbit.ex
gcc (GCC) 3.3.5-20050130 (Gentoo 3.3.5.20050130-r1, ssp-3.3.5.20050130-1, pie-8.7.7.1)
Copyright (C) 2003 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


00010001
00000001


So either this problem is present only on gcc < 3.4 or Gentoo patched it somehow in newer versions...



Thanks,

Grzegorz Kulewski

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