------- Comment #1 from pinskia at gmail dot com 2009-07-04 01:38 -------
Subject: Re: New: Bus error caused by ldd/std instructions in struct copy.
This code is undefined because of alignment requirments differences
for the structs and the union.
Sent from my iPhone
On Jul 3, 2009, at 6:33 PM, "dentongosnell at yahoo dot com"
<gcc-bugzi...@gcc.gnu.org
> wrote:
> $ gcc -v
> Using built-in specs.
> Target: sparc-linux-gnu
> Configured with: ../src/configure -v --with-pkgversion='Debian 4.3.2-1.1
> '
> --with-bugurl=file:///usr/share/doc/gcc-4.3/README.Bugs
> --enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-
> shared
> --with-system-zlib --libexecdir=/usr/lib --without-included-gettext
> --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/
> include/c++/4.3
> --program-suffix=-4.3 --enable-clocale=gnu --enable-libstdcxx-debug
> --enable-objc-gc --enable-mpfr --with-cpu=v8 --with-long-double-128
> --enable-checking=release --build=sparc-linux-gnu --host=sparc-linux-
> gnu
> --target=sparc-linux-gnu
> Thread model: posix
> gcc version 4.3.2 (Debian 4.3.2-1.1)
>
> To trigger the bug :-
>
> $ gcc align_bug.c
> $ ./a.out
> Bus error
> $
>
> Here is align_bug.c :-
>
> ---------
>
> #include <stdio.h>
>
> struct b_one {
> int i;
> double d;
> };
>
> struct b_two {
> int i1;
> int i2;
> };
>
> union myblock {
> struct b_one one;
> struct b_two two;
> };
>
> void myfunc(union myblock *dp1, union myblock *dp2)
> {
> dp2->two = dp1->two;
> }
>
> int main()
> {
> int w;
> struct b_two a = {1,2};
> struct b_two b;
>
> myfunc((union myblock *)&a, (union myblock *)&b);
>
> printf("%d %d\n", b.i1, b.i2);
>
> return 0;
> }
>
> ----------
>
> The problem seems to happen in "myfunc" when the compiled code tries
> to copy the 8-byte structure dp2->two to dp1->two, using a ldd/std
> instruction pair. The problem seems to be that dp1 and dp2 (ie a and
> b in main) aren't strictly enough aligned for that. If you take out
> the redundant "int w" in main then a and b happen to be aligned okay
> and the bus error doesn't happen.
>
> I think the compiler is assuming "union myblock" has the same
> alignment as "struct b_one", which is more strictly aligned than
> "struct b_two" because of its double member.
>
> I realise that casting &a to (union myblock*) in main may technically
> invoke undefined behaviour... but I think the cast is reasonable given
> that union myblock contains the type of a.
>
> One other thing, there is a flag "-mno-faster-structs" which this page
> suggests would prevent this sort of ldd/std use
> (http://gcc.gnu.org/onlinedocs/gcc/SPARC-Options.html). Unfortunately
> this flag doesn't seem to make any difference to this case.
>
>
> --
> Summary: Bus error caused by ldd/std instructions in
> struct copy.
> Product: gcc
> Version: 4.3.2
> Status: UNCONFIRMED
> Severity: normal
> Priority: P3
> Component: c
> AssignedTo: unassigned at gcc dot gnu dot org
> ReportedBy: dentongosnell at yahoo dot com
> GCC build triplet: sparc-linux-gnu
> GCC host triplet: sparc-linux-gnu
> GCC target triplet: sparc-linux-gnu
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40645
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40645
