gcc 4.4.1 and 4.2.4 failed to generate the correct code under -O0; gcc 3.4.6
works fine.
---BEGIN code ---
#include <stdio.h>
int main( )
{
int a, b;
a = 1;
b = 2;
register int *pa = &a;
register int *pb = &b;
/* Swap a and b using pointers pa and pb */
*pa ^= *pb ^= *pa ^= *pb;
printf( "%d %d\n", a, b );
return 0;
}
---END code ---
Symptom
----------------------------
The expected end result is a = 2 and b = 1, but the code generated by gcc 4.4.1
and 4.2.4 produces a = 0 and b = 1.
The Incorrect Assembly code
----------------------------
The relevant portion of the assembly code produced by 'gcc -S -O0' is
---BEGIN code ---
movl $1, -20(%rbp)
movl $2, -24(%rbp)
leaq -20(%rbp), %rbx
.cfi_offset 3, -32
.cfi_offset 12, -24
leaq -24(%rbp), %r12
movl (%rbx), %edx
movl (%r12), %ecx
movl (%rbx), %esi
movl (%r12), %eax
xorl %esi, %eax
movl %eax, (%rbx)
movl (%rbx), %eax
xorl %ecx, %eax
movl %eax, (%r12)
movl (%r12), %eax
xorl %edx, %eax
movl %eax, (%rbx)
---END code ---
The last xorl code is incorrect; the correct one should be "xorl (%rbx) %eax".
The problem with the incorrect code is that even though the value pointed to by
%rbx (or pa in the C code) has been changed, the above xorl code is still using
the old saved value of (%rbx), or *pa, instead of its most current value.
Below is the options given when gcc was built
-----------------------------------------------
Configured with: ../src/configure -v --with-pkgversion='Ubuntu 4.4.1-4ubuntu9'
--with-bugurl=file:///usr/share/doc/gcc-4.4/README.Bugs
--enable-languages=c,c++,fortran,objc,obj-c++ --prefix=/usr --enable-shared
--enable-multiarch --enable-linker-build-id --with-system-zlib
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--with-gxx-include-dir=/usr/include/c++/4.4 --program-suffix=-4.4 --enable-nls
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --disable-werror
--with-arch-32=i486 --with-tune=generic --enable-checking=release
--build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu
--
Summary: gcc failed to generate correct code under -O0
Product: gcc
Version: 4.4.1
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: jasonzou1983 at yahoo dot com
GCC host triplet: x86_64-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43474
