I'm getting a "operation on xxx may be undefined" error in GCC 4.5.1 that I didn't use to get in 4.3.4

[Staffan Tjernstrom <stjernstrom@xxxxxxx>]

The following code is giving a 

operation on p_parent may be undefined warning using 4.5.1 The warning
did not occur on 4.3.4, and although it's probably me being macro-blind,
I can't for the life of me see the conflict. Nothing that I can see in
the while statement should be modifying p_parent?

staffan@bypass:~/src/flop> gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib64/gcc/x86_64-suse-linux/4.5/lto-wrapper
Target: x86_64-suse-linux
Configured with: ../configure --prefix=/usr --infodir=/usr/share/info
--mandir=/usr/share/man --libdir=/usr/lib64 --libexecdir=/usr/lib64
--enable-languages=c,c++,objc,fortran,obj-c++,java,ada
--enable-checking=release --with-gxx-include-dir=/usr/include/c++/4.5
--enable-ssp --disable-libssp --disable-plugin
--with-bugurl=http://bugs.opensuse.org/ --with-pkgversion='SUSE Linux'
--disable-libgcj --disable-libmudflap --with-slibdir=/lib64
--with-system-zlib --enable-__cxa_atexit
--enable-libstdcxx-allocator=new --disable-libstdcxx-pch
--enable-version-specific-runtime-libs --program-suffix=-4.5
--enable-linux-futex --without-system-libunwind --enable-gold
--with-plugin-ld=/usr/bin/gold --with-arch-32=i586 --with-tune=generic
--build=x86_64-suse-linux
Thread model: posix
gcc version 4.5.1 20101208 [gcc-4_5-branch revision 167585] (SUSE
Linux) 

#define rb_parent(r)   ((struct rb_node *)((r)->rb_parent_color & ~3))
#define rb_color(r)   ((r)->rb_parent_color & 1)
#define rb_is_red(r)   (!rb_color(r))
#define rb_is_black(r) rb_color(r)
#define rb_set_red(r)  do { (r)->rb_parent_color &= ~1; } while (0)
#define rb_set_black(r)  do { (r)->rb_parent_color |= 1; } while (0)
#define PHYSICAL_ADDR( r )  ((struct rb_node *)((size_t)(r)+(size_t)((r)?p_base:0)))
#define RELATIVE_ADDR( r ) ((struct rb_node *)((size_t)(r)-(size_t)((r)?p_base:0)))

static void rb_insert_color(struct rb_node *p_node, struct rb_root *p_root)
{
    struct rb_node *p_phys_node =  PHYSICAL_ADDR(p_node);
    struct rb_node *p_parent, *p_phys_parent, *p_grandparent, *p_phys_grandparent;

    while ( ( p_phys_parent = ( PHYSICAL_ADDR( p_parent = rb_parent( p_phys_node ) ) ) ) && rb_is_red( p_phys_parent ) )
    {
    }