value in registers overwritten

Here's some code to reproduce a register allocation bug
where a temporary value overwrites a variable, leading
to wrong results. I have only been able to reproduce
the problem with large memory model.

The variable "next_block" is allocated to R2/R3 and is
used inside a for loop. The loop's execution
overwrites R2/R3 each time the loop is re-run, which
causes the program to assign a wrong value if the
"next_block" variable is used after the first iteration
of the loop.

You can also download a copy of the code here:

http://www.pjrc.com/tmp/test.c

#ifndef SDCC_MODEL_LARGE
#error "This bug only shows up in large memory model"
#endif

struct simple_list_struct {
unsigned int next;
};

static data unsigned int first_block=0;
static xdata at 0x7000 struct simple_list_struct p7;

void test(unsigned int val)
{
unsigned int block, next_block;

next_block = p7.next + 1234; /* assigned to
r2/r3 */

for (block = first_block; block != 0; block =
p7.next) {
if (p7.next == val) {
p7.next = next_block;
return;
}
/* BUG: "block = p7.next" overwrites
r2/r3 */
}
}