[patch] Java definite assignment

Wed Feb 7 03:38:00 GMT 2001

On 6 Feb 2001, Per Bothner wrote:
> Mo DeJong <mdejong@cygnus.com> writes:
>> > Well, here is a much more simple case of the same thing,
>> No, it is very different, because there is no loop.

...
> The point is in:
>> for (;;) {
> if (b) {
> i = 0;
> break;
> }
> }
>> the only way the loop can terminate is by executing the break
> statement, and i *is* definitely assigned at that point. Hence i is
> definitely assigned after the loop. However, i is *not* definitely
> assigned after the 'if' statement, were there another statement there.

After reading up on the rules in 16.2.9, I now think
that you are correct for the cases where the
if statement is true or is not a compile time
constant.
I am still not sure about this case:
 int i;
 while (true) {
 if (false) {
	 i = 0;
 break;
 }
 }
 i++;
It fails the definite assignment
analysis for 16.2.7, but it
satisfies the rules for 16.2.9.
Chapter 16 indicates that the
following 4 results of the
definite assignment analysis
are possible.
1. V is definitely assigned and is not definitely
unassigned. (The flow analysis rules prove that
an assignment to V has occurred.)
2. V is definitely unassigned and is not definitely
assigned.(The flow analysis rules prove that an
assignment to V has not occurred.) 
3. V is not definitely assigned and is not
definitely unassigned.(The rules cannot prove
whether or not an assignment to V has occurred.)
4. V is definitely assigned and is definitely
unassigned.(It is impossible for the statement
or expression to complete normally.)
I think in this case, we have hit #4,
it is both definitely assigned and
definitely unassigned (fun). Do
we need to treat this as
an unreachable statement? For
example, javac pukes on
the following:
 int i;
 while (true) {}
 i++;
tmp7.java:7: unreachable statement
 i++;
 ^
1 error
Is doing this definite assignment
analysis the only way we can
determine if the loop cannot exit?
Mo DeJong
Red Hat Inc