[tree-ssa]: The dangers of keeping constants in phi nodes

[Daniel Berlin <dberlin at dberlin dot org>]

If you rewrite a var which had a constant propagated into a phi node, 
it loses the phi node value (IE it never unpropagates it temporarily, 
or remembers it was there), and thus, you lose the initialization.

There is no warning that this occurred, and unless you noticed it 
otherwise, it's not easy to catch.

This is clearly a bug.

Try the following code, and mark I and J for renaming again at some 
point after dom opts.
You'll see that it I and J will no longer be initialized to 0 anymore, 
and the output code won't run properly.

int main(void)
{
        int I, J;
        const int N = 30;
        const int M = 40;
        for (J = 0; J < N; J++)
        {
                for (I = 0; I < M; I++)
                {
                        printf ("%d %d\n", I, J);
                }
        }
}

I'm not sure what the right solution is here, but the current 
situation, expecting clients to know that variables they rewrite might 
be set to a constant in some phi node somewhere, and thus, you need to 
deal with that, seems wrong.
rewrite_into_ssa needs to notice this, and take care of it.

--Dan