On 10/26/07, J.C. Pizarro <[EMAIL PROTECTED]> wrote:
> What is the matter if the 'b' var. is unused and
> optimally removed by the SSA algorithm?
In this case, it will not be removed. If any of the p_i pointers is
ever dereferenced in this code, that will be considered a use of
variable 'b'.
> int a;
> int b;
>
> a = 2;
> p_4 = phi(a)
Is this 'phi' as in a PHI function or a function in your code? If the
former, then it's wrong, you can never have such a phi function in
this code snippet.
> // b doesn't used here
> if (...)
> p_1 = &a;
> else
> p_2 = &b;
> endif
> p_3 = phi (p_1, p_2)
>
> points-to (p_1) = { a }
> points-to (p_2) = { b }
> points-to (p_3) = { a b }
>
> In this case, should exist hidden p_5 = phi(b) although 'b' is not used
> but yes used his reference to phantom cell 'b'. It's weird for me.
I recommend that you read about the SSA form. PHI nodes are special
constructs that exist only where multiple control flow paths reach a
common join node. The getting started section of the wiki has links
to books and articles about it. Morgan's book on compiler
optimization is fairly good.
> I've not idea WHERE put "hidden p_5 = phi(b)"!
No such thing exists.
> Too it's possible to ocurr *p_2 = c where 'b' will be hidden used through
> the pointer p_2. It's too weird for me.
Yes, that is possible, an that is precisely what alias analysis tells
the compiler. We know from the analysis that reading/writing to
'*p_2' is exactly the same as reading/writing to 'b'.
