Re: [tree-ssa] Remove useless null pointer checks

[Jason Merrill <jason at redhat dot com>]

On Tue, 05 Aug 2003 10:35:37 -0600, law@redhat.com wrote:

> You can see the annoying typecast is gone.  However, you'll also note that
> the LHS of the two marked statements have different types.  One is a 
> void *, the other is a struct mem_attrs *.

Yep.  When I tried changing the type of expressions to remove casts, I ran
into problems with code that, for instance, assumes that &foo has type
typeof(foo) *.

> So the "trick" is to recognize typecasts which are not important from
> a code generation standpoint and enter the proper equivalences into 
> the const_and_copies table.

Yes, I think this is a better approach.

> So given a statement such as
>
> LHS = (TYPE) RHS;
>
> If the main variant of TYPE and RHS is the same, then enter LHS = RHS into
> the const_and_copies table.
>
> If TYPE and RHS's type are both pointers and one is a void *, then enter
> LHS = RHS into the const_and_copies table.

You should also handle casts between pointer and reference type; they are
equivalent outside the front end.

> For integral types, we can safely call STRIP_SIGN_NOPS to remove any
> casts which do not change the underlying mode or sign.
>
> There may be other "rules" we could use for typecasts which are safe to
> ignore in the gimplifier and tree-ssa optimizers. 
>
> Thoughts?

Could we go all the way and allow any cast which doesn't change mode or
sign?

Jason