Re: Thoughts about semantics of BB_VISITED

[Diego Novillo <dnovillo at redhat dot com>]

Zdenek Dvorak wrote:

> it does not change them -- they must be clear on the entry, and they are
> clear on the exit.  Aand this is by far the simplest way.  On contrary,
> I believe that the optimization passes should not use these flags.
> While inside the shared utility there is at least some justification for
> it (it saves you passing bitmap to it and other trouble), I see no
> justification why for example PRE cannot use sbitmap instead of touching
> BB_VISITED.
Absolutely not.  With this approach you are merely muddling the 
interface to helper functions.  A helper function should not rely on 
implicit initialization of global data.  The BB_VISITED bit is a 
globally shared attribute, assuming anything about it inside a helper 
function is a recipe for confusion and hard-to-track bugs.

This is akin to helper functions assuming that TREE_VISITED is clear on 
entry.  Passes use TREE_VISITED in a variety of ways.  Helpers and, 
particularly, read-only helpers should not be modifying shared data.


Diego.