Re: Help! PR18908 vs. PR23522 vs. Ada subtypes?

[Jeffrey A Law <law at redhat dot com>]

On Tue, 2006-03-28 at 10:52 -0700, Roger Sayle wrote:
> It's tree-ssa's structure sharing assumptions that screw this up.
> The new low-level fold_binary interface and the fact that fold
> (should) never modify its arguments means that ideally in the example
> above you could just do.
> 
> 	t = fold_binary (EQ/NE/GT/whatever, type, (T2)a, C);
> 	if (t != NULL_TREE)
> 	  ...  /* fold_binary improved things.  */
> 
> With this interface fold doesn't allocate any memory unless its
> discovers an beneficial transformation.
> No taxation without an improved representation! :)
> 
> 
> The fly in the ointment is that tree-ssa, unlike fold, destructively
> modifies trees assuming that they are never shared.  One solution
> might be to copy_node on the result returned from fold_binary above.
> This will ensure that any remaining/reused pieces of (T)a and C are
> correctly unshared, with only a minor memory wastage.  Indeed, in
> tree-combine, the original stmts are often known to be dead following
> a rewrite, so typically you shouldn't need the tree copy at all.
Hmmm....

It seems to me that in the case where fold_binary returns a
gimple tree that any sharing must inherently already be safe.

Even if we pass in a shared (T2)a the only usable result we get
back is always of the form

a COND C

ie, the cast must always disappear for the result to be useful.
The toplevel node is always a new node, so no sharing there.  The
only things shared would be "a" and "C", which is definitely safe.

Hmm......

Jeff