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Re: [tree-ssa] BIND_EXPR removal
On Sun, 2 Nov 2003 11:49:29 -0800, Richard Henderson <firstname.lastname@example.org> wrote:
> On Sun, Nov 02, 2003 at 12:29:12PM +0100, Zdenek Dvorak wrote:
>> OK, the second attempt. It just records the variables in the gimple
>> lowering phase, then expands those of them that were not optimized
>> out afterwards (the code to determine this is taken from
>> remove_unused_... in tree-cfg.c, so it should do exactly what we do now).
> This seems ok for now.
> About the only thing I'd change is to *not* keep the outermost
> BIND_EXPR of the function, as you do. As far as I can tell, the
> only difference this would make is that gimple_add_tmp_var would
> add the variable to your list instead of either queue it in
> gimplify_ctxp->temps or add it to that now missing bind_expr.
Hmm, I was just about to say the opposite--that we should be moving all the
variables to the outer BIND_EXPR rather than to some random global
variable. Of course, to do that we'd need to support a TREE_LIST for
BIND_EXPR_VARS, but that shouldn't be difficult.
> For the to-look-at-eventually list:
> I've been wondering if we might be able to get better results
> wrt aggregates if we associated basic blocks with scopes, and
> constrained cfg operations to preserve that invariant. We might
> be able to do reasoning such as noticing that
> void f ()
> int x;
> h ();
> We may tail-call to H because while we did leak local stack for
> X into the global memory pool, it is no longer in scope, and so
> a conforming program could not still access it.
Perhaps we could express this by inserting a new statement to express that
X is dead? There are certainly other optimizations which could benefit
from knowing that they don't have to worry about some variables. This
could also apply to dynamically freed/destroyed objects, not just objects
which have gone out of scope.