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Re: [tree-ssa] Too conservative optimization with globalvariables??
- From: Diego Novillo <dnovillo at redhat dot com>
- To: Steven Bosscher <s dot bosscher at student dot tudelft dot nl>
- Cc: "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: 16 Jun 2003 07:11:45 -0400
- Subject: Re: [tree-ssa] Too conservative optimization with globalvariables??
- Organization: Red Hat Canada
- References: <3EEDA327.2070402@student.tudelft.nl>
On Mon, 2003-06-16 at 06:59, Steven Bosscher wrote:
> extern int c;
>
> fubar()
> {
> int T.1;
> int a;
> int b;
>
> c = 3;
> c = c * 2;
> }
>
> Why can't this just be folded to "c = 6;"?
>
Because we do not rename global nor local static variables anymore.
This is to avoid problems in the SSA->normal pass. If two versions of
the same global were live at the same time, we would need to have
copy-in/copy-out operations at entry/exit points to update the original
object.
Adding copy-in/copy-out ops are somewhere in the todo list, but it's not
a priority yet. Inserting those operations blindly seems expensive.
Diego.