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Re: [tree-ssa] vdefs vs DCE
- From: law at redhat dot com
- To: Diego Novillo <dnovillo at redhat dot com>
- Cc: Andrew Macleod <amacleod at redhat dot com>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Date: Thu, 22 May 2003 12:21:24 -0600
- Subject: Re: [tree-ssa] vdefs vs DCE
- Reply-to: law at redhat dot com
In message <1053626175.6937.30.camel@frodo.toronto.redhat.com>, Diego Novillo w
rites:
>I fixed this particular idiocy with the new changes to remove
>INDIRECT_REFs as variables. In my local tree, I get this out of DCE:
>
>foo ()
>{
> (void)0;
> (void)0;
> (void)0;
> (void)0;
> (void)0;
> (void)0;
>
> bar ((char *)"fu", (char *)"bar", (char *)"com")
>}
Good.
>I think that the reason those T.x exist is because of the type casts
>that are going on.
Yes. Type casts cause us numerous problems. It's not just in argument
lists. They (*&@#$ us up in comparisons and just about everywhere else.
I suspect we're going to really need to re-think how we handle NOP
casting.
>It shouldn't be hard to have GIMPLE accept type
>casts in function arguments. Jason?
Well, I've already got a patch which does this. Or more correctly it allows
NOP casting in TREE_LISTs, which happens to be the biggest creator of
offending code like I noted above.
However, when I was looking at that, I realized that simpify_expr is doing
some really dumb things. For example, if you have something like a
NOP cast and your predicate is is_simple_val (which accepts NOP casts
on constants) simplify_expr still insists on mucking things up by
assigning the casted value to a new variable and using the new
variable whereever the cast originally was.
The root of this problem is that simplify_expr doesn't check the predicate
before simplifying. Ugh.
And we can't check the predicate before simplifying due to how we've
defined is_simple_stmt (always returns 1, which would prevent *any*
simplification).
Ideally we'd check that the operand fits the predicate and not simplify
which would make a lot of our casting issues go away. It might even
speed up the compiler (or it may slow it down -- I don't really know).
Arrggh.
Jeff