Re: middle-end/9997: Coelesce stack slots for disjoint scopes.

[Jim Wilson <wilson@tuliptree.org>]

The following reply was made to PR middle-end/9997; it has been noted by GNATS.

From: Jim Wilson <wilson@tuliptree.org>
To: gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: middle-end/9997: Coelesce stack slots for disjoint scopes.
Date: Sun, 09 Mar 2003 10:52:33 -0500

 I believe we have tried this in the past, and ran into aliasing
 problems.  Suppose you have two variables of different types defined in
 different scopes.  Suppose the scheduler reorders instructions causing
 the scopes to overlap.  Now you do an alias test, and the alias code
 says that two references can't possibly alias because they have
 different types, but they do, because they were allocated to the same
 stack slot.  You can't rely on a test for the same address, because we
 may have lost address info in the RTL.
 
 We have different and better alias code now, so perhaps there is a way
 to address this.  Maybe if we add scope nesting level info to the alias
 info, and then assume that locals alias if they are in different scopes
 with a common parent, and neither is a parent of the other.
 
 Jim