Re: [PATCH]: Implement must-def kill operand

[Diego Novillo <dnovillo at redhat dot com>]

On Sun, 2004-10-17 at 22:00, Daniel Berlin wrote:

> This is a revision of the previous patch, the main change is that it no 
> longer requires DCE to go into and out of SSA to rename the virtual 
> must-def operands, as Andrew requested. The renaming it does 
> in DCE is just the 3rd phase of the regular renamer (rewriting reaching 
> definitions),
>
I would rather parameterize the renamer to deal with this situation. 
Just to make sure I understood the problem and the IRC discussion.  Here
we have the following situation:

           # A_4 = V_MUST_DEF <A_3>
        1. A = ...;
        
           # A_5 = V_MUST_DEF <A_4>
        2. A = ...;

A_4 is dead because no statement uses A_4 (the RHS of a V_MUST_DEF is
not a use, it only specifies which version is this V_MUST_DEF killing). 
Once we remove statement (1), we need to tell statement (2) that it no
longer kills A_4, it now kills whichever version was left dominating it
(it may or may not be A_3).

What we need here is a regular renaming pass, but not all of it.  We
only need to (a) detect def sites, (b) walk the dominator the dominator
tree rewriting the operands of V_MUST_DEFs with CURRENT_DEF.  That is,
we need to skip the PHI insertion phase.

So, I guess this involves a new TODO_* flag (TODO_fix_def_def_chains?)
which does (a) and (b) using the existing renamer.  The additional logic
in the renamer should not slow it down one bit, nor make it more
complicated to follow.


Diego.