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

[Daniel Berlin <dberlin@dberlin.org>]

On Oct 20, 2004, at 1:33 PM, Diego Novillo wrote:

> 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).
>
Right.

> 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.

Okay.


>
> 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.

I'll implement this and repost the patch.

I assume nobody really had much problem with the *other* parts of the 
patch?

> Diego.
>