Re: [PR tree-optimization/20460] add phi args to dests ofdce-redirected edges

[Jeffrey A Law <law at redhat dot com>]

On Wed, 2005-03-30 at 02:56 -0300, Alexandre Oliva wrote:
> When remove_dead_stmt() redirects a control stmt, the edge redirection
> reserves space for the phi arg for the new incoming edge in all phi
> nodes, but, instead of filling them in with information obtained from
> the edge redirection, we simply discard this information.  This leaves
> NULL in the phi args, which may cause crashes later on.
> 
> This patch fixes the problem by filling in the phi args using the
> PENDING_STMT list created during edge redirection.  This appears to be
> the intended use for this information, and it is used similarly in
> e.g. loop unrolling.
> 
> Bootstrapping mainline and 4.0 branch on amd64-linux-gnu, and mainline
> on i686-pc-linux-gnu.  Ok to install if bootstrap and regtesting pass?
> 
> The patch below is for the 4.0 branch, but it applies cleanly and
> correctly in mainline as well, since it's just a few lines off.


       /* Redirect the first edge out of BB to reach POST_DOM_BB.  */
       redirect_edge_and_branch (EDGE_SUCC (bb, 0), post_dom_bb);
-      PENDING_STMT (EDGE_SUCC (bb, 0)) = NULL;
+      flush_pending_stmts (EDGE_SUCC (bb, 0));



I'm having trouble seeing how this can be correct.

AFAICT this assumes that EDGE_SUCC (bb, 0)->dest before the redirection
has similar PHI as post_dom_bb and that the PHIs appear in the same
order in both blocks.  I'm not sure you can make that assumption.


This code is triggered rarely, I would expect it to be even rarer still
for POST_DOM_BB to have PHI nodes.  You could probably just ignore dead
control statements where the post dominator has PHI nodes and I doubt
it would ever be noticed.

Jeff