Re: Question re: SSA Aggressive Dead Code Elimination

[law at redhat dot com]

  In message <Pine.LNX.4.33.0106271319510.30643-100000@www.cgsoftware.com>you w
rite:
  > 
  > 
  > On Wed, 27 Jun 2001 law@redhat.com wrote:
  > 
  > Yeah yeah. I need to think more before writing emails so i don't look
  > stupid. It usually hits me about 2 minutes later that I was completely
  > wrong.
You're not the only one.  What had me worried was that in my current condition
(rather ill) that I'd missed something important.  After a night of sleep
things are a lot clearer :-)

  > As to which optimization is wrong, all the dominator optimization did
  > was eliminate a useless  computation, and reuse a register (in effect,
  > copy propping a=b+c into  b=b+c to get b=a), which should not change the
  > importance of the jump node.  It seems to have only because it
  > copy-propped into a phi node.
Right.  And I really don't want to cripple the dominator based optimizations
which appear to be a lot more effective than the aggressive dead code 
elimination (when compared to more traditional dead code elimination).


  > The only way this could happen that i can think of is if you weren't
  > following the control dependences in phi nodes in the dead code
  > elimination.
We follow the source regs in PHI nodes and mark their definition sites as
important.  But that's insufficient in this case as the definition sites
are both in block 0 due to the copy propagation.

As Geoff suggested, I think the way to go is mark the control dependent
edges associated with each alternative in the PHI nodes as important and
everything will just fall out.

Morgan's book doesn't have any special handling for PHI nodes in the
aggressive dead code elimination algorithm (nor does Muchnick or 
Appel have special handling of PHI nodes).

jeff

  > 
  > --Dan
  >