RFA: tree DSE bug

[Jeffrey A Law]

On Thu, 2005-11-10 at 17:45 -0500, Daniel Jacobowitz wrote:
> On Thu, Nov 10, 2005 at 03:43:20PM -0700, Jeffrey A Law wrote:
> > On Thu, 2005-11-10 at 17:23 -0500, Daniel Jacobowitz wrote:
> > > > So a potentially simpler solution may be to not allow walking through
> > > > a PHI when the argument is associated with a back edge in the CFG.
> > > 
> > > This I don't know enough to have an opinion on :-)  But it seems like
> > > it would be unduly pessimal.  For instance, if the array subscript in
> > > my testcase is changed to be a loop invariant, this would prevent the
> > > dead store elimination, even though it would be valid.
> > True, but I really doubt it matters.  DSE doesn't catch a lot of stuff
> > as it is (we've discussed removing it on this list in the past, which
> > I've argued against :-).
> > 
> > I'd wager there isn't a single real world instance where it would miss
> > an optimization due to such a change.
> 
> Would you like me to test this change instead?
I cobbled it together real quick this morning, we take a .5% compile
time hit because we have to mark the backedges in the CFG.  That
was my other concern when I looked at the dumps yesterday.

While I think the CFG solution is better and cleaner, I don't think it's
*that* much cleaner.

jeff