[ tree-ssa] PRE problem

[law@redhat.com]

In message <1077321774.3219.22.camel@localhost.localdomain>, Diego Novillo writ
es:
 >On Fri, 2004-02-20 at 18:48, Daniel Berlin wrote:
 >
 >> Um, Diego, when do you think that code was written?
 >> Last week?
 >> It hasn't been fundamentally touched in ages (I believe someone touched 
 >> the generate_vops part when we moved to having VUSE_OPS_PTR).
 >> 
 >Dan, what is happening here is that we are only *now* discovering that
 >the changes made to the copy propagation of pointers were affecting
 >PRE. 
True.  Basically I suspect this could be summed up as "we weren't aware
that PRE did this kind of thing internally.  If we were aware we certainly
would have taken some action to fix it."  It's somewhat ironic that
Diego and I apparently were poking at this independently.  Diego to
fix a bugzilla bug, and me to fix regressions introduced when improvements
in the dominator optimizer exposed more interesting things for the 
PRE code to do.

Though this does bring up a documentation issue -- I don't think we
discuss these issues anywhere in the documentation (source or external).

I wouldn't be terribly upset if the various propagate_blah
may_propagate_blah, and the inlined updates of annotations in create_temp
all went into their own tiny little file which dealt strictly with
const/copy propagation issues.  We'd also then have a nice place to
stick a block comment about the issues, particularly the assumptions
regarding copies & memory tags.


 >When I introduced propagate_copy and may_propagate_copy, I simply didn't
 >run into this problem.  The fix is simple, just teach PRE to use the
 >value propagation API.
Given that we've got two fixes for these issues, let's get one tested
and installed and get on with our lives.
jeff