Re: [patch] tree-ssa-sink.c: Don't restrict immediate uses that weconsider.

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

On Thu, 2005-04-07 at 10:37 -0400, Kazu Hirata wrote:
> Hi,
> 
> Attached is a patch to teach nearest_common_dominator_of_uses not to
> restrict the set of immediate uses that we consider.
> 
> nearest_common_dominator_of_uses goes through immediate uses to find a
> location to sink a statement into.  It's important that we consider
> all uses so that whatever location we come with dominates all uses.
> It's dangerous to have an "if" statement to restrict the set of
> immediate uses that we consider like so
> 
>   if (PHI_ARG_DEF (usestmt, idx) == var)
> 
> because the location that we come up with may not dominate all uses.
> 
> The patch simply removes the "if" statement.
> 
> Having said this, if an SSA_NAME is used in a PHI argument, it must be
> used there naked.  It cannot possibly occur as a part of an ADDR_EXPR
> because that wouldn't be TREE_INVARIANT, so I don't think removal of
> this "if" statement matters.  However, if we allow an ADDR_EXPR that
> is not TREE_INVARIANT in a PHI argument in future, then this would
> become a problem (although I don't think such an extension will
> actually happen).
> 
> Tested on i686-pc-linux-gnu.  OK to apply?
> 
> Kazu Hirata
> 
> 2005-04-07  Kazu Hirata  <kazu@cs.umass.edu>
> 
> 	* tree-ssa-sink.c (nearest_common_dominator_of_uses): Consider
> 	all immediate uses in PHI nodes.
It seems to me that you're really just removing a test that should
always be true.  Right?  ie, if we have an immediate use which points
to a phi argument, then well, the object in question better appear in
that phi argument, or else something horribly wrong has happened.

Thus we really might consider either checking this as part of our 
standard IL verification (who knows, with Andrew's work we might already
have this check) or we can just turn the IF statement into a gcc_assert.

Thoughts?

jeff