rtl-ssa: Maintain clobber_group invariant [PR121757]

In order to reduce time complexity, rtl-ssa groups consecutive
clobbers together.  Each group of clobbers has a splay tree for
lookup and manipulation purposes.

This arrangement means that we might need to split a group (when
inserting a new non-clobber definition between two clobbers) or
to join consecutive groups together (when deleting an intervening
non-clobber definition).  To reduce the time complexity of these updates,
the back pointer from a clobber to its group is only updated lazily.
The invariant is supposed to be that the first clobber, last clobber,
and splay tree root have the right group at all times, whereas other
members of the group can have identifiably stale group pointers.

However, a lack of abstraction meant that only some splay tree lookups
correctly maintained this invariant.  Others did not update the group
pointer after installing a new root.

This patch adds a helper that maintains the invariant and uses it in
three places, one that was already correct and two that were wrong.
The original lookup_clobber is still used in other code that
manipulates groups as a whole.

gcc/
	PR rtl-optimization/121757
	* rtl-ssa/accesses.h (clobber_group::lookup_clobber): New member
	function.
	* rtl-ssa/accesses.cc (clobber_group::lookup_clobber): Likewise.
	(clobber_group::prev_clobber, clobber_group::next_clobber)
	(function_info::add_clobber): Use it.

gcc/testsuite/
	PR rtl-optimization/121757
	* g++.dg/pr121757.C: New test.

0 files changed, 0 insertions, 0 deletions