Improve compute_control_dep_chain documentation

The following refactors compute_control_dep_chain slightly by
inlining is_loop_exit and factoring the check on the loop
invariant condition.  It also adds a comment as of how I
understand the code and it's current problem.

	* gimple-predicate-analysis.cc (compute_control_dep_chain):
	Inline is_loop_exit and refactor, add comment about
	loop exits.

diff --git a/gcc/gimple-predicate-analysis.cc b/gcc/gimple-predicate-analysis.cc
index 32542f93057142b3a59f11cd127ff0452edc1292..934e9516e7b29b0d99d5fde4758666e32afaac51 100644 (file)
--- a/gcc/gimple-predicate-analysis.cc
+++ b/gcc/gimple-predicate-analysis.cc
@@ -1110,6 +1110,10 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
                           vec<edge> &cur_cd_chain, unsigned *num_calls,
                           unsigned in_region = 0, unsigned depth = 0)
 {
+  /* In our recursive calls this doesn't happen.  */
+  if (single_succ_p (dom_bb))
+    return false;
+
   if (*num_calls > (unsigned)param_uninit_control_dep_attempts)
     {
       if (dump_file)
@@ -1167,7 +1171,21 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
       basic_block cd_bb = e->dest;
       cur_cd_chain.safe_push (e);
       while (!dominated_by_p (CDI_POST_DOMINATORS, dom_bb, cd_bb)
-            || is_loop_exit (dom_bb, cd_bb))
+            /* We want to stop when the CFG merges back from the
+               branch in dom_bb.  The post-dominance check alone
+               falls foul of the case of a loop exit test branch
+               where the path on the loop exit post-dominates
+               the branch block.
+               The following catches this but will not allow
+               exploring the post-dom path further.  For the
+               outermost recursion this means we will fail to
+               reach dep_bb while for others it means at least
+               dropping the loop exit predicate from the path
+               which is problematic as it increases the domain
+               spanned by the resulting predicate.
+               See gcc.dg/uninit-pred-11.c for the first case
+               and PR106754 for the second.  */
+            || single_pred_p (cd_bb))
        {
          if (cd_bb == dep_bb)
            {
@@ -1187,9 +1205,10 @@ compute_control_dep_chain (basic_block dom_bb, const_basic_block dep_bb,
            break;
 
          /* Check if DEP_BB is indirectly control-dependent on DOM_BB.  */
-         if (compute_control_dep_chain (cd_bb, dep_bb, cd_chains,
-                                        num_chains, cur_cd_chain,
-                                        num_calls, in_region, depth + 1))
+         if (!single_succ_p (cd_bb)
+             && compute_control_dep_chain (cd_bb, dep_bb, cd_chains,
+                                           num_chains, cur_cd_chain,
+                                           num_calls, in_region, depth + 1))
            {
              found_cd_chain = true;
              break;