[gcc r13-2164] Speedup path discovery in predicate::use_cannot_happen

[Richard Biener]

https://gcc.gnu.org/g:b6864f4ee87fb56299fc82e1e0ced127b747d87a

commit r13-2164-gb6864f4ee87fb56299fc82e1e0ced127b747d87a
Author: Richard Biener <rguenther@suse.de>
Date:   Tue Aug 23 14:05:34 2022 +0200

    Speedup path discovery in predicate::use_cannot_happen
    
    The following reverts a hunk from r8-5789-g11ef0b22d68cd1 that
    made compute_control_dep_chain start from function entry rather
    than the immediate dominator of the source block of the edge with
    the undefined value on the PHI node.  Reverting at that point
    does not reveal any testsuite FAIL, in particular the added
    testcase still passes.  The following adjusts this to the other
    function that computes predicates that hold on the PHI incoming
    edges with undefined values, predicate::init_from_phi_def, which
    starts at the immediate dominator of the PHI.  That's much less
    likely to run into the CFG walking limit.
    
            * gimple-predicate-analysis.cc (predicate::use_cannot_happen):
            Start the compute_control_dep_chain walk from the immediate
            dominator of the PHI.

Diff:
---
 gcc/gimple-predicate-analysis.cc | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-predicate-analysis.cc b/gcc/gimple-predicate-analysis.cc
index a4291657d0b..ea81daacd4f 100644
--- a/gcc/gimple-predicate-analysis.cc
+++ b/gcc/gimple-predicate-analysis.cc
@@ -1293,6 +1293,12 @@ predicate::use_cannot_happen (gphi *phi, unsigned opnds)
       use_guard = &phi_use_guard_intersection;
     }
 
+  basic_block phi_bb = gimple_bb (phi);
+  /* Find the closest dominating bb to be the control dependence root.  */
+  basic_block cd_root = get_immediate_dominator (CDI_DOMINATORS, phi_bb);
+  if (!cd_root)
+    return false;
+
   /* Look for the control dependencies of all the interesting operands
      and build guard predicates describing them.  */
   unsigned n = gimple_phi_num_args (phi);
@@ -1308,7 +1314,7 @@ predicate::use_cannot_happen (gphi *phi, unsigned opnds)
       unsigned num_calls = 0;
 
       /* Build the control dependency chain for the PHI argument...  */
-      if (!compute_control_dep_chain (ENTRY_BLOCK_PTR_FOR_FN (cfun),
+      if (!compute_control_dep_chain (cd_root,
 				      e->src, dep_chains, &num_chains,
 				      cur_chain, &num_calls))
 	{