tree-optimization/106514 - add --param max-jump-thread-paths

The following adds a limit for the exponential greedy search of
the backwards jump threader.  The idea is to limit the search
space in a way that the paths considered are the same if the search
were in BFS order rather than DFS.  In particular it stops considering
incoming edges into a block if the product of the in-degrees of
blocks on the path exceeds the specified limit.

When considering the low stmt copying limit of 7 (or 1 in the size
optimize case) this means the degenerate case with maximum search
space is a sequence of conditions with no actual code

  B1
   |\
   | empty
   |/
  B2
   |\
   ...
  Bn
   |\

GIMPLE_CONDs are costed 2, an equivalent GIMPLE_SWITCH already 4, so
we reach 7 already with 3 middle conditions (B1 and Bn do not count).
The search space would be 2^4 == 16 to reach this.  The FSM threads
historically allowed for a thread length of 10 but is really looking
for a single multiway branch threaded across the backedge.  I've
chosen the default of the new parameter to 64 which effectively
limits the outdegree of the switch statement (the cases reaching the
backedge) to that number (divided by 2 until I add some special
pruning for FSM threads due to the loop header indegree).  The
testcase ssa-dom-thread-7.c requires 56 at the moment (as said,
some special FSM thread pruning of considered edges would bring
it down to half of that), but we now get one more threading
and quite some more in later threadfull.  This testcase seems to
be difficult to check for expected transforms.

The new testcases add the degenerate case we currently thread
(without deciding whether that's a good idea ...) plus one with
an approripate limit that should prevent the threading.

This obsoletes the mentioned --param max-fsm-thread-length but
I am not removing it as part of this patch.  When the search
space is limited the thread stmt size limit effectively provides
max-fsm-thread-length.

The param with its default does not help PR106514 enough to unleash
path searching with the higher FSM stmt count limit.

	PR tree-optimization/106514
	* params.opt (max-jump-thread-paths): New.
	* doc/invoke.texi (max-jump-thread-paths): Document.
	* tree-ssa-threadbackward.cc (back_threader::find_paths_to_names):
	Honor max-jump-thread-paths, take overall_path argument.
	(back_threader::find_paths): Pass 1 as initial overall_path.

	* gcc.dg/tree-ssa/ssa-thread-16.c: New testcase.
	* gcc.dg/tree-ssa/ssa-thread-17.c: Likewise.
	* gcc.dg/tree-ssa/ssa-dom-thread-7.c: Adjust.

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 92f7aaead74f10e8235a79016367d4dbe88f20eb..f01696696bf4b897bf5e53722f1af3484c661905 100644 (file)
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -14754,6 +14754,13 @@ optimizing.
 Maximum number of statements allowed in a block that needs to be
 duplicated when threading jumps.
 
+@item max-jump-thread-paths
+The maximum number of paths to consider when searching for jump threading
+opportunities.  When arriving at a block incoming edges are only considered
+if the number of paths to be searched sofar multiplied by the incoming
+edge degree does not exhaust the specified maximum number of paths to
+consider.
+
 @item max-fields-for-field-sensitive
 Maximum number of fields in a structure treated in
 a field sensitive manner during pointer analysis.

diff --git a/gcc/params.opt b/gcc/params.opt
index 2f9c9cf27dd22cdb4d575243f53c31e265a009b7..132987343c6c3cf9409d91a089444fd904f5c186 100644 (file)
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -582,6 +582,10 @@ Bound on the number of iterations the brute force # of iterations analysis algor
 Common Joined UInteger Var(param_max_jump_thread_duplication_stmts) Init(15) Param Optimization
 Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
 
+-param=max-jump-thread-paths=
+Common Joined UInteger Var(param_max_jump_thread_paths) Init(64) IntegerRange(1, 65536) Param Optimization
+Search space limit for the backwards jump threader.
+
 -param=max-last-value-rtl=
 Common Joined UInteger Var(param_max_last_value_rtl) Init(10000) Param Optimization
 The maximum number of RTL nodes that can be recorded as combiner's last value.

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
index aa06db5e22359c59801f234ad40ccff7f5384873..47b8fdfa29a585c68f398b7bcfe8f566be211ddf 100644 (file)
--- a/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c
@@ -11,7 +11,7 @@
    to change decisions in switch expansion which in turn can expose new
    jump threading opportunities.  Skip the later tests on aarch64.  */
 /* { dg-final { scan-tree-dump-not "Jumps threaded"  "dom3" { target { ! aarch64*-*-* } } } } */
-/* { dg-final { scan-tree-dump "Jumps threaded: 8"  "thread2" { target { ! aarch64*-*-* } } } } */
+/* { dg-final { scan-tree-dump "Jumps threaded: 9"  "thread2" { target { ! aarch64*-*-* } } } } */
 /* { dg-final { scan-tree-dump "Jumps threaded: 18"  "thread2" { target { aarch64*-*-* } } } } */
 
 enum STATE {

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-16.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-16.c
new file mode 100644 (file)
index 0000000..f96170b
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-16.c
@@ -0,0 +1,24 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-threadfull1-details" } */
+
+int res;
+void foo (int a, int b, int c, int d, int e)
+{
+  if (a > 100)
+    res = 3;
+  if (b != 5)
+    res = 5;
+  if (c == 29)
+    res = 7;
+  if (d < 2)
+    res = 9;
+  /* Accounting whoes makes this not catched.  */
+#if 0
+  if (e != 37)
+    res = 11;
+#endif
+  if (a < 10)
+    res = 13;
+}
+
+/* { dg-final { scan-tree-dump "SUCCESS" "threadfull1" } } */

diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-17.c b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-17.c
new file mode 100644 (file)
index 0000000..94ee666
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-17.c
@@ -0,0 +1,7 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-threadfull1-details --param max-jump-thread-paths=15" } */
+
+#include "ssa-thread-16.c"
+
+/* With limiting the search space we should no longer consider this path.  */
+/* { dg-final { scan-tree-dump-not "SUCCESS" "threadfull1" } } */

diff --git a/gcc/tree-ssa-threadbackward.cc b/gcc/tree-ssa-threadbackward.cc
index 332a1d2a1dd61fb372b13175ec69326867f11b78..a5f8f141071a74f95d98b96486a8171964720c16 100644 (file)
--- a/gcc/tree-ssa-threadbackward.cc
+++ b/gcc/tree-ssa-threadbackward.cc
@@ -90,7 +90,7 @@ private:
   bool debug_counter ();
   edge maybe_register_path ();
   void maybe_register_path_dump (edge taken_edge);
-  void find_paths_to_names (basic_block bb, bitmap imports);
+  void find_paths_to_names (basic_block bb, bitmap imports, unsigned);
   edge find_taken_edge (const vec<basic_block> &path);
   edge find_taken_edge_cond (const vec<basic_block> &path, gcond *);
   edge find_taken_edge_switch (const vec<basic_block> &path, gswitch *);
@@ -337,9 +337,12 @@ back_threader::find_taken_edge_cond (const vec<basic_block> &path,
 // INTERESTING bitmap, and register any such paths.
 //
 // BB is the current path being processed.
+//
+// OVERALL_PATHS is the search space up to this block
 
 void
-back_threader::find_paths_to_names (basic_block bb, bitmap interesting)
+back_threader::find_paths_to_names (basic_block bb, bitmap interesting,
+                                   unsigned overall_paths)
 {
   if (m_visited_bbs.add (bb))
     return;
@@ -352,8 +355,10 @@ back_threader::find_paths_to_names (basic_block bb, bitmap interesting)
          || maybe_register_path ()))
     ;
 
-  // Continue looking for ways to extend the path
-  else
+  // Continue looking for ways to extend the path but limit the
+  // search space along a branch
+  else if ((overall_paths = overall_paths * EDGE_COUNT (bb->preds))
+          <= (unsigned)param_max_jump_thread_paths)
     {
       // For further greedy searching we want to remove interesting
       // names defined in BB but add ones on the PHI edges for the
@@ -407,7 +412,7 @@ back_threader::find_paths_to_names (basic_block bb, bitmap interesting)
                        unwind.quick_push (def);
                      }
                }
-             find_paths_to_names (e->src, new_interesting);
+             find_paths_to_names (e->src, new_interesting, overall_paths);
              // Restore new_interesting.  We leave m_imports alone since
              // we do not prune defs in BB from it and separately keeping
              // track of which bits to unwind isn't worth the trouble.
@@ -417,6 +422,9 @@ back_threader::find_paths_to_names (basic_block bb, bitmap interesting)
            }
        }
     }
+  else if (dump_file && (dump_flags & TDF_DETAILS))
+    fprintf (dump_file, "  FAIL: Search space limit %d reached.\n",
+            param_max_jump_thread_paths);
 
   // Reset things to their original state.
   m_path.pop ();
@@ -447,7 +455,7 @@ back_threader::find_paths (basic_block bb, tree name)
 
       auto_bitmap interesting;
       bitmap_copy (interesting, m_imports);
-      find_paths_to_names (bb, interesting);
+      find_paths_to_names (bb, interesting, 1);
     }
 }