[gcc r13-1998] tree-optimization/106514 - add --param max-jump-thread-paths

[gcc r13-1998] tree-optimization/106514 - add --param max-jump-thread-paths

[Richard Biener]

https://gcc.gnu.org/g:409978d58dafa689c5b3f85013e2786526160f2c

commit r13-1998-g409978d58dafa689c5b3f85013e2786526160f2c
Author: Richard Biener <rguenther@suse.de>
Date:   Mon Aug 8 12:20:04 2022 +0200

    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:
---
 gcc/doc/invoke.texi                              |  7 +++++++
 gcc/params.opt                                   |  4 ++++
 gcc/testsuite/gcc.dg/tree-ssa/ssa-dom-thread-7.c |  2 +-
 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-16.c    | 24 ++++++++++++++++++++++++
 gcc/testsuite/gcc.dg/tree-ssa/ssa-thread-17.c    |  7 +++++++
 gcc/tree-ssa-threadbackward.cc                   | 20 ++++++++++++++------
 6 files changed, 57 insertions(+), 7 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 92f7aaead74..f01696696bf 100644
--- 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 2f9c9cf27dd..132987343c6 100644
--- 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 aa06db5e223..47b8fdfa29a 100644
--- 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
index 00000000000..f96170b073d
--- /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
index 00000000000..94ee6666788
--- /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 332a1d2a1dd..a5f8f141071 100644
--- 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);
     }
 }