switch lowering: limit number of cluster attemps

gcc/ChangeLog:

	PR tree-optimization/96979
	* doc/invoke.texi: Document new param max-switch-clustering-attempts.
	* params.opt: Add new parameter.
	* tree-switch-conversion.c (jump_table_cluster::find_jump_tables):
	Limit number of attempts.
	(bit_test_cluster::find_bit_tests): Likewise.

gcc/testsuite/ChangeLog:

	PR tree-optimization/96979
	* g++.dg/tree-ssa/pr96979.C: New test.

4 files changed, 75 insertions, 0 deletions

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 665c0ff..6a7833b1 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -13452,6 +13452,10 @@ The smallest number of different values for which it is best to use a
 jump-table instead of a tree of conditional branches.  If the value is
 0, use the default for the machine.
 
+@item max-switch-clustering-attempts
+The maximum number of clustering attempts used
+in bit-test and jump-table switch expansion.
+
 @item jump-table-max-growth-ratio-for-size
 The maximum code size growth ratio when expanding
 into a jump table (in percent).  The parameter is used when
diff --git a/gcc/params.opt b/gcc/params.opt
index dcf5e02..5f2e11d 100644
--- a/gcc/params.opt
+++ b/gcc/params.opt
@@ -82,6 +82,10 @@ The maximum length of a constant string for a builtin string cmp call eligible f
 Common Joined UInteger Var(param_case_values_threshold) Param Optimization
 The smallest number of different values for which it is best to use a jump-table instead of a tree of conditional branches, if 0, use the default for the machine.
 
+-param=max-switch-clustering-attempts=
+Common Joined UInteger Var(param_max_switch_clustering_attempts) Param Optimization Init(10000)
+The maximum number of clustering attempts used in bit-test and jump-table switch expansion.
+
 -param=comdat-sharing-probability=
 Common Joined UInteger Var(param_comdat_sharing_probability) Init(20) Param Optimization
 Probability that COMDAT function will be shared with different compilation unit.
diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr96979.C b/gcc/testsuite/g++.dg/tree-ssa/pr96979.C
new file mode 100644
index 0000000..85c703a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/tree-ssa/pr96979.C
@@ -0,0 +1,50 @@
+/* PR tree-optimization/96979 */
+/* { dg-do compile } */
+/* { dg-options "-std=c++17 -O2 -fdump-tree-switchlower1" } */
+
+using u64 = unsigned long long;
+
+constexpr inline u64
+foo (const char *str) noexcept
+{
+  u64 value = 0xcbf29ce484222325ULL;
+  for (u64 i = 0; str[i]; i++)
+    value = (value ^ u64(str[i])) * 0x100000001b3ULL;
+  return value;
+}
+
+struct V
+{
+  enum W
+  {
+#define A(n) n,
+#define B(n) A(n##0) A(n##1) A(n##2) A(n##3) A(n##4) A(n##5) A(n##6) A(n##7) A(n##8) A(n##9)
+#define C(n) B(n##0) B(n##1) B(n##2) B(n##3) B(n##4) B(n##5) B(n##6) B(n##7) B(n##8) B(n##9)
+#define D(n) C(n##0) C(n##1) C(n##2) C(n##3) C(n##4) C(n##5) C(n##6) C(n##7) C(n##8) C(n##9)
+#define E D(foo1) D(foo2) D(foo3)
+    E
+    last
+  };
+
+  constexpr static W
+  bar (const u64 h) noexcept
+  {
+    switch (h)
+      {
+#undef A
+#define F(n) #n
+#define A(n) case foo (F(n)): return n;
+        E
+      }
+    return last;
+  }
+};
+
+int
+baz (const char *s)
+{
+  const u64 h = foo (s);
+  return V::bar (h);
+}
+
+/* { dg-final { scan-tree-dump-times ";; Bail out: --param=max-switch-clustering-attempts reached" 2 "switchlower1" } } */
diff --git a/gcc/tree-switch-conversion.c b/gcc/tree-switch-conversion.c
index 186411f..e6a2c7a 100644
--- a/gcc/tree-switch-conversion.c
+++ b/gcc/tree-switch-conversion.c
@@ -1183,6 +1183,7 @@ jump_table_cluster::find_jump_tables (vec<cluster *> &clusters)
 
   min.quick_push (min_cluster_item (0, 0, 0));
 
+  HOST_WIDE_INT attempts = 0;
   for (unsigned i = 1; i <= l; i++)
     {
       /* Set minimal # of clusters with i-th item to infinite.  */
@@ -1194,6 +1195,14 @@ jump_table_cluster::find_jump_tables (vec<cluster *> &clusters)
 	  if (i - j < case_values_threshold ())
 	    s += i - j;
 
+	  if (attempts++ == param_max_switch_clustering_attempts)
+	    {
+	      if (dump_file)
+		fprintf (dump_file, ";; Bail out: "
+			 "--param=max-switch-clustering-attempts reached\n");
+	      return clusters.copy ();
+	    }
+
 	  /* Prefer clusters with smaller number of numbers covered.  */
 	  if ((min[j].m_count + 1 < min[i].m_count
 	       || (min[j].m_count + 1 == min[i].m_count
@@ -1308,6 +1317,7 @@ bit_test_cluster::find_bit_tests (vec<cluster *> &clusters)
 
   min.quick_push (min_cluster_item (0, 0, 0));
 
+  HOST_WIDE_INT attempts = 0;
   for (unsigned i = 1; i <= l; i++)
     {
       /* Set minimal # of clusters with i-th item to infinite.  */
@@ -1315,6 +1325,13 @@ bit_test_cluster::find_bit_tests (vec<cluster *> &clusters)
 
       for (unsigned j = 0; j < i; j++)
 	{
+	  if (attempts++ == param_max_switch_clustering_attempts)
+	    {
+	      if (dump_file)
+		fprintf (dump_file, ";; Bail out: "
+			 "--param=max-switch-clustering-attempts reached\n");
+	      return clusters.copy ();
+	    }
 	  if (min[j].m_count + 1 < min[i].m_count
 	      && can_be_handled (clusters, j, i - 1))
 	    min[i] = min_cluster_item (min[j].m_count + 1, j, INT_MAX);