switch conversion: make a rapid speed up

gcc/ChangeLog:

	PR tree-optimization/96979
	* tree-switch-conversion.c (jump_table_cluster::can_be_handled):
	Make a fast bail out.
	(bit_test_cluster::can_be_handled): Likewise here.
	* tree-switch-conversion.h (get_range): Use wi::to_wide instead
	of a folding.

gcc/testsuite/ChangeLog:

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

gcc/testsuite/g++.dg/tree-ssa/pr96979.C

@@ -0,0 +1,48 @@
+/* PR tree-optimization/96979 */
+/* { dg-do compile } */
+/* { dg-options "-std=c++17 -O2" } */
+
+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);
+}

gcc/tree-switch-conversion.c

@@ -1268,6 +1268,18 @@ jump_table_cluster::can_be_handled (const vec<cluster *> &clusters,
   if (range == 0)
     return false;
 
+  if (range > HOST_WIDE_INT_M1U / 100)
+    return false;
+
+  unsigned HOST_WIDE_INT lhs = 100 * range;
+  if (lhs < range)
+    return false;
+
+  /* First make quick guess as each cluster
+     can add at maximum 2 to the comparison_count.  */
+  if (lhs > 2 * max_ratio * (end - start + 1))
+    return false;
+
   unsigned HOST_WIDE_INT comparison_count = 0;
   for (unsigned i = start; i <= end; i++)
     {
@@ -1275,10 +1287,6 @@ jump_table_cluster::can_be_handled (const vec<cluster *> &clusters,
       comparison_count += sc->m_range_p ? 2 : 1;
     }
 
-  unsigned HOST_WIDE_INT lhs = 100 * range;
-  if (lhs < range)
-    return false;
-
   return lhs <= max_ratio * comparison_count;
 }
 
@@ -1364,12 +1372,12 @@ bit_test_cluster::can_be_handled (unsigned HOST_WIDE_INT range,
 {
   /* Check overflow.  */
   if (range == 0)
-    return 0;
+    return false;
 
   if (range >= GET_MODE_BITSIZE (word_mode))
     return false;
 
-  return uniq <= 3;
+  return uniq <= m_max_case_bit_tests;
 }
 
 /* Return true when cluster starting at START and ending at END (inclusive)
@@ -1379,6 +1387,7 @@ bool
 bit_test_cluster::can_be_handled (const vec<cluster *> &clusters,
 				  unsigned start, unsigned end)
 {
+  auto_vec<int, m_max_case_bit_tests> dest_bbs;
   /* For algorithm correctness, bit test for a single case must return
      true.  We bail out in is_beneficial if it's called just for
      a single case.  */
@@ -1387,15 +1396,25 @@ bit_test_cluster::can_be_handled (const vec<cluster *> &clusters,
 
   unsigned HOST_WIDE_INT range = get_range (clusters[start]->get_low (),
 					    clusters[end]->get_high ());
-  auto_bitmap dest_bbs;
+
+  /* Make a guess first.  */
+  if (!can_be_handled (range, m_max_case_bit_tests))
+    return false;
 
   for (unsigned i = start; i <= end; i++)
     {
       simple_cluster *sc = static_cast<simple_cluster *> (clusters[i]);
-      bitmap_set_bit (dest_bbs, sc->m_case_bb->index);
+      /* m_max_case_bit_tests is very small integer, thus the operation
+	 is constant. */
+      if (!dest_bbs.contains (sc->m_case_bb->index))
+	{
+	  if (dest_bbs.length () >= m_max_case_bit_tests)
+	    return false;
+	  dest_bbs.quick_push (sc->m_case_bb->index);
+	}
     }
 
-  return can_be_handled (range, bitmap_count_bits (dest_bbs));
+  return true;
 }
 
 /* Return true when COUNT of cases of UNIQ labels is beneficial for bit test

gcc/tree-switch-conversion.h

@@ -84,11 +84,10 @@ public:
      then return 0.  */
   static unsigned HOST_WIDE_INT get_range (tree low, tree high)
   {
-    tree r = fold_build2 (MINUS_EXPR, TREE_TYPE (low), high, low);
-    if (!tree_fits_uhwi_p (r))
+    wide_int w = wi::to_wide (high) - wi::to_wide (low);
+    if (wi::neg_p (w, TYPE_SIGN (TREE_TYPE (low))) || !wi::fits_uhwi_p (w))
       return 0;
-
-    return tree_to_uhwi (r) + 1;
+    return w.to_uhwi () + 1;
   }
 
   /* Case label.  */