Fix intersect of modular set (#2904)

Fix comment bugs and code style

src/arithmetic/modular_set.cc

@@ -36,6 +36,18 @@ struct ModularSetAnalyzer::Entry {
   int64_t coeff{1};
   int64_t base{0};
 
+  Entry() = default;
+
+  Entry(int64_t coeff, int64_t base) {
+    CHECK_GE(coeff, 0);
+    this->coeff = coeff;
+    if (coeff != 0) {
+      base = base % coeff;
+      if (base < 0) base += coeff;
+    }
+    this->base = base;
+  }
+
   bool is_const() const {
     return coeff == 0;
   }
@@ -53,10 +65,7 @@ class ModularSetAnalyzer::Impl :
     if (!override) {
       CHECK(!var_map_.count(var));
     }
-    Entry e;
-    e.coeff = info->coeff;
-    e.base = info->base;
-    var_map_[var] = e;
+    var_map_[var] = Entry(info->coeff, info->base);
   }
 
   // Detect useful constraints and use them in the analysis scope.
@@ -65,9 +74,7 @@ class ModularSetAnalyzer::Impl :
     PVar<Integer> coeff, base;
     // pattern match interesting constraints
     if (((var % coeff) == base).Match(constraint)) {
-      Entry entry;
-      entry.coeff = coeff.Eval()->value;
-      entry.base = base.Eval()->value;
+      Entry entry(coeff.Eval()->value, base.Eval()->value);
       return UpdateByIntersect(var.Eval(), entry);
     }
     return nullptr;
@@ -83,18 +90,12 @@ class ModularSetAnalyzer::Impl :
   }
 
   Entry VisitExpr_(const IntImm* op) final {
-    Entry ret;
-    ret.base = op->value;
-    ret.coeff = 0;
-    return ret;
+    return Entry(0, op->value);
   }
 
   Entry VisitExpr_(const UIntImm* op) final {
     if (op->value < std::numeric_limits<int64_t>::max()) {
-      Entry ret;
-      ret.base = static_cast<int>(op->value);
-      ret.coeff = 0;
-      return ret;
+      return Entry(0, static_cast<int>(op->value));
     } else {
       return Everything();
     }
@@ -103,19 +104,15 @@ class ModularSetAnalyzer::Impl :
   Entry VisitExpr_(const Add* op) final {
     Entry a = VisitExpr(op->a);
     Entry b = VisitExpr(op->b);
-    Entry ret;
-    ret.coeff = ZeroAwareGCD(a.coeff, b.coeff);
-    ret.base = BaseSimplify(a.base + b.base, ret.coeff);
-    return ret;
+    int64_t coeff = ZeroAwareGCD(a.coeff, b.coeff);
+    return Entry(coeff, a.base + b.base);
   }
 
   Entry VisitExpr_(const Sub* op) final {
     Entry a = VisitExpr(op->a);
     Entry b = VisitExpr(op->b);
-    Entry ret;
-    ret.coeff = ZeroAwareGCD(a.coeff, b.coeff);
-    ret.base = BaseSimplify(a.base - b.base, ret.coeff);
-    return ret;
+    int64_t coeff = ZeroAwareGCD(a.coeff, b.coeff);
+    return Entry(coeff, a.base - b.base);
   }
 
   Entry VisitExpr_(const Mul* op) final {
@@ -128,10 +125,8 @@ class ModularSetAnalyzer::Impl :
     int64_t pq = a.coeff * b.coeff;
     int64_t pm = a.coeff * b.base;
     int64_t qn = a.base * b.coeff;
-    Entry ret;
-    ret.coeff = ZeroAwareGCD(pq, ZeroAwareGCD(pm, qn));
-    ret.base = BaseSimplify(a.base * b.base, ret.coeff);
-    return ret;
+    int64_t coeff = ZeroAwareGCD(pq, ZeroAwareGCD(pm, qn));
+    return Entry(coeff, a.base * b.base);
   }
 
   Entry DivByConst(const Expr& lhs,
@@ -140,20 +135,15 @@ class ModularSetAnalyzer::Impl :
     Entry a = VisitExpr(lhs);
     CHECK_NE(val, 0);
     if (a.coeff % val == 0) {
-      Entry ret;
       if (a.base == 0) {
         // a c x  / c -> a x
-        ret.coeff = std::abs(a.coeff / val);
-        ret.base = 0;
-        return ret;
+        return Entry(std::abs(a.coeff / val), 0);
       }
       // positive division have a clear rounding mode.
       // Only handle case where we clearly know we need to round down.
       if (a.base > 0 && val > 0 &&
           (round_down || parent_->CanProveGreaterEqual(lhs, 0))) {
-        ret.coeff = a.coeff / val;
-        ret.base = a.base / val;
-        return ret;
+        return Entry(a.coeff / val, a.base / val);
       }
     }
     return Everything();
@@ -251,41 +241,80 @@ class ModularSetAnalyzer::Impl :
     }
     int64_t base0 = a.base % coeff;
     int64_t base1 = b.base % coeff;
-    Entry ret;
     if (base0 == base1) {
-      ret.coeff = coeff;
-      ret.base = base0;
-      return ret;
+      return Entry(coeff, base0);
     } else {
-      ret.coeff = ZeroAwareGCD(ZeroAwareGCD(base0, base1), coeff);
-      ret.base = 0;
-      return ret;
+      return Entry(ZeroAwareGCD(ZeroAwareGCD(base0, base1), coeff), base0);
     }
   }
   /*!
+   * \brief Use Extended Euclidean algorithm to solve ax + by = gcd(a, b)
+   * \param a The first coefficient.
+   * \param b The second coefficient.
+   * \param x The solution of x.
+   * \param y The solution of y.
+   * \return The GCD of a and b.
+   */
+  static int64_t ExtendedEuclidean(int64_t a, int64_t b, int64_t* x, int64_t* y) {
+    // Extended Euclidean algorithm
+    // if a < 0, the problem can be convert into
+    // |a|* (-x) + b * y = gcd(|a|, b)
+    //
+    // initial condition:
+    // a * 0 + b * 1 = b
+    // a * 1 + b * 0 = a
+    int64_t s = 0, old_s = 1;
+    int64_t r = b, old_r = a >= 0 ? a : -a;
+    // Iteration (r2 < r1):
+    // a * x1 + b * y1 = r1
+    // a * x2 + b * y2 = r2
+    // The above two eqs can derive the following eq (q = r1 / r2)
+    // a * (x1 - x2 * q) + b * (y1 - y2 * q) = r1 - r2 * q = r3
+    // Because r3 < r2, the iteration can eventually terminate
+    while (r != 0) {
+      int64_t q = old_r / r;
+      int64_t tmp = old_r;
+      old_r = r;
+      r = tmp - q * r;
+      tmp = old_s;
+      old_s = s;
+      s = tmp - q * s;
+    }
+
+    *x = a >= 0 ? old_s : -old_s;
+    if (b != 0) {
+      *y = (old_r - (*x) * a) / b;
+    } else {
+      *y = 1;
+    }
+
+    return old_r;
+  }
+  /*!
    * \brief Create interect of two sets.
    * \param a The left operand.
    * \param b the right operand.
    */
   static Entry Intersect(Entry a, Entry b) {
-    // simple rule for now: pick higher constraints.
-    // TODO(team-team): Use extended euclidean algorithm.
-    if (a.coeff == 0) return a;
-    if (b.coeff == 0) return b;
-    if (a.coeff >= b.coeff) return a;
-    return b;
-  }
-  /*!
-   * \brief Simplify base so that it is in [0, coeff) when coeff != 0.
-   * \param base The base value.
-   * \param coeff The coeff value.
-   * \return The simplified base.
-   */
-  static int64_t BaseSimplify(int64_t base, int64_t coeff) {
-    if (coeff == 0) return base;
-    base = base % coeff;
-    if (base < 0) base += coeff;
-    return base;
+    int64_t x, y;
+    int64_t c1 = a.coeff, b1 = a.base, c2 = b.coeff, b2 = b.base;
+    // z = c1 * p + b1
+    // z = c2 * q + b2
+    // c1 * x + c2 * y = gcd(c1, c2)
+    // -> c1 * p - c2 * q = b2 - b1
+    // -> p = (b2 - b1) / gcd * x
+    // -> q = (b2 - b1) / gcd * (-y)
+    // -> z = LCM(x, y) * k + (c1 * p + b1)
+    int64_t gcd = ExtendedEuclidean(c1, c2, &x, &y);
+    int64_t v = b2 - b1;
+    if (v % gcd == 0) {
+      x = v / gcd * x;
+      y = v / gcd * (-y);
+      int64_t coeff = c1 / gcd * c2;
+      return Entry(coeff, x * c1 + b1);
+    } else {
+      return Nothing();
+    }
   }
   /*!
    * \brief Take GCD of a and b.
@@ -311,9 +340,14 @@ class ModularSetAnalyzer::Impl :
    * \return Bound that represent everything dtype can represent.
    */
   static Entry Everything() {
-    Entry ret;
-    ret.coeff = 1; ret.base = 0;
-    return ret;
+    return Entry(1, 0);
+  }
+  /*!
+   * \brief return an empty set
+   * \return Bound that represent everything dtype can represent.
+   */
+  static Entry Nothing() {
+      return Entry(0, 1);
   }
 };
 

tests/python/unittest/test_arith_modular_set.py

@@ -117,6 +117,22 @@ def test_constraint_scope():
     assert m.coeff == 1
     assert m.base == 0
 
+def test_intersect():
+    a = tvm.var("a")
+    analyzer = tvm.arith.Analyzer()
+    with analyzer.constraint_scope(a % 4 == 1):
+        with analyzer.constraint_scope(a % 3 == 1):
+            m = analyzer.modular_set(a)
+            assert m.coeff == 12
+            assert m.base == 1
+
+    with analyzer.constraint_scope(a % 3 == 2):
+        with analyzer.constraint_scope(a % 5 == 3):
+            with analyzer.constraint_scope(a % 7 == 2):
+                m = analyzer.modular_set(a)
+                assert m.coeff == 105
+                assert m.base == 23
+
 
 if __name__ == "__main__":
     test_cast()
@@ -126,3 +142,4 @@ if __name__ == "__main__":
     test_min_max_select()
     test_mix_index()
     test_constraint_scope()
+    test_intersect()