[ARITH] Add Lowering rule for FloorDiv/Mod (#3976)

* [ARITH] Add Lowering rule for FloorDiv/Mod

* add comment about constant folding

include/tvm/expr_operator.h

@@ -333,6 +333,30 @@ TVM_DLL Expr operator||(Expr a, Expr b);
  */
 TVM_DLL Expr operator!(Expr a);
 /*!
+ * \brief compute trunc(a / b)
+ *
+ * This is the default integer division behavior in C.
+ *
+ * \param a left operand
+ * \param b right operand
+ * \return The result expression.
+ * \note this function does eager constant folding for
+ *       index types(int32, int64) when possible.
+ */
+TVM_DLL Expr truncdiv(Expr a, Expr b);
+/*!
+ * \brief compute the remainder of truncdiv
+ *
+ * This is the default integer division behavior in C.
+ *
+ * \param a left operand
+ * \param b right operand
+ * \return The result expression.
+ * \note this function does eager constant folding for
+ *       index types(int32, int64) when possible.
+ */
+TVM_DLL Expr truncmod(Expr a, Expr b);
+/*!
  * \brief compute floor(a / b)
  *
  * \param a left operand

python/tvm/api.py

@@ -891,6 +891,52 @@ def comm_reducer(fcombine, fidentity, name="reduce"):
     return reducer
 
 
+def truncdiv(a, b):
+    """Compute the truncdiv of two expressions.
+
+    Parameters
+    ----------
+    a : Expr
+        The left hand operand
+
+    b : Expr
+        The right hand operand
+
+    Returns
+    -------
+    res : Expr
+        The result expression.
+
+    Note
+    ----
+    This is the default integer division behavior in C.
+    """
+    return _make._OpTruncDiv(a, b)
+
+
+def truncmod(a, b):
+    """Compute the truncmod of two expressions.
+
+    Parameters
+    ----------
+    a : Expr
+        The left hand operand
+
+    b : Expr
+        The right hand operand
+
+    Returns
+    -------
+    res : Expr
+        The result expression.
+
+    Note
+    ----
+    This is the default integer division behavior in C.
+    """
+    return _make._OpTruncMod(a, b)
+
+
 def floordiv(a, b):
     """Compute the floordiv of two expressions.
 

src/api/api_ir.cc

@@ -196,6 +196,8 @@ REGISTER_MAKE_BINARY_OP(_OpDiv, operator/);
 REGISTER_MAKE_BINARY_OP(_OpMod, operator%);
 REGISTER_MAKE_BINARY_OP(_OpFloorDiv, floordiv);
 REGISTER_MAKE_BINARY_OP(_OpFloorMod, floormod);
+REGISTER_MAKE_BINARY_OP(_OpTruncDiv, truncdiv);
+REGISTER_MAKE_BINARY_OP(_OpTruncMod, truncmod);
 REGISTER_MAKE_BINARY_OP(_OpPow, pow);
 REGISTER_MAKE_BINARY_OP(_OpMin, min);
 REGISTER_MAKE_BINARY_OP(_OpMax, max);

src/arithmetic/const_fold.h

@@ -18,7 +18,6 @@
  */
 
 /*!
- *  Copyright (c) 2019 by Contributors
  * \file const_fold.h
  * \brief Centralized location for constant folding.
  */

src/arithmetic/int_operator.h

@@ -99,11 +99,12 @@ inline bool WillOverflow<ir::Mod>(int64_t x,
  * \return the result.
  */
 inline int64_t floordiv(int64_t x, int64_t y) {
-  bool round_down =
-      (x >= 0 && y >= 0) ||
-      (x <= 0 && y <= 0) ||
-      (x % y == 0);
-  return round_down ? (x / y) : (x / y - 1);
+  int64_t rdiv = x / y;
+  int64_t rmod = x % y;
+  bool is_floor_div =
+      (y >= 0 && rmod >= 0) ||
+      (y < 0 && rmod <= 0);
+  return is_floor_div ? rdiv : (rdiv - 1);
 }
 
 
@@ -114,11 +115,11 @@ inline int64_t floordiv(int64_t x, int64_t y) {
  * \return the result.
  */
 inline int64_t floormod(int64_t x, int64_t y) {
-  bool round_down =
-      (x >= 0 && y >= 0) ||
-      (x <= 0 && y <= 0) ||
-      (x % y == 0);
-  return round_down ? (x % y) : (x % y + y);
+  int64_t rmod = x % y;
+  bool is_floor_div =
+      (y >= 0 && rmod >= 0) ||
+      (y < 0 && rmod <= 0);
+  return is_floor_div ? rmod : rmod + y;
 }
 
 }  // namespace arith

src/arithmetic/ir_mutator_with_analyzer.cc

@@ -41,8 +41,9 @@ Mutate_(const LetStmt* op, const Stmt& s) {
   Expr value = this->Mutate(op->value);
   if (!ir::HasSideEffect(value)) {
     analyzer_->Bind(op->var, value);
-    return this->Mutate(op->body);
   }
+  // We keep the let-binding here
+  // as sub-class may or maynot choose to replace it.
   Stmt body = this->Mutate(op->body);
   if (value.same_as(op->value) &&
       body.same_as(op->body)) {
@@ -152,8 +153,9 @@ Mutate_(const Let* op, const Expr& self) {
   Expr value = this->Mutate(op->value);
   if (!ir::HasSideEffect(value)) {
     analyzer_->Bind(op->var, value);
-    return this->Mutate(op->body);
   }
+  // We keep the let-binding here
+  // as sub-class may or maynot choose to replace it.
   Expr body = this->Mutate(op->body);
   if (value.same_as(op->value) &&
       body.same_as(op->body)) {

src/arithmetic/ir_mutator_with_analyzer.h

@@ -45,6 +45,8 @@ class IRMutatorWithAnalyzer : public ir::IRMutator {
   explicit IRMutatorWithAnalyzer(Analyzer* analyzer)
       : analyzer_(analyzer) {}
 
+  using IRMutator::Mutate_;
+
   // override functions that need to populate the context information.
   Stmt Mutate_(const ir::For* op, const Stmt& self) override;
   Stmt Mutate_(const ir::LetStmt* op, const Stmt& self) override;

src/arithmetic/pattern_match.h

@@ -18,7 +18,6 @@
  */
 
 /*!
- *  Copyright (c) 2019 by Contributors
  * \file tvm/arithmetic/pattern_match.h
  *
  * \brief Internal tool for expression-template based pattern matching.
@@ -326,6 +325,8 @@ TVM_PATTERN_BINARY_OP(operator/, ir::Div);
 TVM_PATTERN_BINARY_OP(operator%, ir::Mod);
 TVM_PATTERN_BINARY_OP(min, ir::Min);
 TVM_PATTERN_BINARY_OP(max, ir::Max);
+TVM_PATTERN_BINARY_OP(truncdiv, ir::Div);
+TVM_PATTERN_BINARY_OP(truncmod, ir::Mod);
 TVM_PATTERN_BINARY_OP(floordiv, ir::FloorDiv);
 TVM_PATTERN_BINARY_OP(floormod, ir::FloorMod);
 

src/arithmetic/rewrite_simplify.cc

@@ -1674,6 +1674,16 @@ Mutate_(const Call* op, const Expr& self) {
   if (op == nullptr) return ret;
   if (op->is_intrinsic(Call::likely) && is_const(op->args[0])) {
     return op->args[0];
+  } else if (op->is_intrinsic(Call::shift_right)) {
+    if (op->args[0].as<IntImm>() && op->args[1].as<IntImm>()) {
+      // the operator overload will eagerly constant fold.
+      return op->args[0] >> op->args[1];
+    }
+  } else if (op->is_intrinsic(Call::bitwise_and)) {
+    if (op->args[0].as<IntImm>() && op->args[1].as<IntImm>()) {
+      // the operator overload will eagerly constant fold.
+      return op->args[0] & op->args[1];
+    }
   }
   return ret;
 }
@@ -1695,6 +1705,24 @@ Mutate_(const Cast* op, const Expr& self) {
   return cast(op->type, op->value);
 }
 
+Expr RewriteSimplifier::Impl::
+Mutate_(const Let* op, const Expr& self) {
+  Expr value = this->Mutate(op->value);
+  if (!ir::HasSideEffect(value)) {
+    // it is fine to discard the let binding
+    // because the value will always be inlined in the simplifier.
+    analyzer_->Bind(op->var, value);
+    return this->Mutate(op->body);
+  }
+  Expr body = this->Mutate(op->body);
+  if (value.same_as(op->value) &&
+      body.same_as(op->body)) {
+    return self;
+  } else {
+    return Let::make(op->var, value, body);
+  }
+}
+
 Expr RewriteSimplifier::operator()(const Expr& expr) {
   // Run simplification in post order
   Expr res = expr;

src/arithmetic/rewrite_simplify.h

@@ -72,6 +72,7 @@ class RewriteSimplifier::Impl : public IRMutatorWithAnalyzer {
   Expr Mutate_(const Call* op, const Expr& self) override;
   Expr Mutate_(const Variable* op, const Expr& self) override;
   Expr Mutate_(const Cast* op, const Expr& self) override;
+  Expr Mutate_(const Let* op, const Expr& self) override;
 
  protected:
   /*! \brief internal structure for comparison. */

src/arithmetic/stmt_simplify.cc

@@ -51,6 +51,23 @@ class StmtSimplifier : public IRMutatorWithAnalyzer {
     return Mutate(stmt);
   }
 
+  Stmt Mutate_(const LetStmt* op, const Stmt& s) {
+    Expr value = this->Mutate(op->value);
+    if (!ir::HasSideEffect(value)) {
+      // it is fine to discard the let binding
+      // because the call to simplify will always inline the var.
+      analyzer_->Bind(op->var, value);
+      return Mutate(op->body);
+    }
+    Stmt body = this->Mutate(op->body);
+    if (value.same_as(op->value) &&
+        body.same_as(op->body)) {
+      return s;
+    } else {
+      return LetStmt::make(op->var, value, body);
+    }
+  }
+
   // eliminate useless stores
   Stmt Mutate_(const Store* op, const Stmt& s) final {
     Stmt stmt = IRMutator::Mutate_(op, s);

src/lang/expr_operator.cc

@@ -178,20 +178,28 @@ Expr operator*(Expr a, Expr b) {
   return ir::Mul::make(a, b);
 }
 
-Expr operator/(Expr a, Expr b) {
+Expr truncdiv(Expr a, Expr b) {
   BinaryOpMatchTypes(a, b);
   Expr ret = arith::TryConstFold<ir::Div>(a, b);
   if (ret.defined()) return ret;
   return ir::Div::make(a, b);
 }
 
-Expr operator%(Expr a, Expr b) {
+Expr truncmod(Expr a, Expr b) {
   BinaryOpMatchTypes(a, b);
   Expr ret = arith::TryConstFold<ir::Mod>(a, b);
   if (ret.defined()) return ret;
   return ir::Mod::make(a, b);
 }
 
+Expr operator/(Expr a, Expr b) {
+  return truncdiv(a, b);
+}
+
+Expr operator%(Expr a, Expr b) {
+  return truncmod(a, b);
+}
+
 Expr floordiv(Expr a, Expr b) {
   BinaryOpMatchTypes(a, b);
   Expr ret = arith::TryConstFold<ir::FloorDiv>(a, b);

src/pass/lower_intrin.cc

@@ -6,9 +6,9 @@
  * to you under the Apache License, Version 2.0 (the
  * "License"); you may not use this file except in compliance
  * with the License.  You may obtain a copy of the License at
- * 
+ *
  *   http://www.apache.org/licenses/LICENSE-2.0
- * 
+ *
  * Unless required by applicable law or agreed to in writing,
  * software distributed under the License is distributed on an
  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
@@ -18,23 +18,28 @@
  */
 
 /*!
- *  Copyright (c) 2017 by Contributors
- *  Lower intrinsic calls to device specific ir when possible.
+ *  Lower intrinsic calls and ops to device specific ir when possible.
  * \file lower_intrin.cc
  */
 #include <tvm/ir.h>
 #include <tvm/ir_mutator.h>
 #include <tvm/ir_pass.h>
 #include <tvm/api_registry.h>
+#include <tvm/expr_operator.h>
 #include <unordered_set>
 #include "ir_util.h"
+#include "../arithmetic/pattern_match.h"
+#include "../arithmetic/ir_mutator_with_analyzer.h"
 
 namespace tvm {
 namespace ir {
 
-class IntrinInjecter : public IRMutator {
+class IntrinInjecter : public arith::IRMutatorWithAnalyzer {
  public:
-  explicit IntrinInjecter(std::string target) {
+  using IRMutatorWithAnalyzer::Mutate_;
+
+  IntrinInjecter(arith::Analyzer* analyzer, std::string target)
+      : IRMutatorWithAnalyzer(analyzer) {
     std::istringstream is(target);
     std::string starget;
     is >> starget;
@@ -61,6 +66,118 @@ class IntrinInjecter : public IRMutator {
     return IRMutator::Mutate_(op, e);
   }
 
+  // We use floordiv for integer analysis,
+  // but will need to lower them to native truncdiv instructions
+  Expr Mutate_(const FloorDiv* op, const Expr& e) final {
+    Expr ret = IRMutatorWithAnalyzer::Mutate_(op, e);
+    op = ret.as<FloorDiv>();
+    if (op == nullptr) return ret;
+    int shift;
+    const DataType& dtype = op->type;
+    if (dtype.is_float()) {
+      return floor(Div::make(op->a, op->b));
+    }
+    CHECK(dtype.is_int() || !dtype.is_uint());
+
+    if (is_const_power_of_two_integer(op->b, &shift)) {
+      // lower to right shift if possible.
+      return op->a >> make_const(dtype, shift);
+    }
+
+    if (analyzer_->CanProveGreaterEqual(op->b, 0)) {
+      // Common path, positive divisor
+      if (analyzer_->CanProveGreaterEqual(op->a, 0) ||
+          analyzer_->CanProveGreaterEqual(e, 0)) {
+        return truncdiv(op->a, op->b);
+      } else {
+        DLOG(INFO) << "LowerFloorDiv: Cannot decide the sign of divident";
+        Expr rdiv = truncdiv(op->a, op->b);
+        Expr rmod = truncmod(op->a, op->b);
+        // condition on b >= 0.
+        // truncmod(a, b) < 0 will implies ceildiv,
+        // So we need to correct these cases.
+        if (dtype == Int(32) || dtype == Int(64)) {
+          // equivalent to rdiv + (rmod >= 0 ? 0: -1);
+          return rdiv + (rmod >> make_const(dtype, dtype.bits() - 1));
+        } else {
+          return ir::Select::make(rmod >= 0 , rdiv, rdiv - make_const(dtype, 1));
+        }
+      }
+    } else {
+      // uncommon case
+      DLOG(INFO) << "LowerFloorDiv: Cannot decide the sign of divisor";
+      // b >= 0 => (rmod >=0 ? rdiv : rdiv - 1)
+      // b < 0  => (rmod <= 0 ? rdiv : rdiv - 1)
+      Expr rdiv = truncdiv(op->a, op->b);
+      Expr rmod = truncmod(op->a, op->b);
+      return ir::Select::make(
+          (op->b >= 0 && rmod >= 0) || (op->b < 0 && rmod <= 0),
+          rdiv, rdiv - make_const(dtype, 1));
+    }
+  }
+
+  Expr Mutate_(const FloorMod* op, const Expr& e) final {
+    Expr ret = IRMutatorWithAnalyzer::Mutate_(op, e);
+    op = ret.as<FloorMod>();
+    if (op == nullptr) return ret;
+    // Lower floordiv to native truncdiv.
+    int shift;
+    const DataType& dtype = op->type;
+    CHECK(dtype.is_int() || !dtype.is_uint());
+
+    if (is_const_power_of_two_integer(op->b, &shift)) {
+      // lower to masking if possible.
+      int64_t mask = (
+          static_cast<int64_t>(1) << static_cast<int64_t>(shift)) - 1;
+      return op->a & make_const(dtype, mask);
+    }
+
+    if (analyzer_->CanProveGreaterEqual(op->b, 0)) {
+      // Common pass, positive divisor
+      if (analyzer_->CanProveGreaterEqual(op->a, 0) ||
+          analyzer_->CanProveGreaterEqual(e, 0)) {
+        return truncmod(op->a, op->b);
+      } else {
+        DLOG(INFO) << "LowerFloorMod: Cannot decide the sign of divident";
+        // NOTE:condition on b >= 0.
+        // mod(a, b) < 0 will imply we are doing ceildiv,
+        // So we need to correct these cases.
+        Expr rmod = truncmod(op->a, op->b);
+        if (dtype == Int(32) || dtype == Int(64)) {
+          // (rmod >> shift) & b
+          // -> (rmod >= 0 ? 0: -1) & b
+          // -> rmod >= 0 ? 0 : b
+          return rmod + (op->b & (rmod >> make_const(dtype, dtype.bits() - 1)));
+        } else {
+          return ir::Select::make(rmod >= 0, rmod, rmod + op->b);
+        }
+      }
+    } else {
+      // uncommon case
+      DLOG(INFO) << "LowerFloorMod: Cannot decide the sign of divsor and divident";
+      Expr rmod = truncmod(op->a, op->b);
+      // b > 0 && rmod >= 0 -> rmod
+      // b > 0 && rmod < 0  -> rmod + b
+      // b < 0 && rmod < 0 -> rmod
+      // b < 0 && rmod > 0 -> rmod + b
+      return ir::Select::make(
+          (op->b >= 0 && rmod >= 0) || (op->b < 0 && rmod <= 0),
+          rmod, rmod + op->b);
+    }
+  }
+
+  Expr Mutate_(const Max* op, const Expr& e) final {
+    using namespace arith;
+    PVar<Expr> x, y;
+    PVar<Integer> c;
+    if (max(floordiv(x, y), c).Match(e) &&
+        c.Eval()->value >= 0 &&
+        analyzer_->CanProveGreaterEqual(y.Eval(), 0)) {
+      return max(Mutate(truncdiv(x, y).Eval()), c.Eval());
+    }
+    return IRMutatorWithAnalyzer::Mutate_(op, e);
+  }
+
  private:
   Expr SwapBroadcastCast(const Expr& e) {
     // Try to change broadcast(cast(x)) to cast(broadcast(x))
@@ -132,17 +249,27 @@ class IntrinInjecter : public IRMutator {
     }
     return Expr();
   }
+
   // patterns
   std::vector<std::string> patterns_;
   const PackedFunc* fma_{nullptr};
 };
 
+Stmt LowerIntrinStmt(Stmt stmt, const std::string& target) {
+  arith::Analyzer analyzer;
+  return IntrinInjecter(&analyzer, target).Mutate(stmt);
+}
+
 LoweredFunc
 LowerIntrin(LoweredFunc f, const std::string& target) {
   auto n = make_node<LoweredFuncNode>(*f.operator->());
-  n->body = IntrinInjecter(target).Mutate(n->body);
+  n->body = LowerIntrinStmt(n->body, target);
   return LoweredFunc(n);
 }
 
+// Register the api only for test purposes
+TVM_REGISTER_API("ir_pass._LowerIntrinStmt")
+.set_body_typed(LowerIntrinStmt);
+
 }  // namespace ir
 }  // namespace tvm

tests/python/unittest/test_codegen_llvm.py

@@ -87,6 +87,7 @@ def test_llvm_lookup_intrin():
     func = tvm.ir_pass.MakeAPI(body, "ctpop", [A], 1, True)
     fcode = tvm.build(func, None, "llvm")
 
+
 def test_llvm_add_pipeline():
     nn = 1024
     n = tvm.convert(nn)

tests/python/unittest/test_pass_lower_intrin.py

+ # Licensed to the Apache Software Foundation (ASF) under one
+ # or more contributor license agreements.  See the NOTICE file
+ # distributed with this work for additional information
+ # regarding copyright ownership.  The ASF licenses this file
+ # to you under the Apache License, Version 2.0 (the
+ # "License"); you may not use this file except in compliance
+ # with the License.  You may obtain a copy of the License at
+ #
+ #   http://www.apache.org/licenses/LICENSE-2.0
+ #
+ # Unless required by applicable law or agreed to in writing,
+ # software distributed under the License is distributed on an
+ # "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ # KIND, either express or implied.  See the License for the
+ # specific language governing permissions and limitations
+ # under the License.
+import tvm
+import numpy as np
+
+def lower_intrin(stmt):
+    """wrapper to call transformation in stmt"""
+    lower_expr = isinstance(stmt, tvm.expr.Expr)
+    stmt = tvm.stmt.Evaluate(stmt) if lower_expr else stmt
+    stmt = tvm.ir_pass.CanonicalSimplify(stmt)
+    stmt  = tvm.ir_pass._LowerIntrinStmt(stmt, "llvm")
+    return stmt.value if lower_expr else stmt.body
+
+
+def check_value(expr, vx, vy, data, fref):
+    n = len(data)
+    A = tvm.placeholder((n,), name="A", dtype=expr.dtype)
+    B = tvm.placeholder((n,), name="B", dtype=expr.dtype)
+
+    def make_binds(i):
+        x = expr
+        x = tvm.expr.Let(vx, A[i], x)
+        x = tvm.expr.Let(vy, B[i], x)
+        return x
+
+    C = tvm.compute((n,), make_binds)
+    s = tvm.create_schedule([C.op])
+
+    if not tvm.module.enabled("llvm"):
+        return
+
+    f = tvm.build(s, [A, B, C], "llvm")
+    a = tvm.nd.array(np.array([x for x, y in data], dtype=expr.dtype))
+    b = tvm.nd.array(np.array([y for x, y in data], dtype=expr.dtype))
+    c = tvm.nd.array(np.zeros(len(data), dtype=expr.dtype))
+    f(a, b, c)
+    cref = np.array([fref(x, y) for x, y in data])
+    np.testing.assert_equal(c.asnumpy(), cref)
+
+
+
+def get_ref_data():
+    """Get reference data for every pairs"""
+    import itertools
+    x = range(-10, 10)
+    y = list(range(-10, 10))
+    y.remove(0)
+    return list(itertools.product(x, y))
+
+
+def test_lower_floordiv():
+    data = get_ref_data()
+    for dtype in ["int32", "int64", "int16"]:
+        x = tvm.var("x", dtype=dtype)
+        y = tvm.var("y", dtype=dtype)
+        zero = tvm.const(0, dtype)
+        # no constraints
+        res = lower_intrin(tvm.floordiv(x, y))
+        check_value(res, x, y, data, lambda a, b: a // b)
+        # rhs >= 0
+        res = lower_intrin(tvm.expr.Select(y >= 0, tvm.floordiv(x, y), zero))
+        check_value(res, x, y, data, lambda a, b: a // b if b > 0 else 0)
+        # involves max
+        res = lower_intrin(tvm.expr.Select(y >= 0, tvm.max(tvm.floordiv(x, y), zero), zero))
+        check_value(res, x, y, data, lambda a, b: max(a // b, 0) if b > 0 else 0)
+        # lhs >= 0
+        res = lower_intrin(tvm.expr.Select(tvm.all(y >= 0, x >= 0), tvm.floordiv(x, y), zero))
+        check_value(res, x, y, data, lambda a, b: a // b if b > 0 and a >= 0 else 0)
+        # const power of two
+        res = lower_intrin(tvm.floordiv(x, tvm.const(8, dtype=dtype)))
+        check_value(res, x, y, [(a, b) for a, b in data if b == 8], lambda a, b: a // b)
+
+
+def test_lower_floormod():
+    data = get_ref_data()
+    for dtype in ["int32", "int64", "int16"]:
+        x = tvm.var("x", dtype=dtype)
+        y = tvm.var("y", dtype=dtype)
+        zero = tvm.const(0, dtype)
+        # no constraints
+        res = lower_intrin(tvm.floormod(x, y))
+        check_value(res, x, y, data, lambda a, b: a % b)
+        # rhs >= 0
+        res = lower_intrin(tvm.expr.Select(y >= 0, tvm.floormod(x, y), zero))
+        check_value(res, x, y, data, lambda a, b: a % b if b > 0 else 0)
+        # lhs >= 0
+        res = lower_intrin(tvm.expr.Select(tvm.all(y >= 0, x >= 0), tvm.floormod(x, y), zero))
+        check_value(res, x, y, data, lambda a, b: a % b if b > 0 and a >= 0 else 0)
+        # const power of two
+        res = lower_intrin(tvm.floormod(x, tvm.const(8, dtype=dtype)))
+        check_value(res, x, y, [(a, b) for a, b in data if b == 8], lambda a, b: a % b)
+
+
+
+if __name__ == "__main__":
+    test_lower_floordiv()
+    test_lower_floormod()