Generalized division via multiplication.

src/base/division-by-constant.h

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_BASE_DIVISION_BY_CONSTANT_H_
+#define V8_BASE_DIVISION_BY_CONSTANT_H_
+
+namespace v8 {
+namespace base {
+
+// ----------------------------------------------------------------------------
+
+// The magic numbers for division via multiplication, see Warren's "Hacker's
+// Delight", chapter 10. The template parameter must be one of the unsigned
+// integral types.
+template <class T>
+struct MagicNumbersForDivision {
+  MagicNumbersForDivision(T m, unsigned s, bool a)
+      : multiplier(m), shift(s), add(a) {}
+  bool operator==(const MagicNumbersForDivision& rhs) const;
+
+  T multiplier;
+  unsigned shift;
+  bool add;
+};
+
+
+// Calculate the multiplier and shift for signed division via multiplication.
+// The divisor must not be -1, 0 or 1 when interpreted as a signed value.
+template <class T>
+MagicNumbersForDivision<T> SignedDivisionByConstant(T d);
+
+
+// Calculate the multiplier and shift for unsigned division via multiplication,
+// see Warren's "Hacker's Delight", chapter 10. The divisor must not be 0 and
+// leading_zeros can be used to speed up the calculation if the given number of
+// upper bits of the dividend value are known to be zero.
+template <class T>
+MagicNumbersForDivision<T> UnsignedDivisionByConstant(
+    T d, unsigned leading_zeros = 0);
+
+}  // namespace base
+}  // namespace v8
+
+#endif  // V8_BASE_DIVISION_BY_CONSTANT_H_