Safe numeric library: base/numerics (copied from Chromium)

webrtc/base/numerics/safe_conversions_impl.h

+/*
+ *  Copyright (c) 2016 The WebRTC 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 in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ *
+ */
+
+// Borrowed from Chromium's src/base/numerics/safe_conversions_impl.h.
+// - Modified to work in WebRTC (paths, namespace, use of webrtc/base).
+//   Based on 'chromium_revision': 'ee311243eae6aef9c907543663754ff38f1f4f40'.
+
+#ifndef WEBRTC_BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
+#define WEBRTC_BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
+
+#include <stdint.h>
+
+#include <limits.h>

[kwiberg-webrtc, 2016/03/05 00:47:26]

The original includes these two in the other order, without a blank line in
between.

[hbos, 2016/03/07 09:18:02]

Done. (I think after the presubmit made me use limits instead of limits.h it
then asked me to change the order, something about c/c++ headers, forgot to
change back after ignoring the first warning.)

+
+#include "webrtc/base/checks.h"
+
+namespace rtc {
+namespace internal {
+
+// The std library doesn't provide a binary max_exponent for integers, however
+// we can compute one by adding one to the number of non-sign bits. This allows
+// for accurate range comparisons between floating point and integer types.
+template <typename NumericType>
+struct MaxExponent {
+  static const int value = std::numeric_limits<NumericType>::is_iec559
+                               ? std::numeric_limits<NumericType>::max_exponent
+                               : (sizeof(NumericType) * 8 + 1 -
+                                  std::numeric_limits<NumericType>::is_signed);
+};
+
+enum IntegerRepresentation {
+  INTEGER_REPRESENTATION_UNSIGNED,
+  INTEGER_REPRESENTATION_SIGNED
+};
+
+// A range for a given nunmeric Src type is contained for a given numeric Dst
+// type if both numeric_limits<Src>::max() <= numeric_limits<Dst>::max() and
+// numeric_limits<Src>::min() >= numeric_limits<Dst>::min() are true.
+// We implement this as template specializations rather than simple static
+// comparisons to ensure type correctness in our comparisons.
+enum NumericRangeRepresentation {
+  NUMERIC_RANGE_NOT_CONTAINED,
+  NUMERIC_RANGE_CONTAINED
+};
+
+// Helper templates to statically determine if our destination type can contain
+// maximum and minimum values represented by the source type.
+
+template <
+    typename Dst,
+    typename Src,
+    IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed
+                                            ? INTEGER_REPRESENTATION_SIGNED
+                                            : INTEGER_REPRESENTATION_UNSIGNED,
+    IntegerRepresentation SrcSign =
+        std::numeric_limits<Src>::is_signed
+            ? INTEGER_REPRESENTATION_SIGNED
+            : INTEGER_REPRESENTATION_UNSIGNED >
+struct StaticDstRangeRelationToSrcRange;
+
+// Same sign: Dst is guaranteed to contain Src only if its range is equal or
+// larger.
+template <typename Dst, typename Src, IntegerRepresentation Sign>
+struct StaticDstRangeRelationToSrcRange<Dst, Src, Sign, Sign> {
+  static const NumericRangeRepresentation value =
+      MaxExponent<Dst>::value >= MaxExponent<Src>::value
+          ? NUMERIC_RANGE_CONTAINED
+          : NUMERIC_RANGE_NOT_CONTAINED;
+};
+
+// Unsigned to signed: Dst is guaranteed to contain source only if its range is
+// larger.
+template <typename Dst, typename Src>
+struct StaticDstRangeRelationToSrcRange<Dst,
+                                        Src,
+                                        INTEGER_REPRESENTATION_SIGNED,
+                                        INTEGER_REPRESENTATION_UNSIGNED> {
+  static const NumericRangeRepresentation value =
+      MaxExponent<Dst>::value > MaxExponent<Src>::value
+          ? NUMERIC_RANGE_CONTAINED
+          : NUMERIC_RANGE_NOT_CONTAINED;
+};
+
+// Signed to unsigned: Dst cannot be statically determined to contain Src.
+template <typename Dst, typename Src>
+struct StaticDstRangeRelationToSrcRange<Dst,
+                                        Src,
+                                        INTEGER_REPRESENTATION_UNSIGNED,
+                                        INTEGER_REPRESENTATION_SIGNED> {
+  static const NumericRangeRepresentation value = NUMERIC_RANGE_NOT_CONTAINED;
+};
+
+enum RangeConstraint {
+  RANGE_VALID = 0x0,  // Value can be represented by the destination type.
+  RANGE_UNDERFLOW = 0x1,  // Value would overflow.
+  RANGE_OVERFLOW = 0x2,  // Value would underflow.
+  RANGE_INVALID = RANGE_UNDERFLOW | RANGE_OVERFLOW  // Invalid (i.e. NaN).
+};
+
+// Helper function for coercing an int back to a RangeContraint.
+inline RangeConstraint GetRangeConstraint(int integer_range_constraint) {
+  RTC_DCHECK(integer_range_constraint >= RANGE_VALID &&
+             integer_range_constraint <= RANGE_INVALID);
+  return static_cast<RangeConstraint>(integer_range_constraint);
+}
+
+// This function creates a RangeConstraint from an upper and lower bound
+// check by taking advantage of the fact that only NaN can be out of range in
+// both directions at once.
+inline RangeConstraint GetRangeConstraint(bool is_in_upper_bound,
+                                   bool is_in_lower_bound) {
+  return GetRangeConstraint((is_in_upper_bound ? 0 : RANGE_OVERFLOW) |
+                            (is_in_lower_bound ? 0 : RANGE_UNDERFLOW));
+}
+
+// The following helper template addresses a corner case in range checks for
+// conversion from a floating-point type to an integral type of smaller range
+// but larger precision (e.g. float -> unsigned). The problem is as follows:
+//   1. Integral maximum is always one less than a power of two, so it must be
+//      truncated to fit the mantissa of the floating point. The direction of
+//      rounding is implementation defined, but by default it's always IEEE
+//      floats, which round to nearest and thus result in a value of larger
+//      magnitude than the integral value.
+//      Example: float f = UINT_MAX; // f is 4294967296f but UINT_MAX
+//                                   // is 4294967295u.
+//   2. If the floating point value is equal to the promoted integral maximum
+//      value, a range check will erroneously pass.
+//      Example: (4294967296f <= 4294967295u) // This is true due to a precision
+//                                            // loss in rounding up to float.
+//   3. When the floating point value is then converted to an integral, the
+//      resulting value is out of range for the target integral type and
+//      thus is implementation defined.
+//      Example: unsigned u = (float)INT_MAX; // u will typically overflow to 0.
+// To fix this bug we manually truncate the maximum value when the destination
+// type is an integral of larger precision than the source floating-point type,
+// such that the resulting maximum is represented exactly as a floating point.
+template <typename Dst, typename Src>
+struct NarrowingRange {
+  typedef typename std::numeric_limits<Src> SrcLimits;
+  typedef typename std::numeric_limits<Dst> DstLimits;
+
+  static Dst max() {
+    // The following logic avoids warnings where the max function is
+    // instantiated with invalid values for a bit shift (even though
+    // such a function can never be called).
+    static const int shift =
+        (MaxExponent<Src>::value > MaxExponent<Dst>::value &&
+         SrcLimits::digits < DstLimits::digits && SrcLimits::is_iec559 &&
+         DstLimits::is_integer)
+            ? (DstLimits::digits - SrcLimits::digits)
+            : 0;
+
+    // We use UINTMAX_C below to avoid compiler warnings about shifting floating
+    // points. Since it's a compile time calculation, it shouldn't have any
+    // performance impact.
+    return DstLimits::max() - static_cast<Dst>((UINTMAX_C(1) << shift) - 1);
+  }
+
+  static Dst min() {
+    return std::numeric_limits<Dst>::is_iec559 ? -DstLimits::max()
+                                               : DstLimits::min();
+  }
+};
+
+template <
+    typename Dst,
+    typename Src,
+    IntegerRepresentation DstSign = std::numeric_limits<Dst>::is_signed
+                                            ? INTEGER_REPRESENTATION_SIGNED
+                                            : INTEGER_REPRESENTATION_UNSIGNED,
+    IntegerRepresentation SrcSign = std::numeric_limits<Src>::is_signed
+                                            ? INTEGER_REPRESENTATION_SIGNED
+                                            : INTEGER_REPRESENTATION_UNSIGNED,
+    NumericRangeRepresentation DstRange =
+        StaticDstRangeRelationToSrcRange<Dst, Src>::value >
+struct DstRangeRelationToSrcRangeImpl;
+
+// The following templates are for ranges that must be verified at runtime. We
+// split it into checks based on signedness to avoid confusing casts and
+// compiler warnings on signed an unsigned comparisons.
+
+// Dst range is statically determined to contain Src: Nothing to check.
+template <typename Dst,
+          typename Src,
+          IntegerRepresentation DstSign,
+          IntegerRepresentation SrcSign>
+struct DstRangeRelationToSrcRangeImpl<Dst,
+                                      Src,
+                                      DstSign,
+                                      SrcSign,
+                                      NUMERIC_RANGE_CONTAINED> {
+  static RangeConstraint Check(Src value) { return RANGE_VALID; }
+};
+
+// Signed to signed narrowing: Both the upper and lower boundaries may be
+// exceeded.
+template <typename Dst, typename Src>
+struct DstRangeRelationToSrcRangeImpl<Dst,
+                                      Src,
+                                      INTEGER_REPRESENTATION_SIGNED,
+                                      INTEGER_REPRESENTATION_SIGNED,
+                                      NUMERIC_RANGE_NOT_CONTAINED> {
+  static RangeConstraint Check(Src value) {
+    return GetRangeConstraint((value <= NarrowingRange<Dst, Src>::max()),
+                              (value >= NarrowingRange<Dst, Src>::min()));
+  }
+};
+
+// Unsigned to unsigned narrowing: Only the upper boundary can be exceeded.
+template <typename Dst, typename Src>
+struct DstRangeRelationToSrcRangeImpl<Dst,
+                                      Src,
+                                      INTEGER_REPRESENTATION_UNSIGNED,
+                                      INTEGER_REPRESENTATION_UNSIGNED,
+                                      NUMERIC_RANGE_NOT_CONTAINED> {
+  static RangeConstraint Check(Src value) {
+    return GetRangeConstraint(value <= NarrowingRange<Dst, Src>::max(), true);
+  }
+};
+
+// Unsigned to signed: The upper boundary may be exceeded.
+template <typename Dst, typename Src>
+struct DstRangeRelationToSrcRangeImpl<Dst,
+                                      Src,
+                                      INTEGER_REPRESENTATION_SIGNED,
+                                      INTEGER_REPRESENTATION_UNSIGNED,
+                                      NUMERIC_RANGE_NOT_CONTAINED> {
+  static RangeConstraint Check(Src value) {
+    return sizeof(Dst) > sizeof(Src)
+               ? RANGE_VALID
+               : GetRangeConstraint(
+                     value <= static_cast<Src>(NarrowingRange<Dst, Src>::max()),
+                     true);
+  }
+};
+
+// Signed to unsigned: The upper boundary may be exceeded for a narrower Dst,
+// and any negative value exceeds the lower boundary.
+template <typename Dst, typename Src>
+struct DstRangeRelationToSrcRangeImpl<Dst,
+                                      Src,
+                                      INTEGER_REPRESENTATION_UNSIGNED,
+                                      INTEGER_REPRESENTATION_SIGNED,
+                                      NUMERIC_RANGE_NOT_CONTAINED> {
+  static RangeConstraint Check(Src value) {
+    return (MaxExponent<Dst>::value >= MaxExponent<Src>::value)
+               ? GetRangeConstraint(true, value >= static_cast<Src>(0))
+               : GetRangeConstraint(
+                     value <= static_cast<Src>(NarrowingRange<Dst, Src>::max()),
+                     value >= static_cast<Src>(0));
+  }
+};
+
+template <typename Dst, typename Src>
+inline RangeConstraint DstRangeRelationToSrcRange(Src value) {
+  static_assert(std::numeric_limits<Src>::is_specialized,
+                "Argument must be numeric.");
+  static_assert(std::numeric_limits<Dst>::is_specialized,
+                "Result must be numeric.");
+  return DstRangeRelationToSrcRangeImpl<Dst, Src>::Check(value);
+}
+
+}  // namespace internal
+}  // namespace rtc
+
+#endif  // WEBRTC_BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_