Convert base/numerics conversions to constexpr

base/numerics/safe_conversions_impl.h

 // Copyright 2014 The Chromium 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 BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 #define BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_
 
+#include <assert.h>

[brucedawson, 2016/02/04 20:39:58]

Delete - presumably not needed?

 #include <limits.h>
 #include <stdint.h>
 
 namespace base {
 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>
@@ skipping 67 matching lines @@
 };
 
 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) {
-  DCHECK(integer_range_constraint >= RANGE_VALID &&
-         integer_range_constraint <= RANGE_INVALID);
+inline constexpr RangeConstraint GetRangeConstraint(
+    int integer_range_constraint) {
+  // TODO(jschuh): Once we get full C++14 support we want this
+  // assert(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) {
+constexpr 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;
+  // 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;
 
-  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;
-
+  static constexpr Dst max() {
     // 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() {
+  static constexpr 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
@@ skipping 12 matching lines @@
 // 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; }
+  static constexpr 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) {
+  static constexpr 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) {
+  static constexpr 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) {
+  static constexpr 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) {
+  static constexpr 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) {
+inline constexpr 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 base
 
 #endif  // BASE_NUMERICS_SAFE_CONVERSIONS_IMPL_H_