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Common/Bitfield: store value as unsigned type 

Storing signed type causes the following behaviour: extractValue can do overflow/negative left shift. Now it only relies on two implementation-defined behaviours (which are almost always defined as we want): unsigned->signed conversion and signed right shift
This commit is contained in:
Weiyi Wang 2018-11-10 23:41:30 -05:00 committed by fearlessTobi
parent d104478f1b
commit 786995a81e
1 changed files with 10 additions and 9 deletions
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19
src/common/bit_field.h
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@ -117,21 +117,21 @@ private:
// We don't delete it because we want BitField to be trivially copyable.
constexpr BitField& operator=(const BitField&) = default;
// StorageType is T for non-enum types and the underlying type of T if
// UnderlyingType is T for non-enum types and the underlying type of T if
// T is an enumeration. Note that T is wrapped within an enable_if in the
// former case to workaround compile errors which arise when using
// std::underlying_type<T>::type directly.
using StorageType = typename std::conditional_t<std::is_enum<T>::value, std::underlying_type<T>,
std::enable_if<true, T>>::type;
using UnderlyingType = typename std::conditional_t<std::is_enum_v<T>, std::underlying_type<T>,
std::enable_if<true, T>>::type;
// Unsigned version of StorageType
using StorageTypeU = std::make_unsigned_t<StorageType>;
// We store the value as the unsigned type to avoid undefined behaviour on value shifting
using StorageType = std::make_unsigned_t<UnderlyingType>;
public:
/// Constants to allow limited introspection of fields if needed
static constexpr std::size_t position = Position;
static constexpr std::size_t bits = Bits;
static constexpr StorageType mask = (((StorageTypeU)~0) >> (8 * sizeof(T) - bits)) << position;
static constexpr StorageType mask = (((StorageType)~0) >> (8 * sizeof(T) - bits)) << position;
/**
* Formats a value by masking and shifting it according to the field parameters. A value
@ -148,11 +148,12 @@ public:
* union in a constexpr context.
*/
static constexpr FORCE_INLINE T ExtractValue(const StorageType& storage) {
if (std::numeric_limits<T>::is_signed) {
if constexpr (std::numeric_limits<UnderlyingType>::is_signed) {
std::size_t shift = 8 * sizeof(T) - bits;
return (T)((storage << (shift - position)) >> shift);
return static_cast<T>(static_cast<UnderlyingType>(storage << (shift - position)) >>
shift);
} else {
return (T)((storage & mask) >> position);
return static_cast<T>((storage & mask) >> position);
}
}