[webkit-dev] Hash table empty value

[Maciej Stachowiak]

> On Dec 19, 2018, at 12:54 PM, Michael Catanzaro <mcatanzaro at igalia.com> wrote:
> 
> On Tue, Dec 18, 2018 at 2:31 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:
>> I tend to agree but then we'd come up with other numbers for the empty & deleted values.
>> I've been thinking that we could use -1 and -2 but that's also somewhat arbitrary restriction.
> 
> Using min/max values seems much safer. E.g. we already have in HashTraits.h:

I think that’s true for integers, but for pointers, 0 and -1 are likely safer (if you don’t need to store null pointers) or -1 and -2 if you do, since all 1 bits is less likely to be a valid address. There’s also some optimization for the special case where the empty value is 0, but probably not that important if we often need to store null pointers. 

> // Default unsigned traits disallow both 0 and max as keys -- use these traits to allow zero and disallow max - 1.
> template<typename T> struct UnsignedWithZeroKeyHashTraits : GenericHashTraits<T> {
>   static const bool emptyValueIsZero = false;
>   static T emptyValue() { return std::numeric_limits<T>::max(); }
>   static void constructDeletedValue(T& slot) { slot = std::numeric_limits<T>::max() - 1; }
>   static bool isDeletedValue(T value) { return value == std::numeric_limits<T>::max() - 1; }
> };
> 
> And:
> 
> template<typename T> struct SignedWithZeroKeyHashTraits : GenericHashTraits<T> {
>   static const bool emptyValueIsZero = false;
>   static T emptyValue() { return std::numeric_limits<T>::min(); }
>   static void constructDeletedValue(T& slot) { slot = std::numeric_limits<T>::max(); }
>   static bool isDeletedValue(T value) { return value == std::numeric_limits<T>::max(); }
> };
> 
> Which both seem much safer than the current default:
> 
> // Default integer traits disallow both 0 and -1 as keys (max value instead of -1 for unsigned).
> template<typename T> struct GenericHashTraitsBase<true, T> : GenericHashTraitsBase<false, T> {
>   static const bool emptyValueIsZero = true;
>   static void constructDeletedValue(T& slot) { slot = static_cast<T>(-1); }
>   static bool isDeletedValue(T value) { return value == static_cast<T>(-1); }
> };
> 
> Michael
> 
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