[LLVMdev] We need better hashing

[Talin]

OK here's a patch with the latest, including unit tests. I've also tried to
make the comments clear that this is intended for the case of "generic" key
types, where you are either hashing multiple data types together, or you
don't know in advance what the data type will be - for cases such as
strings where a tailored algorithm is available, you should use that
instead of this.

There's a couple of things I don't like: First, there's too many levels of
inlined function calls - my experience is that compilers aren't as good at
inlining nested calls as is often advertised, however I couldn't figure out
a way to get the same effect without the nested calls.

Similarly the addBitsImpl helper types are a little too complex and magical
for my taste, but again I couldn't think of a better way to automatically
detect whether to use the aligned vs. unaligned hashing routine.

On Wed, Feb 15, 2012 at 12:01 PM, Chris Lattner <clattner at apple.com> wrote:

> On Feb 14, 2012, at 10:47 PM, Talin wrote:
>
>  /// Add a pointer value
>>   template<typename T>
>>   void add(const T *PtrVal) {
>>     addImpl(
>>         reinterpret_cast<const uint32_t *>(&PtrVal),
>>         reinterpret_cast<const uint32_t *>(&PtrVal + 1));
>>   }
>>
>> This violates TBAA rules and looks pretty dangerous to expose as public
>> API.  Is this really needed?  Also, addImpl is dereferencing the pointers
>> as uint32_t's, but there is nothing that guarantees that T is a multiple of
>> 4 bytes.  The ArrayRef version has the same problem.
>>
>> So as far as hashing pointer values is concerned, I was just copying the
> code from FoldingSet. Since a lot of the keys that we're going to be
> dealing with are uniqued pointers, it makes sense to be able to calculate a
> hash of the bit-value of the pointer, rather than hashing the thing pointed
> to. That being said, renaming it to "addPointer" and adding a comment might
> be in order. Similarly, I can make the ArrayRef version 'addPointers' and
> have it take an ArrayRef<T*>.
>
>
> Ah, Jay was right, I misread this code!
>
> Though it is more verbose, I think it would be better to expose a template
>> specialization approach to getting the hash_value of T.
>>
>>   /// Add a float
>>   void add(float Data) {
>>     addImpl(
>>       reinterpret_cast<const uint32_t *>(&Data),
>>       reinterpret_cast<const uint32_t *>(&Data + 1));
>>   }
>>
>>   /// Add a double
>>   void add(double Data) {
>>     addImpl(
>>       reinterpret_cast<const uint32_t *>(&Data),
>>       reinterpret_cast<const uint32_t *>(&Data + 1));
>>   }
>>
>> Similarly, these need to go through a union to avoid TBAA problems.
>>
>> I'm not sure how that works. Can you give an example?
>
>
> Just use:
>
> void add(double Data) {
>   union {
>      double D; uint64_t I;
>   };
>   D = Data;
>   add(I);
> }
>
>
>>  void add(StringRef StrVal) {
>>     addImpl(StrVal.data(), StrVal.size());
>>   }
>>
>> I'm contradicting my stance above about not caring about the
>> implementation :), but is MurmurHash a good hash for string data?
>>  The Bernstein hash function works really well and is much cheaper to
>> compute than Murmur.  It is used by HashString (and thus by StringMap).
>>
>> So, MurmurHash is intended for blocks of arbitrary binary data, which may
> contain character data, integers, or whatever - it's designed to do such a
> thorough job of mixing the bits that it really doesn't matter what data
> types you feed it. You are correct that for purely string data, you'd want
> to use a less expensive algorithm (I'm partial to FNV-1, which is as cheap
> as the Bernstein hash and is AFAIK more mathematically sound.)
>
>
> Ok, so what's the answer? :)   We can do different things for
> ArrayRef<char> and StringRef.
>
> -Chris
>
>
>
>


-- 
-- Talin
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