[LLVMdev] We need better hashing

[Jeffrey Yasskin]

On Tue, Feb 14, 2012 at 2:44 AM, Chris Lattner <clattner at apple.com> wrote:
> On Feb 13, 2012, at 2:00 AM, Talin wrote:
>>
>> Just out of curiosity, why not MurmurHash3 ? This page seems to
>> suggest that #2 has some flaw, and #3 is better all round:
>>
>> https://sites.google.com/site/murmurhash/
>>
> The main reason is because there's no incremental version of 3.
>
>
> I think that that is a great reason.
>
> LLVM's needs, on the other hand, are fairly modest. I'm guessing that most
> DenseMaps contain less than a few thousand entries. Even a bad hash function
> wouldn't hurt performance that much, and the time taken to calculate the
> hash is probably more of a factor in overall performance than getting the
> optimum distribution of hash values.
>
>
> Indeed.  It can't be hard to be better than our existing adhoc stuff :).
>  That said, please do not change the hash function used by StringMap without
> do careful performance analysis of the clang preprocessor.  The identifier
> uniquing is a very hot path in "clang -E" performance.
>
>>
>> Would it be possible to use CityHash instead for strings?
>>
>> http://code.google.com/p/cityhash/
>>
> OK by me. My intention is that "Hashing.h" should contain a variety of
> different hashing algorithms for various specific needs. Right now, I am
> mainly focusing on hashing of mixed data types - that is, you have some
> structure which contains pointers, ints, strings, and you want to calculate
> a hash of the entire struct. I need this because I'm working on improving
> the uniquing of constants and similar data structures. My next target is to
> improve the uniquing of MDNodes, but I want to get the hashing stuff squared
> away first.
>
> It is also my intent that some person who is more of an expert in hashing
> than I am can do detailed performance analysis under real-world loads (such
> as compiling actual programs with clang) and tweak and optimize the hashing
> algorithm, without needing to modify the API and/or all of the places that
> call it.
>
>
> I think that this is a great way to stage it.  I personally care more about
> the interface than the implementation.  Someone can tweak and tune it after
> enough code is using the API to get reasonable performance numbers.
>
>
> #include "llvm/ADT/ArrayRef.h"
> #include "llvm/ADT/StringRef.h"
> #include "llvm/Support/Compiler.h"
> #include "llvm/Support/DataTypes.h"
> #include "llvm/Support/PointerLikeTypeTraits.h"
> #include "llvm/Support/type_traits.h"
>
> Do you actually need all of these includes?  PointerLikeTypeTraits doesn't
> seem necessary.  Is type_traits?
>
>   enum {
>     BufferSize = 32,
>
> BufferSize is dead.
>
>
>  /// 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.
>
> 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.

These are just wrong: they'll hash +0 and -0 to different values even
though they compare ==.

>
>  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.

Have you seen benchmarks saying that, or are you just guessing from
the length of the code? The benchmarks I've seen say the opposite:
http://code.google.com/p/smhasher/wiki/MurmurHash3#Bulk_speed_test,_hashing_an_8-byte-aligned_256k_block
and http://code.google.com/p/cityhash/source/browse/trunk/README.

> It
> is used by HashString (and thus by StringMap).
>
>   // Add a possibly unaligned sequence of bytes.
>   void addImpl(const char *I, size_t Length) {
>
> This should probably be moved out of line to avoid code bloat.
>
> Overall, the design of the class is making sense to me!  Thanks for tackling
> this!
>
> -Chris
>
>
>
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