Rewrite LLVM's generalized support library for hashing to follow the API

of the proposed standard hashing interfaces (N3333), and to use
a modified and tuned version of the CityHash algorithm.

Some of the highlights of this change:
 -- Significantly higher quality hashing algorithm with very well
    distributed results, and extremely few collisions. Should be close to
    a checksum for up to 64-bit keys. Very little clustering or clumping of
    hash codes, to better distribute load on probed hash tables.
 -- Built-in support for reserved values.
 -- Simplified API that composes cleanly with other C++ idioms and APIs.
 -- Better scaling performance as keys grow. This is the fastest
    algorithm I've found and measured for moderately sized keys (such as
    show up in some of the uniquing and folding use cases)
 -- Support for enabling per-execution seeds to prevent table ordering
    or other artifacts of hashing algorithms to impact the output of
    LLVM. The seeding would make each run different and highlight these
    problems during bootstrap.

This implementation was tested extensively using the SMHasher test
suite, and pased with flying colors, doing better than the original
CityHash algorithm even.

I've included a unittest, although it is somewhat minimal at the moment.
I've also added (or refactored into the proper location) type traits
necessary to implement this, and converted users of GeneralHash over.

My only immediate concerns with this implementation is the performance
of hashing small keys. I've already started working to improve this, and
will continue to do so. Currently, the only algorithms faster produce
lower quality results, but it is likely there is a better compromise
than the current one.

Many thanks to Jeffrey Yasskin who did most of the work on the N3333
paper, pair-programmed some of this code, and reviewed much of it. Many
thanks also go to Geoff Pike Pike and Jyrki Alakuijala, the original
authors of CityHash on which this is heavily based, and Austin Appleby
who created MurmurHash and the SMHasher test suite.

Also thanks to Nadav, Tobias, Howard, Jay, Nick, Ahmed, and Duncan for
all of the review comments! If there are further comments or concerns,
please let me know and I'll jump on 'em.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151822 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 53 insertions, 18 deletions

diff --git a/include/llvm/Support/system_error.h b/include/llvm/Support/system_error.h
index b309732..af81206 100644
--- a/include/llvm/Support/system_error.h
+++ b/include/llvm/Support/system_error.h
@@ -470,17 +470,6 @@ template <> struct hash<std::error_code>;
 
 namespace llvm {
 
-template <class T, T v>
-struct integral_constant {
-  typedef T value_type;
-  static const value_type value = v;
-  typedef integral_constant<T,v> type;
-  operator value_type() { return value; }
-};
-
-typedef integral_constant<bool, true> true_type;
-typedef integral_constant<bool, false> false_type;
-
 // is_error_code_enum
 
 template <class Tp> struct is_error_code_enum : public false_type {};
diff --git a/include/llvm/Support/type_traits.h b/include/llvm/Support/type_traits.h
index 515295b..03f85e9 100644
--- a/include/llvm/Support/type_traits.h
+++ b/include/llvm/Support/type_traits.h
@@ -17,6 +17,7 @@
 #ifndef LLVM_SUPPORT_TYPE_TRAITS_H
 #define LLVM_SUPPORT_TYPE_TRAITS_H
 
+#include "llvm/Support/DataTypes.h"
 #include <utility>
 
 // This is actually the conforming implementation which works with abstract
@@ -68,17 +69,62 @@ struct isPodLike<std::pair<T, U> > {
 };
   
 
+template <class T, T v>
+struct integral_constant {
+  typedef T value_type;
+  static const value_type value = v;
+  typedef integral_constant<T,v> type;
+  operator value_type() { return value; }
+};
+
+typedef integral_constant<bool, true> true_type;
+typedef integral_constant<bool, false> false_type;
+
 /// \brief Metafunction that determines whether the two given types are 
 /// equivalent.
-template<typename T, typename U>
-struct is_same {
-  static const bool value = false;
-};
+template<typename T, typename U> struct is_same       : public false_type {};
+template<typename T>             struct is_same<T, T> : public true_type {};
+
+/// \brief Metafunction that removes const qualification from a type.
+template <typename T> struct remove_const          { typedef T type; };
+template <typename T> struct remove_const<const T> { typedef T type; };
 
-template<typename T>
-struct is_same<T, T> {
-  static const bool value = true;
+/// \brief Metafunction that removes volatile qualification from a type.
+template <typename T> struct remove_volatile             { typedef T type; };
+template <typename T> struct remove_volatile<volatile T> { typedef T type; };
+
+/// \brief Metafunction that removes both const and volatile qualification from
+/// a type.
+template <typename T> struct remove_cv {
+  typedef typename remove_const<typename remove_volatile<T>::type>::type type;
 };
+
+/// \brief Helper to implement is_integral metafunction.
+template <typename T> struct is_integral_impl           : false_type {};
+template <> struct is_integral_impl<         bool>      : true_type {};
+template <> struct is_integral_impl<         char>      : true_type {};
+template <> struct is_integral_impl<  signed char>      : true_type {};
+template <> struct is_integral_impl<unsigned char>      : true_type {};
+template <> struct is_integral_impl<         wchar_t>   : true_type {};
+template <> struct is_integral_impl<         short>     : true_type {};
+template <> struct is_integral_impl<unsigned short>     : true_type {};
+template <> struct is_integral_impl<         int>       : true_type {};
+template <> struct is_integral_impl<unsigned int>       : true_type {};
+template <> struct is_integral_impl<         long>      : true_type {};
+template <> struct is_integral_impl<unsigned long>      : true_type {};
+template <> struct is_integral_impl<         long long> : true_type {};
+template <> struct is_integral_impl<unsigned long long> : true_type {};
+
+/// \brief Metafunction that determines whether the given type is an integral
+/// type.
+template <typename T>
+struct is_integral : is_integral_impl<T> {};
+
+/// \brief Metafunction that determines whether the given type is a pointer
+/// type.
+template <typename T> struct is_pointer : false_type {};
+template <typename T> struct is_pointer<T*> : true_type {};
+
   
 // enable_if_c - Enable/disable a template based on a metafunction
 template<bool Cond, typename T = void>