From ae27c7a4bc810b04daef024ff4647ee71a9547a8 Mon Sep 17 00:00:00 2001
From: Corentin Wallez <cwallez@chromium.org>
Date: Mon, 7 May 2018 17:33:48 -0400
Subject: [PATCH] Move HashCombine to HashUtils and make it work better on
 64bits

HashCombine will be used in more than just BindGroupLayout caching so we
extract it to a separate header. Add a better mixing constant for 64bit
systems.
---
 src/backend/BindGroupLayout.cpp | 24 ++------------
 src/common/CMakeLists.txt       |  1 +
 src/common/HashUtils.h          | 55 +++++++++++++++++++++++++++++++++
 3 files changed, 58 insertions(+), 22 deletions(-)
 create mode 100644 src/common/HashUtils.h

diff --git a/src/backend/BindGroupLayout.cpp b/src/backend/BindGroupLayout.cpp
index 64d4941470..e9f6704392 100644
--- a/src/backend/BindGroupLayout.cpp
+++ b/src/backend/BindGroupLayout.cpp
@@ -15,39 +15,19 @@
 #include "backend/BindGroupLayout.h"
 
 #include "backend/Device.h"
+#include "common/HashUtils.h"
 
 #include <functional>
 
 namespace backend {
 
     namespace {
-
-        // Workaround for Chrome's stdlib having a broken std::hash for enums and bitsets
-        template <typename T>
-        typename std::enable_if<std::is_enum<T>::value, size_t>::type Hash(T value) {
-            using Integral = typename nxt::UnderlyingType<T>::type;
-            return std::hash<Integral>()(static_cast<Integral>(value));
-        }
-
-        template <size_t N>
-        size_t Hash(const std::bitset<N>& value) {
-            static_assert(N <= sizeof(unsigned long long) * 8, "");
-            return std::hash<unsigned long long>()(value.to_ullong());
-        }
-
-        // TODO(cwallez@chromium.org): see if we can use boost's hash combined or some equivalent
-        // this currently assumes that size_t is 64 bits
-        void CombineHashes(size_t* h1, size_t h2) {
-            *h1 ^= (h2 << 7) + (h2 >> (sizeof(size_t) * 8 - 7)) + 0x304975;
-        }
-
         size_t HashBindingInfo(const BindGroupLayoutBase::LayoutBindingInfo& info) {
             size_t hash = Hash(info.mask);
 
             for (size_t binding = 0; binding < kMaxBindingsPerGroup; ++binding) {
                 if (info.mask[binding]) {
-                    CombineHashes(&hash, Hash(info.visibilities[binding]));
-                    CombineHashes(&hash, Hash(info.types[binding]));
+                    HashCombine(&hash, info.visibilities[binding], info.types[binding]);
                 }
             }
 
diff --git a/src/common/CMakeLists.txt b/src/common/CMakeLists.txt
index 7eed43c19e..91e8a03f50 100644
--- a/src/common/CMakeLists.txt
+++ b/src/common/CMakeLists.txt
@@ -21,6 +21,7 @@ list(APPEND COMMON_SOURCES
     ${COMMON_DIR}/Compiler.h
     ${COMMON_DIR}/DynamicLib.cpp
     ${COMMON_DIR}/DynamicLib.h
+    ${COMMON_DIR}/HashUtils.h
     ${COMMON_DIR}/Math.cpp
     ${COMMON_DIR}/Math.h
     ${COMMON_DIR}/Platform.h
diff --git a/src/common/HashUtils.h b/src/common/HashUtils.h
new file mode 100644
index 0000000000..34d15982c8
--- /dev/null
+++ b/src/common/HashUtils.h
@@ -0,0 +1,55 @@
+// Copyright 2018 The NXT Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef COMMON_HASHUTILS_H_
+#define COMMON_HASHUTILS_H_
+
+#include "common/Platform.h"
+
+#include <functional>
+
+// Wrapper around std::hash to make it a templated function instead of a functor. It is marginally
+// nicer, and avoids adding to the std namespace to add hashing of other types.
+template <typename T>
+size_t Hash(const T& value) {
+    return std::hash<T>()(value);
+}
+
+// When hashing sparse structures we want to iteratively build a hash value with only parts of the
+// data. HashCombine "hashes" together an existing hash and hashable values.
+//
+// Example usage to compute the hash of a mask and values corresponding to the mask:
+//
+//    size_t hash = Hash(mask):
+//    for (uint32_t i : IterateBitSet(mask)) { HashCombine(&hash, hashables[i]); }
+//    return hash;
+template <typename T>
+void HashCombine(size_t* hash, const T& value) {
+#if defined(NXT_PLATFORM_64_BIT)
+    const size_t offset = 0x9e3779b97f4a7c16;
+#elif defined(NXT_PLATFORM_32_BIT)
+    const size_t offset = 0x9e3779b9;
+#else
+#    error "Unsupported platform"
+#endif
+    *hash ^= Hash(value) + offset + (*hash << 6) + (*hash >> 2);
+}
+
+template <typename T, typename... Args>
+void HashCombine(size_t* hash, const T& value, const Args&... args) {
+    HashCombine(hash, value);
+    HashCombine(hash, args...);
+}
+
+#endif  // COMMON_HASHUTILS_H_