From a521ace63ad1fa624aa692617c48b287dd2cfa5d Mon Sep 17 00:00:00 2001
From: dzhwinter <dzhwinter@gmail.com>
Date: Sun, 24 Dec 2017 03:03:58 -0800
Subject: [PATCH] "remove hash combine"

---
 paddle/framework/op_kernel_type.h | 32 +++++++++++--------------------
 paddle/platform/device_context.h  |  4 ++--
 2 files changed, 13 insertions(+), 23 deletions(-)

diff --git a/paddle/framework/op_kernel_type.h b/paddle/framework/op_kernel_type.h
index 45bbbe580..a1dea0d9d 100644
--- a/paddle/framework/op_kernel_type.h
+++ b/paddle/framework/op_kernel_type.h
@@ -22,33 +22,23 @@ limitations under the License. */
 namespace paddle {
 namespace framework {
 
-/*
-Refer to https://stackoverflow.com/questions/35985960/
-c-why-is-boosthash-combine-the-best-way-to-combine-hash-values
-*/
-template <class T>
-inline void HashCombine(const T& v, std::size_t* seed) {
-  std::hash<T> hasher;
-  *seed ^= hasher(v) + 0x9e3779b9 + (*seed << 6) + (*seed >> 2);
-}
-
 struct OpKernelType {
   struct Hash {
     size_t operator()(const OpKernelType& key) const {
-      int place = key.place_.which();
-      int data_type = static_cast<int>(key.data_type_);
-      int data_layout = static_cast<int>(key.data_layout_);
-      int library_type = static_cast<int>(key.library_type_);
-
-      size_t seed = 0;
-      HashCombine(place, &seed);
-      HashCombine(data_type, &seed);
-      HashCombine(data_layout, &seed);
-      HashCombine(library_type, &seed);
-      return seed;
+      int place = key.place_.which() + (1 << LEFT_SHIFT);
+      int data_type =
+          static_cast<int>(key.data_type_) + (1 << (LEFT_SHIFT + 1));
+      int data_layout =
+          static_cast<int>(key.data_layout_) + (1 << (LEFT_SHIFT + 2));
+      int library_type =
+          static_cast<int>(key.library_type_) + (1 << (LEFT_SHIFT + 3));
+      std::hash<int> hasher;
+      return hasher(place + data_type + data_layout + library_type);
     }
   };
 
+  // place, data_type, library_type kinds less than 2^8
+  constexpr static int LEFT_SHIFT = 8;
   proto::DataType data_type_;
   DataLayout data_layout_;
   platform::Place place_;
diff --git a/paddle/platform/device_context.h b/paddle/platform/device_context.h
index 1d46ce5c7..9b958f7c9 100644
--- a/paddle/platform/device_context.h
+++ b/paddle/platform/device_context.h
@@ -137,11 +137,11 @@ class DeviceContextPool {
 
  private:
   static DeviceContextPool* pool;
+  constexpr static int LEFT_SHIFT = 8;
   struct Hash {
     std::hash<int> hash_;
     size_t operator()(const platform::Place& place) const {
-      int pre_hash = place.which()
-                     << (sizeof(int) * 8 - NUM_PLACE_TYPE_LIMIT_IN_BIT);
+      int pre_hash = place.which() + (1 << LEFT_SHIFT);
       if (platform::is_gpu_place(place)) {
         pre_hash += boost::get<platform::GPUPlace>(place).GetDeviceId();
       }
-- 
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