initial commit, 4.5 stable

thirdparty/embree/common/algorithms/parallel_prefix_sum.h

@@ -0,0 +1,85 @@
+// Copyright 2009-2021 Intel Corporation
+// SPDX-License-Identifier: Apache-2.0
+
+#pragma once
+
+#include "parallel_for.h"
+
+namespace embree
+{
+  template<typename Value>
+    struct ParallelPrefixSumState 
+  {
+    enum { MAX_TASKS = 64 };
+    Value counts[MAX_TASKS];
+    Value sums  [MAX_TASKS];
+  };
+
+  template<typename Index, typename Value, typename Func, typename Reduction>
+    __forceinline Value parallel_prefix_sum( ParallelPrefixSumState<Value>& state, Index first, Index last, Index minStepSize, const Value& identity, const Func& func, const Reduction& reduction)
+  {
+    /* calculate number of tasks to use */
+    const size_t numThreads = TaskScheduler::threadCount();
+    const size_t numBlocks  = (last-first+minStepSize-1)/minStepSize;
+    const size_t taskCount  = min(numThreads,numBlocks,size_t(ParallelPrefixSumState<Value>::MAX_TASKS));
+
+    /* perform parallel prefix sum */
+    parallel_for(taskCount, [&](const size_t taskIndex)
+    {
+      const size_t i0 = first+(taskIndex+0)*(last-first)/taskCount;
+      const size_t i1 = first+(taskIndex+1)*(last-first)/taskCount;
+      state.counts[taskIndex] = func(range<size_t>(i0,i1),state.sums[taskIndex]);
+    });
+
+    /* calculate prefix sum */
+    Value sum=identity;
+    for (size_t i=0; i<taskCount; i++) 
+    {
+      const Value c = state.counts[i];
+      state.sums[i] = sum;
+      sum=reduction(sum,c);
+    }
+
+    return sum;
+  }
+
+  /*! parallel calculation of prefix sums */
+  template<typename SrcArray, typename DstArray, typename Value, typename Add>
+    __forceinline Value parallel_prefix_sum(const SrcArray& src, DstArray& dst, size_t N, const Value& identity, const Add& add, const size_t SINGLE_THREAD_THRESHOLD = 4096) 
+  {
+    /* perform single threaded prefix operation for small N */
+    if (N < SINGLE_THREAD_THRESHOLD) 
+    {
+      Value sum=identity;
+      for (size_t i=0; i<N; sum=add(sum,src[i++])) dst[i] = sum;
+      return sum;
+    }
+    
+    /* perform parallel prefix operation for large N */
+    else 
+    {
+      ParallelPrefixSumState<Value> state;
+      
+      /* initial run just sets up start values for subtasks */
+      parallel_prefix_sum( state, size_t(0), size_t(N), size_t(1024), identity, [&](const range<size_t>& r, const Value& sum) -> Value {
+          
+          Value s = identity;
+          for (size_t i=r.begin(); i<r.end(); i++) s = add(s,src[i]);
+          return s;
+          
+        }, add);
+      
+      /* final run calculates prefix sum */
+      return parallel_prefix_sum( state, size_t(0), size_t(N), size_t(1024), identity, [&](const range<size_t>& r, const Value& sum) -> Value {
+          
+          Value s = identity;
+          for (size_t i=r.begin(); i<r.end(); i++) {
+            dst[i] = add(sum,s);
+            s = add(s,src[i]);
+          }
+          return s;
+          
+        }, add);
+    }
+  }
+}