use serial prefix sum in lscsr (#37)

Author: meyerzinn

libgalois/include/galois/graphs/LS_LC_CSR_Graph.h

@@ -97,31 +97,19 @@ class LS_LC_CSR_Graph : private boost::noncopyable {
    * Prefix Sum utilities
    */
   std::vector<uint64_t> m_pfx_sum_cache;
-  static uint64_t transmute(const VertexMetadata& vertex_meta) {
-    return vertex_meta.degree();
-  }
-  static uint64_t scan_op(const VertexMetadata& p, const uint64_t& l) {
-    return p.degree() + l;
-  }
-  static uint64_t combiner(const uint64_t& f, const uint64_t& s) {
-    return f + s;
-  }
-  PrefixSum<VertexMetadata, uint64_t, transmute, scan_op, combiner,
-            CacheLinePaddedArr>
-      m_pfx{&m_vertices[0], &m_pfx_sum_cache[0]};
 
   alignas(hardware_destructive_interference_size)
       std::atomic<bool> m_prefix_valid = ATOMIC_VAR_INIT(false);
 
-  void resetPrefixSum() {
-    m_pfx_sum_cache.resize(m_vertices.size());
-    m_pfx.src = &m_vertices[0];
-    m_pfx.dst = &m_pfx_sum_cache[0];
-  }
+  void resetPrefixSum() { m_pfx_sum_cache.resize(m_vertices.size()); }
 
   // Compute the prefix sum using the two level method
   void computePrefixSum() {
-    m_pfx.computePrefixSum(m_vertices.size());
+    // todo: switch to parallel prefix sum when `galois::PrefixSum` is fixed
+    std::transform_inclusive_scan(
+        m_vertices.begin(), m_vertices.end(), m_pfx_sum_cache.begin(),
+        std::plus<uint64_t>(),
+        [](VertexMetadata const& v) { return v.degree(); }, 0ul);
     m_prefix_valid.store(true, std::memory_order_release);
   }
 

libgalois/test/graph-compile-lscsr.cpp

@@ -116,5 +116,14 @@ int main() {
   // ...
   GALOIS_ASSERT(g[8] == 7);
 
+  uint64_t num_vertices = (1 << 22) + 67;
+  galois::graphs::LS_LC_CSR_Graph<void, void> big(num_vertices);
+  for (uint64_t i = 0; i < num_vertices; ++i) {
+    big.addEdgesTopologyOnly(i, {(i + 1) % num_vertices});
+  }
+  for (uint64_t i = 0; i < num_vertices; ++i) {
+    GALOIS_ASSERT(big[i] == i + 1);
+  }
+
   return 0;
 }