llvm/lib/Analysis/SyntheticCountsUtils.cpp

   1 //===--- SyntheticCountsUtils.cpp - synthetic counts propagation utils ---===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file defines utilities for propagating synthetic counts.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "llvm/Analysis/SyntheticCountsUtils.h"
  14 #include "llvm/ADT/DenseSet.h"
  15 #include "llvm/ADT/SCCIterator.h"
  16 #include "llvm/Analysis/CallGraph.h"
  17 #include "llvm/IR/ModuleSummaryIndex.h"
  18
  19 using namespace llvm;
  20
  21 // Given an SCC, propagate entry counts along the edge of the SCC nodes.
  22 template <typename CallGraphType>
  23 void SyntheticCountsUtils<CallGraphType>::propagateFromSCC(
  24     const SccTy &SCC, GetProfCountTy GetProfCount, AddCountTy AddCount) {
  25
  26   DenseSet<NodeRef> SCCNodes;
  27   SmallVector<std::pair<NodeRef, EdgeRef>, 8> SCCEdges, NonSCCEdges;
  28
  29   for (auto &Node : SCC)
  30     SCCNodes.insert(Node);
  31
  32   // Partition the edges coming out of the SCC into those whose destination is
  33   // in the SCC and the rest.
  34   for (const auto &Node : SCCNodes) {
  35     for (auto &E : children_edges<CallGraphType>(Node)) {
  36       if (SCCNodes.count(CGT::edge_dest(E)))
  37         SCCEdges.emplace_back(Node, E);
  38       else
  39         NonSCCEdges.emplace_back(Node, E);
  40     }
  41   }
  42
  43   // For nodes in the same SCC, update the counts in two steps:
  44   // 1. Compute the additional count for each node by propagating the counts
  45   // along all incoming edges to the node that originate from within the same
  46   // SCC and summing them up.
  47   // 2. Add the additional counts to the nodes in the SCC.
  48   // This ensures that the order of
  49   // traversal of nodes within the SCC doesn't affect the final result.
  50
  51   DenseMap<NodeRef, Scaled64> AdditionalCounts;
  52   for (auto &E : SCCEdges) {
  53     auto OptProfCount = GetProfCount(E.first, E.second);
  54     if (!OptProfCount)
  55       continue;
  56     auto Callee = CGT::edge_dest(E.second);
  57     AdditionalCounts[Callee] += *OptProfCount;
  58   }
  59
  60   // Update the counts for the nodes in the SCC.
  61   for (auto &Entry : AdditionalCounts)
  62     AddCount(Entry.first, Entry.second);
  63
  64   // Now update the counts for nodes outside the SCC.
  65   for (auto &E : NonSCCEdges) {
  66     auto OptProfCount = GetProfCount(E.first, E.second);
  67     if (!OptProfCount)
  68       continue;
  69     auto Callee = CGT::edge_dest(E.second);
  70     AddCount(Callee, *OptProfCount);
  71   }
  72 }
  73
  74 /// Propgate synthetic entry counts on a callgraph \p CG.
  75 ///
  76 /// This performs a reverse post-order traversal of the callgraph SCC. For each
  77 /// SCC, it first propagates the entry counts to the nodes within the SCC
  78 /// through call edges and updates them in one shot. Then the entry counts are
  79 /// propagated to nodes outside the SCC. This requires \p GraphTraits
  80 /// to have a specialization for \p CallGraphType.
  81
  82 template <typename CallGraphType>
  83 void SyntheticCountsUtils<CallGraphType>::propagate(const CallGraphType &CG,
  84                                                     GetProfCountTy GetProfCount,
  85                                                     AddCountTy AddCount) {
  86   std::vector<SccTy> SCCs;
  87
  88   // Collect all the SCCs.
  89   for (auto I = scc_begin(CG); !I.isAtEnd(); ++I)
  90     SCCs.push_back(*I);
  91
  92   // The callgraph-scc needs to be visited in top-down order for propagation.
  93   // The scc iterator returns the scc in bottom-up order, so reverse the SCCs
  94   // and call propagateFromSCC.
  95   for (auto &SCC : reverse(SCCs))
  96     propagateFromSCC(SCC, GetProfCount, AddCount);
  97 }
  98
  99 template class llvm::SyntheticCountsUtils<const CallGraph *>;
 100 template class llvm::SyntheticCountsUtils<ModuleSummaryIndex *>;