lib/LTO/SummaryBasedOptimizations.cpp

   1 //==-SummaryBasedOptimizations.cpp - Optimizations based on ThinLTO summary-==//
   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 implements optimizations that are based on the module summaries.
  10 // These optimizations are performed during the thinlink phase of the
  11 // compilation.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/LTO/SummaryBasedOptimizations.h"
  16 #include "llvm/Analysis/SyntheticCountsUtils.h"
  17 #include "llvm/IR/ModuleSummaryIndex.h"
  18
  19 using namespace llvm;
  20
  21 static cl::opt<bool> ThinLTOSynthesizeEntryCounts(
  22     "thinlto-synthesize-entry-counts", cl::init(false), cl::Hidden,
  23     cl::desc("Synthesize entry counts based on the summary"));
  24
  25 extern cl::opt<int> InitialSyntheticCount;
  26
  27 static void initializeCounts(ModuleSummaryIndex &Index) {
  28   auto Root = Index.calculateCallGraphRoot();
  29   // Root is a fake node. All its successors are the actual roots of the
  30   // callgraph.
  31   // FIXME: This initializes the entry counts of only the root nodes. This makes
  32   // sense when compiling a binary with ThinLTO, but for libraries any of the
  33   // non-root nodes could be called from outside.
  34   for (auto &C : Root.calls()) {
  35     auto &V = C.first;
  36     for (auto &GVS : V.getSummaryList()) {
  37       auto S = GVS.get()->getBaseObject();
  38       auto *F = cast<FunctionSummary>(S);
  39       F->setEntryCount(InitialSyntheticCount);
  40     }
  41   }
  42 }
  43
  44 void llvm::computeSyntheticCounts(ModuleSummaryIndex &Index) {
  45   if (!ThinLTOSynthesizeEntryCounts)
  46     return;
  47
  48   using Scaled64 = ScaledNumber<uint64_t>;
  49   initializeCounts(Index);
  50   auto GetCallSiteRelFreq = [](FunctionSummary::EdgeTy &Edge) {
  51     return Scaled64(Edge.second.RelBlockFreq, -CalleeInfo::ScaleShift);
  52   };
  53   auto GetEntryCount = [](ValueInfo V) {
  54     if (V.getSummaryList().size()) {
  55       auto S = V.getSummaryList().front().get()->getBaseObject();
  56       auto *F = cast<FunctionSummary>(S);
  57       return F->entryCount();
  58     } else {
  59       return UINT64_C(0);
  60     }
  61   };
  62   auto AddToEntryCount = [](ValueInfo V, Scaled64 New) {
  63     if (!V.getSummaryList().size())
  64       return;
  65     for (auto &GVS : V.getSummaryList()) {
  66       auto S = GVS.get()->getBaseObject();
  67       auto *F = cast<FunctionSummary>(S);
  68       F->setEntryCount(
  69           SaturatingAdd(F->entryCount(), New.template toInt<uint64_t>()));
  70     }
  71   };
  72
  73   auto GetProfileCount = [&](ValueInfo V, FunctionSummary::EdgeTy &Edge) {
  74     auto RelFreq = GetCallSiteRelFreq(Edge);
  75     Scaled64 EC(GetEntryCount(V), 0);
  76     return RelFreq * EC;
  77   };
  78   // After initializing the counts in initializeCounts above, the counts have to
  79   // be propagated across the combined callgraph.
  80   // SyntheticCountsUtils::propagate takes care of this propagation on any
  81   // callgraph that specialized GraphTraits.
  82   SyntheticCountsUtils<ModuleSummaryIndex *>::propagate(&Index, GetProfileCount,
  83                                                         AddToEntryCount);
  84   Index.setHasSyntheticEntryCounts();
  85 }