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 #include "llvm/Support/CommandLine.h"
19 
20 using namespace llvm;
21 
22 static cl::opt<bool> ThinLTOSynthesizeEntryCounts(
23     "thinlto-synthesize-entry-counts", cl::init(false), cl::Hidden,
24     cl::desc("Synthesize entry counts based on the summary"));
25 
26 namespace llvm {
27 extern cl::opt<int> InitialSyntheticCount;
28 }
29 
30 static void initializeCounts(ModuleSummaryIndex &Index) {
31   auto Root = Index.calculateCallGraphRoot();
32   // Root is a fake node. All its successors are the actual roots of the
33   // callgraph.
34   // FIXME: This initializes the entry counts of only the root nodes. This makes
35   // sense when compiling a binary with ThinLTO, but for libraries any of the
36   // non-root nodes could be called from outside.
37   for (auto &C : Root.calls()) {
38     auto &V = C.first;
39     for (auto &GVS : V.getSummaryList()) {
40       auto S = GVS.get()->getBaseObject();
41       auto *F = cast<FunctionSummary>(S);
42       F->setEntryCount(InitialSyntheticCount);
43     }
44   }
45 }
46 
47 void llvm::computeSyntheticCounts(ModuleSummaryIndex &Index) {
48   if (!ThinLTOSynthesizeEntryCounts)
49     return;
50 
51   using Scaled64 = ScaledNumber<uint64_t>;
52   initializeCounts(Index);
53   auto GetCallSiteRelFreq = [](FunctionSummary::EdgeTy &Edge) {
54     return Scaled64(Edge.second.RelBlockFreq, -CalleeInfo::ScaleShift);
55   };
56   auto GetEntryCount = [](ValueInfo V) {
57     if (V.getSummaryList().size()) {
58       auto S = V.getSummaryList().front().get()->getBaseObject();
59       auto *F = cast<FunctionSummary>(S);
60       return F->entryCount();
61     } else {
62       return UINT64_C(0);
63     }
64   };
65   auto AddToEntryCount = [](ValueInfo V, Scaled64 New) {
66     if (!V.getSummaryList().size())
67       return;
68     for (auto &GVS : V.getSummaryList()) {
69       auto S = GVS.get()->getBaseObject();
70       auto *F = cast<FunctionSummary>(S);
71       F->setEntryCount(
72           SaturatingAdd(F->entryCount(), New.template toInt<uint64_t>()));
73     }
74   };
75 
76   auto GetProfileCount = [&](ValueInfo V, FunctionSummary::EdgeTy &Edge) {
77     auto RelFreq = GetCallSiteRelFreq(Edge);
78     Scaled64 EC(GetEntryCount(V), 0);
79     return RelFreq * EC;
80   };
81   // After initializing the counts in initializeCounts above, the counts have to
82   // be propagated across the combined callgraph.
83   // SyntheticCountsUtils::propagate takes care of this propagation on any
84   // callgraph that specialized GraphTraits.
85   SyntheticCountsUtils<ModuleSummaryIndex *>::propagate(&Index, GetProfileCount,
86                                                         AddToEntryCount);
87   Index.setHasSyntheticEntryCounts();
88 }
89