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