1 //=- SyntheticCountsPropagation.cpp - Propagate function counts --*- C++ -*-=//
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 a transformation that synthesizes entry counts for
10 // functions and attaches !prof metadata to functions with the synthesized
11 // counts. The presence of !prof metadata with counter name set to
12 // 'synthesized_function_entry_count' indicate that the value of the counter is
13 // an estimation of the likely execution count of the function. This transform
14 // is applied only in non PGO mode as functions get 'real' profile-based
15 // function entry counts in the PGO mode.
16 //
17 // The transformation works by first assigning some initial values to the entry
18 // counts of all functions and then doing a top-down traversal of the
19 // callgraph-scc to propagate the counts. For each function the set of callsites
20 // and their relative block frequency is gathered. The relative block frequency
21 // multiplied by the entry count of the caller and added to the callee's entry
22 // count. For non-trivial SCCs, the new counts are computed from the previous
23 // counts and updated in one shot.
24 //
25 //===----------------------------------------------------------------------===//
26 
27 #include "llvm/Transforms/IPO/SyntheticCountsPropagation.h"
28 #include "llvm/Analysis/BlockFrequencyInfo.h"
29 #include "llvm/Analysis/CallGraph.h"
30 #include "llvm/Analysis/SyntheticCountsUtils.h"
31 #include "llvm/IR/Function.h"
32 #include "llvm/IR/Instructions.h"
33 #include "llvm/IR/Module.h"
34 #include "llvm/Support/CommandLine.h"
35 
36 using namespace llvm;
37 using Scaled64 = ScaledNumber<uint64_t>;
38 using ProfileCount = Function::ProfileCount;
39 
40 #define DEBUG_TYPE "synthetic-counts-propagation"
41 
42 namespace llvm {
43 cl::opt<int>
44     InitialSyntheticCount("initial-synthetic-count", cl::Hidden, cl::init(10),
45                           cl::desc("Initial value of synthetic entry count"));
46 } // namespace llvm
47 
48 /// Initial synthetic count assigned to inline functions.
49 static cl::opt<int> InlineSyntheticCount(
50     "inline-synthetic-count", cl::Hidden, cl::init(15),
51     cl::desc("Initial synthetic entry count for inline functions."));
52 
53 /// Initial synthetic count assigned to cold functions.
54 static cl::opt<int> ColdSyntheticCount(
55     "cold-synthetic-count", cl::Hidden, cl::init(5),
56     cl::desc("Initial synthetic entry count for cold functions."));
57 
58 // Assign initial synthetic entry counts to functions.
59 static void
60 initializeCounts(Module &M, function_ref<void(Function *, uint64_t)> SetCount) {
61   auto MayHaveIndirectCalls = [](Function &F) {
62     for (auto *U : F.users()) {
63       if (!isa<CallInst>(U) && !isa<InvokeInst>(U))
64         return true;
65     }
66     return false;
67   };
68 
69   for (Function &F : M) {
70     uint64_t InitialCount = InitialSyntheticCount;
71     if (F.isDeclaration())
72       continue;
73     if (F.hasFnAttribute(Attribute::AlwaysInline) ||
74         F.hasFnAttribute(Attribute::InlineHint)) {
75       // Use a higher value for inline functions to account for the fact that
76       // these are usually beneficial to inline.
77       InitialCount = InlineSyntheticCount;
78     } else if (F.hasLocalLinkage() && !MayHaveIndirectCalls(F)) {
79       // Local functions without inline hints get counts only through
80       // propagation.
81       InitialCount = 0;
82     } else if (F.hasFnAttribute(Attribute::Cold) ||
83                F.hasFnAttribute(Attribute::NoInline)) {
84       // Use a lower value for noinline and cold functions.
85       InitialCount = ColdSyntheticCount;
86     }
87     SetCount(&F, InitialCount);
88   }
89 }
90 
91 PreservedAnalyses SyntheticCountsPropagation::run(Module &M,
92                                                   ModuleAnalysisManager &MAM) {
93   FunctionAnalysisManager &FAM =
94       MAM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
95   DenseMap<Function *, Scaled64> Counts;
96   // Set initial entry counts.
97   initializeCounts(
98       M, [&](Function *F, uint64_t Count) { Counts[F] = Scaled64(Count, 0); });
99 
100   // Edge includes information about the source. Hence ignore the first
101   // parameter.
102   auto GetCallSiteProfCount = [&](const CallGraphNode *,
103                                   const CallGraphNode::CallRecord &Edge) {
104     Optional<Scaled64> Res = None;
105     if (!Edge.first)
106       return Res;
107     CallBase &CB = *cast<CallBase>(*Edge.first);
108     Function *Caller = CB.getCaller();
109     auto &BFI = FAM.getResult<BlockFrequencyAnalysis>(*Caller);
110 
111     // Now compute the callsite count from relative frequency and
112     // entry count:
113     BasicBlock *CSBB = CB.getParent();
114     Scaled64 EntryFreq(BFI.getEntryFreq(), 0);
115     Scaled64 BBCount(BFI.getBlockFreq(CSBB).getFrequency(), 0);
116     BBCount /= EntryFreq;
117     BBCount *= Counts[Caller];
118     return Optional<Scaled64>(BBCount);
119   };
120 
121   CallGraph CG(M);
122   // Propgate the entry counts on the callgraph.
123   SyntheticCountsUtils<const CallGraph *>::propagate(
124       &CG, GetCallSiteProfCount, [&](const CallGraphNode *N, Scaled64 New) {
125         auto F = N->getFunction();
126         if (!F || F->isDeclaration())
127           return;
128 
129         Counts[F] += New;
130       });
131 
132   // Set the counts as metadata.
133   for (auto Entry : Counts) {
134     Entry.first->setEntryCount(ProfileCount(
135         Entry.second.template toInt<uint64_t>(), Function::PCT_Synthetic));
136   }
137 
138   return PreservedAnalyses::all();
139 }
140