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
36using namespace llvm;
37using Scaled64 = ScaledNumber<uint64_t>;
38using ProfileCount = Function::ProfileCount;
39
40#define DEBUG_TYPE "synthetic-counts-propagation"
41
42namespace llvm {
43cl::opt<int>
44 InitialSyntheticCount("initial-synthetic-count", cl::Hidden, cl::init(Val: 10),
45 cl::desc("Initial value of synthetic entry count"));
46} // namespace llvm
47
48/// Initial synthetic count assigned to inline functions.
49static cl::opt<int> InlineSyntheticCount(
50 "inline-synthetic-count", cl::Hidden, cl::init(Val: 15),
51 cl::desc("Initial synthetic entry count for inline functions."));
52
53/// Initial synthetic count assigned to cold functions.
54static cl::opt<int> ColdSyntheticCount(
55 "cold-synthetic-count", cl::Hidden, cl::init(Val: 5),
56 cl::desc("Initial synthetic entry count for cold functions."));
57
58// Assign initial synthetic entry counts to functions.
59static void
60initializeCounts(Module &M, function_ref<void(Function *, uint64_t)> SetCount) {
61 auto MayHaveIndirectCalls = [](Function &F) {
62 for (auto *U : F.users()) {
63 if (!isa<CallInst>(Val: U) && !isa<InvokeInst>(Val: 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(Kind: Attribute::AlwaysInline) ||
74 F.hasFnAttribute(Kind: 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(Kind: Attribute::Cold) ||
83 F.hasFnAttribute(Kind: Attribute::NoInline)) {
84 // Use a lower value for noinline and cold functions.
85 InitialCount = ColdSyntheticCount;
86 }
87 SetCount(&F, InitialCount);
88 }
89}
90
91PreservedAnalyses SyntheticCountsPropagation::run(Module &M,
92 ModuleAnalysisManager &MAM) {
93 FunctionAnalysisManager &FAM =
94 MAM.getResult<FunctionAnalysisManagerModuleProxy>(IR&: M).getManager();
95 DenseMap<Function *, Scaled64> Counts;
96 // Set initial entry counts.
97 initializeCounts(
98 M, SetCount: [&](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 std::optional<Scaled64> Res;
105 if (!Edge.first)
106 return Res;
107 CallBase &CB = *cast<CallBase>(Val: *Edge.first);
108 Function *Caller = CB.getCaller();
109 auto &BFI = FAM.getResult<BlockFrequencyAnalysis>(IR&: *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().getFrequency(), 0);
115 Scaled64 BBCount(BFI.getBlockFreq(BB: CSBB).getFrequency(), 0);
116 BBCount /= EntryFreq;
117 BBCount *= Counts[Caller];
118 return std::optional<Scaled64>(BBCount);
119 };
120
121 CallGraph CG(M);
122 // Propgate the entry counts on the callgraph.
123 SyntheticCountsUtils<const CallGraph *>::propagate(
124 CG: &CG, GetProfCount: GetCallSiteProfCount, AddCount: [&](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(Count: ProfileCount(
135 Entry.second.template toInt<uint64_t>(), Function::PCT_Synthetic));
136 }
137
138 return PreservedAnalyses::all();
139}
140