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//=- SyntheticCountsPropagation.cpp - Propagate function counts --*- C++ -*-=//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements a transformation that synthesizes entry counts for
// functions and attaches !prof metadata to functions with the synthesized
// counts. The presence of !prof metadata with counter name set to
// 'synthesized_function_entry_count' indicate that the value of the counter is
// an estimation of the likely execution count of the function. This transform
// is applied only in non PGO mode as functions get 'real' profile-based
// function entry counts in the PGO mode.
// The transformation works by first assigning some initial values to the entry
// counts of all functions and then doing a top-down traversal of the
// callgraph-scc to propagate the counts. For each function the set of callsites
// and their relative block frequency is gathered. The relative block frequency
// multiplied by the entry count of the caller and added to the callee's entry
// count. For non-trivial SCCs, the new counts are computed from the previous
// counts and updated in one shot.
#include "llvm/Transforms/IPO/SyntheticCountsPropagation.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/SyntheticCountsUtils.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CommandLine.h"
using namespace llvm;
using Scaled64 = ScaledNumber<uint64_t>;
using ProfileCount = Function::ProfileCount;
#define DEBUG_TYPE "synthetic-counts-propagation"
namespace llvm {
InitialSyntheticCount("initial-synthetic-count", cl::Hidden, cl::init(10),
cl::desc("Initial value of synthetic entry count"));
} // namespace llvm
/// Initial synthetic count assigned to inline functions.
static cl::opt<int> InlineSyntheticCount(
"inline-synthetic-count", cl::Hidden, cl::init(15),
cl::desc("Initial synthetic entry count for inline functions."));
/// Initial synthetic count assigned to cold functions.
static cl::opt<int> ColdSyntheticCount(
"cold-synthetic-count", cl::Hidden, cl::init(5),
cl::desc("Initial synthetic entry count for cold functions."));
// Assign initial synthetic entry counts to functions.
static void
initializeCounts(Module &M, function_ref<void(Function *, uint64_t)> SetCount) {
auto MayHaveIndirectCalls = [](Function &F) {
for (auto *U : F.users()) {
if (!isa<CallInst>(U) && !isa<InvokeInst>(U))
return true;
return false;
for (Function &F : M) {
uint64_t InitialCount = InitialSyntheticCount;
if (F.isDeclaration())
if (F.hasFnAttribute(Attribute::AlwaysInline) ||
F.hasFnAttribute(Attribute::InlineHint)) {
// Use a higher value for inline functions to account for the fact that
// these are usually beneficial to inline.
InitialCount = InlineSyntheticCount;
} else if (F.hasLocalLinkage() && !MayHaveIndirectCalls(F)) {
// Local functions without inline hints get counts only through
// propagation.
InitialCount = 0;
} else if (F.hasFnAttribute(Attribute::Cold) ||
F.hasFnAttribute(Attribute::NoInline)) {
// Use a lower value for noinline and cold functions.
InitialCount = ColdSyntheticCount;
SetCount(&F, InitialCount);
PreservedAnalyses SyntheticCountsPropagation::run(Module &M,
ModuleAnalysisManager &MAM) {
FunctionAnalysisManager &FAM =
DenseMap<Function *, Scaled64> Counts;
// Set initial entry counts.
M, [&](Function *F, uint64_t Count) { Counts[F] = Scaled64(Count, 0); });
// Edge includes information about the source. Hence ignore the first
// parameter.
auto GetCallSiteProfCount = [&](const CallGraphNode *,
const CallGraphNode::CallRecord &Edge) {
std::optional<Scaled64> Res;
if (!Edge.first)
return Res;
CallBase &CB = *cast<CallBase>(*Edge.first);
Function *Caller = CB.getCaller();
auto &BFI = FAM.getResult<BlockFrequencyAnalysis>(*Caller);
// Now compute the callsite count from relative frequency and
// entry count:
BasicBlock *CSBB = CB.getParent();
Scaled64 EntryFreq(BFI.getEntryFreq().getFrequency(), 0);
Scaled64 BBCount(BFI.getBlockFreq(CSBB).getFrequency(), 0);
BBCount /= EntryFreq;
BBCount *= Counts[Caller];
return std::optional<Scaled64>(BBCount);
CallGraph CG(M);
// Propgate the entry counts on the callgraph.
SyntheticCountsUtils<const CallGraph *>::propagate(
&CG, GetCallSiteProfCount, [&](const CallGraphNode *N, Scaled64 New) {
auto F = N->getFunction();
if (!F || F->isDeclaration())
Counts[F] += New;
// Set the counts as metadata.
for (auto Entry : Counts) {
Entry.second.template toInt<uint64_t>(), Function::PCT_Synthetic));
return PreservedAnalyses::all();