1//===-- MachineFunctionSplitter.cpp - Split machine functions //-----------===//
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// \file
10// Uses profile information to split out cold blocks.
11//
12// This pass splits out cold machine basic blocks from the parent function. This
13// implementation leverages the basic block section framework. Blocks marked
14// cold by this pass are grouped together in a separate section prefixed with
15// ".text.unlikely.*". The linker can then group these together as a cold
16// section. The split part of the function is a contiguous region identified by
17// the symbol "foo.cold". Grouping all cold blocks across functions together
18// decreases fragmentation and improves icache and itlb utilization. Note that
19// the overall changes to the binary size are negligible; only a small number of
20// additional jump instructions may be introduced.
21//
22// For the original RFC of this pass please see
23// https://groups.google.com/d/msg/llvm-dev/RUegaMg-iqc/wFAVxa6fCgAJ
24//===----------------------------------------------------------------------===//
25
26#include "llvm/ADT/SmallVector.h"
27#include "llvm/Analysis/BranchProbabilityInfo.h"
28#include "llvm/Analysis/EHUtils.h"
29#include "llvm/Analysis/ProfileSummaryInfo.h"
30#include "llvm/CodeGen/BasicBlockSectionUtils.h"
31#include "llvm/CodeGen/BasicBlockSectionsProfileReader.h"
32#include "llvm/CodeGen/MachineBasicBlock.h"
33#include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
34#include "llvm/CodeGen/MachineFunction.h"
35#include "llvm/CodeGen/MachineFunctionPass.h"
36#include "llvm/CodeGen/MachineModuleInfo.h"
37#include "llvm/CodeGen/Passes.h"
38#include "llvm/CodeGen/TargetInstrInfo.h"
39#include "llvm/IR/Function.h"
40#include "llvm/InitializePasses.h"
41#include "llvm/Support/CommandLine.h"
42#include <optional>
43
44using namespace llvm;
45
46// FIXME: This cutoff value is CPU dependent and should be moved to
47// TargetTransformInfo once we consider enabling this on other platforms.
48// The value is expressed as a ProfileSummaryInfo integer percentile cutoff.
49// Defaults to 999950, i.e. all blocks colder than 99.995 percentile are split.
50// The default was empirically determined to be optimal when considering cutoff
51// values between 99%-ile to 100%-ile with respect to iTLB and icache metrics on
52// Intel CPUs.
53static cl::opt<unsigned>
54 PercentileCutoff("mfs-psi-cutoff",
55 cl::desc("Percentile profile summary cutoff used to "
56 "determine cold blocks. Unused if set to zero."),
57 cl::init(Val: 999950), cl::Hidden);
58
59static cl::opt<unsigned> ColdCountThreshold(
60 "mfs-count-threshold",
61 cl::desc(
62 "Minimum number of times a block must be executed to be retained."),
63 cl::init(Val: 1), cl::Hidden);
64
65static cl::opt<bool> SplitAllEHCode(
66 "mfs-split-ehcode",
67 cl::desc("Splits all EH code and it's descendants by default."),
68 cl::init(Val: false), cl::Hidden);
69
70namespace {
71
72class MachineFunctionSplitter : public MachineFunctionPass {
73public:
74 static char ID;
75 MachineFunctionSplitter() : MachineFunctionPass(ID) {
76 initializeMachineFunctionSplitterPass(*PassRegistry::getPassRegistry());
77 }
78
79 StringRef getPassName() const override {
80 return "Machine Function Splitter Transformation";
81 }
82
83 void getAnalysisUsage(AnalysisUsage &AU) const override;
84
85 bool runOnMachineFunction(MachineFunction &F) override;
86};
87} // end anonymous namespace
88
89/// setDescendantEHBlocksCold - This splits all EH pads and blocks reachable
90/// only by EH pad as cold. This will help mark EH pads statically cold
91/// instead of relying on profile data.
92static void setDescendantEHBlocksCold(MachineFunction &MF) {
93 DenseSet<MachineBasicBlock *> EHBlocks;
94 computeEHOnlyBlocks(F&: MF, EHBlocks);
95 for (auto Block : EHBlocks) {
96 Block->setSectionID(MBBSectionID::ColdSectionID);
97 }
98}
99
100static void finishAdjustingBasicBlocksAndLandingPads(MachineFunction &MF) {
101 auto Comparator = [](const MachineBasicBlock &X, const MachineBasicBlock &Y) {
102 return X.getSectionID().Type < Y.getSectionID().Type;
103 };
104 llvm::sortBasicBlocksAndUpdateBranches(MF, MBBCmp: Comparator);
105 llvm::avoidZeroOffsetLandingPad(MF);
106}
107
108static bool isColdBlock(const MachineBasicBlock &MBB,
109 const MachineBlockFrequencyInfo *MBFI,
110 ProfileSummaryInfo *PSI) {
111 std::optional<uint64_t> Count = MBFI->getBlockProfileCount(MBB: &MBB);
112 // For instrumentation profiles and sample profiles, we use different ways
113 // to judge whether a block is cold and should be split.
114 if (PSI->hasInstrumentationProfile() || PSI->hasCSInstrumentationProfile()) {
115 // If using instrument profile, which is deemed "accurate", no count means
116 // cold.
117 if (!Count)
118 return true;
119 if (PercentileCutoff > 0)
120 return PSI->isColdCountNthPercentile(PercentileCutoff, C: *Count);
121 // Fallthrough to end of function.
122 } else if (PSI->hasSampleProfile()) {
123 // For sample profile, no count means "do not judege coldness".
124 if (!Count)
125 return false;
126 }
127
128 return (*Count < ColdCountThreshold);
129}
130
131bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
132 // Do not split functions when -basic-block-sections=all is specified.
133 if (MF.getTarget().getBBSectionsType() == llvm::BasicBlockSection::All)
134 return false;
135 // We target functions with profile data. Static information in the form
136 // of exception handling code may be split to cold if user passes the
137 // mfs-split-ehcode flag.
138 bool UseProfileData = MF.getFunction().hasProfileData();
139 if (!UseProfileData && !SplitAllEHCode)
140 return false;
141
142 const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
143 if (!TII.isFunctionSafeToSplit(MF))
144 return false;
145
146 // Do not split functions with BasicBlockSections profiles as they will
147 // be split by the BasicBlockSections pass.
148 auto BBSectionsProfile =
149 getAnalysisIfAvailable<BasicBlockSectionsProfileReaderWrapperPass>();
150 if (BBSectionsProfile != nullptr &&
151 BBSectionsProfile->getBBSPR().isFunctionHot(FuncName: MF.getName()))
152 return false;
153
154 // Renumbering blocks here preserves the order of the blocks as
155 // sortBasicBlocksAndUpdateBranches uses the numeric identifier to sort
156 // blocks. Preserving the order of blocks is essential to retaining decisions
157 // made by prior passes such as MachineBlockPlacement.
158 MF.RenumberBlocks();
159 MF.setBBSectionsType(BasicBlockSection::Preset);
160
161 MachineBlockFrequencyInfo *MBFI = nullptr;
162 ProfileSummaryInfo *PSI = nullptr;
163 if (UseProfileData) {
164 MBFI = &getAnalysis<MachineBlockFrequencyInfoWrapperPass>().getMBFI();
165 PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
166 // If we don't have a good profile (sample profile is not deemed
167 // as a "good profile") and the function is not hot, then early
168 // return. (Because we can only trust hot functions when profile
169 // quality is not good.)
170 if (PSI->hasSampleProfile() && !PSI->isFunctionHotInCallGraph(F: &MF, BFI&: *MBFI)) {
171 // Split all EH code and it's descendant statically by default.
172 if (SplitAllEHCode)
173 setDescendantEHBlocksCold(MF);
174 finishAdjustingBasicBlocksAndLandingPads(MF);
175 return true;
176 }
177 }
178
179 SmallVector<MachineBasicBlock *, 2> LandingPads;
180 for (auto &MBB : MF) {
181 if (MBB.isEntryBlock())
182 continue;
183
184 if (MBB.isEHPad())
185 LandingPads.push_back(Elt: &MBB);
186 else if (UseProfileData && isColdBlock(MBB, MBFI, PSI) &&
187 TII.isMBBSafeToSplitToCold(MBB) && !SplitAllEHCode)
188 MBB.setSectionID(MBBSectionID::ColdSectionID);
189 }
190
191 // Split all EH code and it's descendant statically by default.
192 if (SplitAllEHCode)
193 setDescendantEHBlocksCold(MF);
194 // We only split out eh pads if all of them are cold.
195 else {
196 // Here we have UseProfileData == true.
197 bool HasHotLandingPads = false;
198 for (const MachineBasicBlock *LP : LandingPads) {
199 if (!isColdBlock(MBB: *LP, MBFI, PSI) || !TII.isMBBSafeToSplitToCold(MBB: *LP))
200 HasHotLandingPads = true;
201 }
202 if (!HasHotLandingPads) {
203 for (MachineBasicBlock *LP : LandingPads)
204 LP->setSectionID(MBBSectionID::ColdSectionID);
205 }
206 }
207
208 finishAdjustingBasicBlocksAndLandingPads(MF);
209 return true;
210}
211
212void MachineFunctionSplitter::getAnalysisUsage(AnalysisUsage &AU) const {
213 AU.addRequired<MachineModuleInfoWrapperPass>();
214 AU.addRequired<MachineBlockFrequencyInfoWrapperPass>();
215 AU.addRequired<ProfileSummaryInfoWrapperPass>();
216 AU.addUsedIfAvailable<BasicBlockSectionsProfileReaderWrapperPass>();
217}
218
219char MachineFunctionSplitter::ID = 0;
220INITIALIZE_PASS(MachineFunctionSplitter, "machine-function-splitter",
221 "Split machine functions using profile information", false,
222 false)
223
224MachineFunctionPass *llvm::createMachineFunctionSplitterPass() {
225 return new MachineFunctionSplitter();
226}
227