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/BlockFrequencyInfo.h"
28#include "llvm/Analysis/BranchProbabilityInfo.h"
29#include "llvm/Analysis/EHUtils.h"
30#include "llvm/Analysis/ProfileSummaryInfo.h"
31#include "llvm/CodeGen/BasicBlockSectionUtils.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
113 // Temporary hack to cope with AArch64's jump table encoding
114 const TargetInstrInfo &TII = *MBB.getParent()->getSubtarget().getInstrInfo();
115 if (!TII.isMBBSafeToSplitToCold(MBB))
116 return false;
117
118 // For instrumentation profiles and sample profiles, we use different ways
119 // to judge whether a block is cold and should be split.
120 if (PSI->hasInstrumentationProfile() || PSI->hasCSInstrumentationProfile()) {
121 // If using instrument profile, which is deemed "accurate", no count means
122 // cold.
123 if (!Count)
124 return true;
125 if (PercentileCutoff > 0)
126 return PSI->isColdCountNthPercentile(PercentileCutoff, C: *Count);
127 // Fallthrough to end of function.
128 } else if (PSI->hasSampleProfile()) {
129 // For sample profile, no count means "do not judege coldness".
130 if (!Count)
131 return false;
132 }
133
134 return (*Count < ColdCountThreshold);
135}
136
137bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
138 // We target functions with profile data. Static information in the form
139 // of exception handling code may be split to cold if user passes the
140 // mfs-split-ehcode flag.
141 bool UseProfileData = MF.getFunction().hasProfileData();
142 if (!UseProfileData && !SplitAllEHCode)
143 return false;
144
145 const TargetInstrInfo &TII = *MF.getSubtarget().getInstrInfo();
146 if (!TII.isFunctionSafeToSplit(MF))
147 return false;
148
149 // Renumbering blocks here preserves the order of the blocks as
150 // sortBasicBlocksAndUpdateBranches uses the numeric identifier to sort
151 // blocks. Preserving the order of blocks is essential to retaining decisions
152 // made by prior passes such as MachineBlockPlacement.
153 MF.RenumberBlocks();
154 MF.setBBSectionsType(BasicBlockSection::Preset);
155
156 MachineBlockFrequencyInfo *MBFI = nullptr;
157 ProfileSummaryInfo *PSI = nullptr;
158 if (UseProfileData) {
159 MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
160 PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
161 // If we don't have a good profile (sample profile is not deemed
162 // as a "good profile") and the function is not hot, then early
163 // return. (Because we can only trust hot functions when profile
164 // quality is not good.)
165 if (PSI->hasSampleProfile() && !PSI->isFunctionHotInCallGraph(F: &MF, BFI&: *MBFI)) {
166 // Split all EH code and it's descendant statically by default.
167 if (SplitAllEHCode)
168 setDescendantEHBlocksCold(MF);
169 finishAdjustingBasicBlocksAndLandingPads(MF);
170 return true;
171 }
172 }
173
174 SmallVector<MachineBasicBlock *, 2> LandingPads;
175 for (auto &MBB : MF) {
176 if (MBB.isEntryBlock())
177 continue;
178
179 if (MBB.isEHPad())
180 LandingPads.push_back(Elt: &MBB);
181 else if (UseProfileData && isColdBlock(MBB, MBFI, PSI) && !SplitAllEHCode)
182 MBB.setSectionID(MBBSectionID::ColdSectionID);
183 }
184
185 // Split all EH code and it's descendant statically by default.
186 if (SplitAllEHCode)
187 setDescendantEHBlocksCold(MF);
188 // We only split out eh pads if all of them are cold.
189 else {
190 // Here we have UseProfileData == true.
191 bool HasHotLandingPads = false;
192 for (const MachineBasicBlock *LP : LandingPads) {
193 if (!isColdBlock(MBB: *LP, MBFI, PSI))
194 HasHotLandingPads = true;
195 }
196 if (!HasHotLandingPads) {
197 for (MachineBasicBlock *LP : LandingPads)
198 LP->setSectionID(MBBSectionID::ColdSectionID);
199 }
200 }
201
202 finishAdjustingBasicBlocksAndLandingPads(MF);
203 return true;
204}
205
206void MachineFunctionSplitter::getAnalysisUsage(AnalysisUsage &AU) const {
207 AU.addRequired<MachineModuleInfoWrapperPass>();
208 AU.addRequired<MachineBlockFrequencyInfo>();
209 AU.addRequired<ProfileSummaryInfoWrapperPass>();
210}
211
212char MachineFunctionSplitter::ID = 0;
213INITIALIZE_PASS(MachineFunctionSplitter, "machine-function-splitter",
214 "Split machine functions using profile information", false,
215 false)
216
217MachineFunctionPass *llvm::createMachineFunctionSplitterPass() {
218 return new MachineFunctionSplitter();
219}
220

source code of llvm/lib/CodeGen/MachineFunctionSplitter.cpp