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/ProfileSummaryInfo.h"
28 #include "llvm/CodeGen/BasicBlockSectionUtils.h"
29 #include "llvm/CodeGen/MachineBasicBlock.h"
30 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h"
31 #include "llvm/CodeGen/MachineFunction.h"
32 #include "llvm/CodeGen/MachineFunctionPass.h"
33 #include "llvm/CodeGen/MachineModuleInfo.h"
34 #include "llvm/CodeGen/Passes.h"
35 #include "llvm/IR/Function.h"
36 #include "llvm/InitializePasses.h"
37 #include "llvm/Support/CommandLine.h"
38 #include <optional>
39 
40 using namespace llvm;
41 
42 // FIXME: This cutoff value is CPU dependent and should be moved to
43 // TargetTransformInfo once we consider enabling this on other platforms.
44 // The value is expressed as a ProfileSummaryInfo integer percentile cutoff.
45 // Defaults to 999950, i.e. all blocks colder than 99.995 percentile are split.
46 // The default was empirically determined to be optimal when considering cutoff
47 // values between 99%-ile to 100%-ile with respect to iTLB and icache metrics on
48 // Intel CPUs.
49 static cl::opt<unsigned>
50     PercentileCutoff("mfs-psi-cutoff",
51                      cl::desc("Percentile profile summary cutoff used to "
52                               "determine cold blocks. Unused if set to zero."),
53                      cl::init(999950), cl::Hidden);
54 
55 static cl::opt<unsigned> ColdCountThreshold(
56     "mfs-count-threshold",
57     cl::desc(
58         "Minimum number of times a block must be executed to be retained."),
59     cl::init(1), cl::Hidden);
60 
61 static cl::opt<bool> SplitAllEHCode(
62     "mfs-split-ehcode",
63     cl::desc("Splits all EH code and it's descendants by default."),
64     cl::init(false), cl::Hidden);
65 
66 namespace {
67 
68 class MachineFunctionSplitter : public MachineFunctionPass {
69 public:
70   static char ID;
71   MachineFunctionSplitter() : MachineFunctionPass(ID) {
72     initializeMachineFunctionSplitterPass(*PassRegistry::getPassRegistry());
73   }
74 
75   StringRef getPassName() const override {
76     return "Machine Function Splitter Transformation";
77   }
78 
79   void getAnalysisUsage(AnalysisUsage &AU) const override;
80 
81   bool runOnMachineFunction(MachineFunction &F) override;
82 };
83 } // end anonymous namespace
84 
85 /// setDescendantEHBlocksCold - This splits all EH pads and blocks reachable
86 /// only by EH pad as cold. This will help mark EH pads statically cold instead
87 /// of relying on profile data.
88 static void
89 setDescendantEHBlocksCold(SmallVectorImpl<MachineBasicBlock *> &EHBlocks,
90                           MachineFunction &MF) {
91   MachineBasicBlock *StartBlock = &MF.front();
92   // A block can be unknown if its not reachable from anywhere
93   // EH if its only reachable from start blocks via some path through EH pads
94   // NonEH if it's reachable from Non EH blocks as well.
95   enum Status { Unknown = 0, EH = 1, NonEH = 2 };
96   DenseSet<MachineBasicBlock *> WorkList;
97   DenseMap<MachineBasicBlock *, Status> Statuses;
98 
99   auto getStatus = [&](MachineBasicBlock *MBB) {
100     if (Statuses.find(MBB) != Statuses.end())
101       return Statuses[MBB];
102     else
103       return Unknown;
104   };
105 
106   auto checkPredecessors = [&](MachineBasicBlock *MBB, Status Stat) {
107     for (auto *PredMBB : MBB->predecessors()) {
108       Status PredStatus = getStatus(PredMBB);
109       // If status of predecessor block has gone above current block
110       // we update current blocks status.
111       if (PredStatus > Stat)
112         Stat = PredStatus;
113     }
114     return Stat;
115   };
116 
117   auto addSuccesors = [&](MachineBasicBlock *MBB) {
118     for (auto *SuccMBB : MBB->successors()) {
119       if (!SuccMBB->isEHPad())
120         WorkList.insert(SuccMBB);
121     }
122   };
123 
124   // Insert the successors of start block
125   // and landing pads successor.
126   Statuses[StartBlock] = NonEH;
127   addSuccesors(StartBlock);
128   for (auto *LP : EHBlocks) {
129     addSuccesors(LP);
130     Statuses[LP] = EH;
131   }
132 
133   // Worklist iterative algorithm.
134   while (!WorkList.empty()) {
135     auto *MBB = *WorkList.begin();
136     WorkList.erase(MBB);
137 
138     Status OldStatus = getStatus(MBB);
139 
140     // Check on predecessors and check for
141     // Status update.
142     Status NewStatus = checkPredecessors(MBB, OldStatus);
143 
144     // Did the block status change?
145     bool changed = OldStatus != NewStatus;
146     if (changed) {
147       addSuccesors(MBB);
148       Statuses[MBB] = NewStatus;
149     }
150   }
151 
152   for (auto Entry : Statuses) {
153     if (Entry.second == EH)
154       Entry.first->setSectionID(MBBSectionID::ColdSectionID);
155   }
156 }
157 
158 static bool isColdBlock(const MachineBasicBlock &MBB,
159                         const MachineBlockFrequencyInfo *MBFI,
160                         ProfileSummaryInfo *PSI) {
161   std::optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB);
162   if (!Count)
163     return true;
164 
165   if (PercentileCutoff > 0) {
166     return PSI->isColdCountNthPercentile(PercentileCutoff, *Count);
167   }
168   return (*Count < ColdCountThreshold);
169 }
170 
171 bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) {
172   // We target functions with profile data. Static information in the form
173   // of exception handling code may be split to cold if user passes the
174   // mfs-split-ehcode flag.
175   bool UseProfileData = MF.getFunction().hasProfileData();
176   if (!UseProfileData && !SplitAllEHCode)
177     return false;
178 
179   // TODO: We don't split functions where a section attribute has been set
180   // since the split part may not be placed in a contiguous region. It may also
181   // be more beneficial to augment the linker to ensure contiguous layout of
182   // split functions within the same section as specified by the attribute.
183   if (MF.getFunction().hasSection() ||
184       MF.getFunction().hasFnAttribute("implicit-section-name"))
185     return false;
186 
187   // We don't want to proceed further for cold functions
188   // or functions of unknown hotness. Lukewarm functions have no prefix.
189   std::optional<StringRef> SectionPrefix = MF.getFunction().getSectionPrefix();
190   if (SectionPrefix &&
191       (*SectionPrefix == "unlikely" || *SectionPrefix == "unknown")) {
192     return false;
193   }
194 
195   // Renumbering blocks here preserves the order of the blocks as
196   // sortBasicBlocksAndUpdateBranches uses the numeric identifier to sort
197   // blocks. Preserving the order of blocks is essential to retaining decisions
198   // made by prior passes such as MachineBlockPlacement.
199   MF.RenumberBlocks();
200   MF.setBBSectionsType(BasicBlockSection::Preset);
201 
202   MachineBlockFrequencyInfo *MBFI = nullptr;
203   ProfileSummaryInfo *PSI = nullptr;
204   if (UseProfileData) {
205     MBFI = &getAnalysis<MachineBlockFrequencyInfo>();
206     PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI();
207   }
208 
209   SmallVector<MachineBasicBlock *, 2> LandingPads;
210   for (auto &MBB : MF) {
211     if (MBB.isEntryBlock())
212       continue;
213 
214     if (MBB.isEHPad())
215       LandingPads.push_back(&MBB);
216     else if (UseProfileData && isColdBlock(MBB, MBFI, PSI) && !SplitAllEHCode)
217       MBB.setSectionID(MBBSectionID::ColdSectionID);
218   }
219 
220   // Split all EH code and it's descendant statically by default.
221   if (SplitAllEHCode)
222     setDescendantEHBlocksCold(LandingPads, MF);
223   // We only split out eh pads if all of them are cold.
224   else {
225     bool HasHotLandingPads = false;
226     for (const MachineBasicBlock *LP : LandingPads) {
227       if (!isColdBlock(*LP, MBFI, PSI))
228         HasHotLandingPads = true;
229     }
230     if (!HasHotLandingPads) {
231       for (MachineBasicBlock *LP : LandingPads)
232         LP->setSectionID(MBBSectionID::ColdSectionID);
233     }
234   }
235   auto Comparator = [](const MachineBasicBlock &X, const MachineBasicBlock &Y) {
236     return X.getSectionID().Type < Y.getSectionID().Type;
237   };
238   llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator);
239   llvm::avoidZeroOffsetLandingPad(MF);
240   return true;
241 }
242 
243 void MachineFunctionSplitter::getAnalysisUsage(AnalysisUsage &AU) const {
244   AU.addRequired<MachineModuleInfoWrapperPass>();
245   AU.addRequired<MachineBlockFrequencyInfo>();
246   AU.addRequired<ProfileSummaryInfoWrapperPass>();
247 }
248 
249 char MachineFunctionSplitter::ID = 0;
250 INITIALIZE_PASS(MachineFunctionSplitter, "machine-function-splitter",
251                 "Split machine functions using profile information", false,
252                 false)
253 
254 MachineFunctionPass *llvm::createMachineFunctionSplitterPass() {
255   return new MachineFunctionSplitter();
256 }
257