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/Statistic.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/IR/Module.h" 37 #include "llvm/InitializePasses.h" 38 #include "llvm/Support/CommandLine.h" 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 namespace { 62 63 class MachineFunctionSplitter : public MachineFunctionPass { 64 public: 65 static char ID; 66 MachineFunctionSplitter() : MachineFunctionPass(ID) { 67 initializeMachineFunctionSplitterPass(*PassRegistry::getPassRegistry()); 68 } 69 70 StringRef getPassName() const override { 71 return "Machine Function Splitter Transformation"; 72 } 73 74 void getAnalysisUsage(AnalysisUsage &AU) const override; 75 76 bool runOnMachineFunction(MachineFunction &F) override; 77 }; 78 } // end anonymous namespace 79 80 static bool isColdBlock(MachineBasicBlock &MBB, 81 const MachineBlockFrequencyInfo *MBFI, 82 ProfileSummaryInfo *PSI) { 83 Optional<uint64_t> Count = MBFI->getBlockProfileCount(&MBB); 84 if (!Count.hasValue()) 85 return true; 86 87 if (PercentileCutoff > 0) { 88 return PSI->isColdCountNthPercentile(PercentileCutoff, *Count); 89 } 90 return (*Count < ColdCountThreshold); 91 } 92 93 bool MachineFunctionSplitter::runOnMachineFunction(MachineFunction &MF) { 94 // TODO: We only target functions with profile data. Static information may 95 // also be considered but we don't see performance improvements yet. 96 if (!MF.getFunction().hasProfileData()) 97 return false; 98 99 // TODO: We don't split functions where a section attribute has been set 100 // since the split part may not be placed in a contiguous region. It may also 101 // be more beneficial to augment the linker to ensure contiguous layout of 102 // split functions within the same section as specified by the attribute. 103 if (!MF.getFunction().getSection().empty()) 104 return false; 105 106 // We don't want to proceed further for cold functions 107 // or functions of unknown hotness. Lukewarm functions have no prefix. 108 Optional<StringRef> SectionPrefix = MF.getFunction().getSectionPrefix(); 109 if (SectionPrefix.hasValue() && 110 (SectionPrefix.getValue().equals("unlikely") || 111 SectionPrefix.getValue().equals("unknown"))) { 112 return false; 113 } 114 115 // Renumbering blocks here preserves the order of the blocks as 116 // sortBasicBlocksAndUpdateBranches uses the numeric identifier to sort 117 // blocks. Preserving the order of blocks is essential to retaining decisions 118 // made by prior passes such as MachineBlockPlacement. 119 MF.RenumberBlocks(); 120 MF.setBBSectionsType(BasicBlockSection::Preset); 121 auto *MBFI = &getAnalysis<MachineBlockFrequencyInfo>(); 122 auto *PSI = &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI(); 123 124 for (auto &MBB : MF) { 125 // FIXME: We retain the entry block and conservatively keep all landing pad 126 // blocks as part of the original function. Once D73739 is submitted, we can 127 // improve the handling of ehpads. 128 if ((MBB.pred_empty() || MBB.isEHPad())) 129 continue; 130 if (isColdBlock(MBB, MBFI, PSI)) 131 MBB.setSectionID(MBBSectionID::ColdSectionID); 132 } 133 134 auto Comparator = [](const MachineBasicBlock &X, const MachineBasicBlock &Y) { 135 return X.getSectionID().Type < Y.getSectionID().Type; 136 }; 137 llvm::sortBasicBlocksAndUpdateBranches(MF, Comparator); 138 139 return true; 140 } 141 142 void MachineFunctionSplitter::getAnalysisUsage(AnalysisUsage &AU) const { 143 AU.addRequired<MachineModuleInfoWrapperPass>(); 144 AU.addRequired<MachineBlockFrequencyInfo>(); 145 AU.addRequired<ProfileSummaryInfoWrapperPass>(); 146 } 147 148 char MachineFunctionSplitter::ID = 0; 149 INITIALIZE_PASS(MachineFunctionSplitter, "machine-function-splitter", 150 "Split machine functions using profile information", false, 151 false) 152 153 MachineFunctionPass *llvm::createMachineFunctionSplitterPass() { 154 return new MachineFunctionSplitter(); 155 } 156