1 //===--- WebAssemblyOptimizeLiveIntervals.cpp - LiveInterval processing ---===// 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 /// Optimize LiveIntervals for use in a post-RA context. 11 // 12 /// LiveIntervals normally runs before register allocation when the code is 13 /// only recently lowered out of SSA form, so it's uncommon for registers to 14 /// have multiple defs, and when they do, the defs are usually closely related. 15 /// Later, after coalescing, tail duplication, and other optimizations, it's 16 /// more common to see registers with multiple unrelated defs. This pass 17 /// updates LiveIntervals to distribute the value numbers across separate 18 /// LiveIntervals. 19 /// 20 //===----------------------------------------------------------------------===// 21 22 #include "WebAssembly.h" 23 #include "WebAssemblyMachineFunctionInfo.h" 24 #include "WebAssemblySubtarget.h" 25 #include "llvm/CodeGen/LiveIntervals.h" 26 #include "llvm/CodeGen/MachineBlockFrequencyInfo.h" 27 #include "llvm/CodeGen/MachineRegisterInfo.h" 28 #include "llvm/CodeGen/Passes.h" 29 #include "llvm/Support/Debug.h" 30 #include "llvm/Support/raw_ostream.h" 31 using namespace llvm; 32 33 #define DEBUG_TYPE "wasm-optimize-live-intervals" 34 35 namespace { 36 class WebAssemblyOptimizeLiveIntervals final : public MachineFunctionPass { 37 StringRef getPassName() const override { 38 return "WebAssembly Optimize Live Intervals"; 39 } 40 41 void getAnalysisUsage(AnalysisUsage &AU) const override { 42 AU.setPreservesCFG(); 43 AU.addRequired<LiveIntervals>(); 44 AU.addPreserved<MachineBlockFrequencyInfo>(); 45 AU.addPreserved<SlotIndexes>(); 46 AU.addPreserved<LiveIntervals>(); 47 AU.addPreservedID(LiveVariablesID); 48 AU.addPreservedID(MachineDominatorsID); 49 MachineFunctionPass::getAnalysisUsage(AU); 50 } 51 52 bool runOnMachineFunction(MachineFunction &MF) override; 53 54 public: 55 static char ID; // Pass identification, replacement for typeid 56 WebAssemblyOptimizeLiveIntervals() : MachineFunctionPass(ID) {} 57 }; 58 } // end anonymous namespace 59 60 char WebAssemblyOptimizeLiveIntervals::ID = 0; 61 INITIALIZE_PASS(WebAssemblyOptimizeLiveIntervals, DEBUG_TYPE, 62 "Optimize LiveIntervals for WebAssembly", false, false) 63 64 FunctionPass *llvm::createWebAssemblyOptimizeLiveIntervals() { 65 return new WebAssemblyOptimizeLiveIntervals(); 66 } 67 68 bool WebAssemblyOptimizeLiveIntervals::runOnMachineFunction( 69 MachineFunction &MF) { 70 LLVM_DEBUG(dbgs() << "********** Optimize LiveIntervals **********\n" 71 "********** Function: " 72 << MF.getName() << '\n'); 73 74 MachineRegisterInfo &MRI = MF.getRegInfo(); 75 auto &LIS = getAnalysis<LiveIntervals>(); 76 77 // We don't preserve SSA form. 78 MRI.leaveSSA(); 79 80 assert(MRI.tracksLiveness() && "OptimizeLiveIntervals expects liveness"); 81 82 // Split multiple-VN LiveIntervals into multiple LiveIntervals. 83 SmallVector<LiveInterval *, 4> SplitLIs; 84 for (unsigned I = 0, E = MRI.getNumVirtRegs(); I < E; ++I) { 85 unsigned Reg = Register::index2VirtReg(I); 86 auto &TRI = *MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo(); 87 88 if (MRI.reg_nodbg_empty(Reg)) 89 continue; 90 91 LIS.splitSeparateComponents(LIS.getInterval(Reg), SplitLIs); 92 if (Reg == TRI.getFrameRegister(MF) && SplitLIs.size() > 0) { 93 // The live interval for the frame register was split, resulting in a new 94 // VReg. For now we only support debug info output for a single frame base 95 // value for the function, so just use the last one. It will certainly be 96 // wrong for some part of the function, but until we are able to track 97 // values through live-range splitting and stackification, it will have to 98 // do. 99 MF.getInfo<WebAssemblyFunctionInfo>()->setFrameBaseVreg( 100 SplitLIs.back()->reg); 101 } 102 SplitLIs.clear(); 103 } 104 105 // In PrepareForLiveIntervals, we conservatively inserted IMPLICIT_DEF 106 // instructions to satisfy LiveIntervals' requirement that all uses be 107 // dominated by defs. Now that LiveIntervals has computed which of these 108 // defs are actually needed and which are dead, remove the dead ones. 109 for (auto MII = MF.begin()->begin(), MIE = MF.begin()->end(); MII != MIE;) { 110 MachineInstr *MI = &*MII++; 111 if (MI->isImplicitDef() && MI->getOperand(0).isDead()) { 112 LiveInterval &LI = LIS.getInterval(MI->getOperand(0).getReg()); 113 LIS.removeVRegDefAt(LI, LIS.getInstructionIndex(*MI).getRegSlot()); 114 LIS.RemoveMachineInstrFromMaps(*MI); 115 MI->eraseFromParent(); 116 } 117 } 118 119 return true; 120 } 121