1 //===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 /// 10 /// \file 11 /// \brief This file contains the WebAssembly implementation of 12 /// TargetFrameLowering class. 13 /// 14 /// On WebAssembly, there aren't a lot of things to do here. There are no 15 /// callee-saved registers to save, and no spill slots. 16 /// 17 /// The stack grows downward. 18 /// 19 //===----------------------------------------------------------------------===// 20 21 #include "WebAssemblyFrameLowering.h" 22 #include "MCTargetDesc/WebAssemblyMCTargetDesc.h" 23 #include "WebAssemblyInstrInfo.h" 24 #include "WebAssemblyMachineFunctionInfo.h" 25 #include "WebAssemblySubtarget.h" 26 #include "WebAssemblyTargetMachine.h" 27 #include "llvm/CodeGen/MachineFrameInfo.h" 28 #include "llvm/CodeGen/MachineFunction.h" 29 #include "llvm/CodeGen/MachineInstrBuilder.h" 30 #include "llvm/CodeGen/MachineModuleInfo.h" 31 #include "llvm/CodeGen/MachineRegisterInfo.h" 32 #include "llvm/Support/Debug.h" 33 using namespace llvm; 34 35 #define DEBUG_TYPE "wasm-frame-info" 36 37 // TODO: wasm64 38 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions 39 40 /// Return true if the specified function should have a dedicated frame pointer 41 /// register. 42 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const { 43 const MachineFrameInfo *MFI = MF.getFrameInfo(); 44 const auto *RegInfo = 45 MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo(); 46 return MFI->isFrameAddressTaken() || MFI->hasVarSizedObjects() || 47 MFI->hasStackMap() || MFI->hasPatchPoint() || 48 RegInfo->needsStackRealignment(MF); 49 } 50 51 /// Under normal circumstances, when a frame pointer is not required, we reserve 52 /// argument space for call sites in the function immediately on entry to the 53 /// current function. This eliminates the need for add/sub sp brackets around 54 /// call sites. Returns true if the call frame is included as part of the stack 55 /// frame. 56 bool WebAssemblyFrameLowering::hasReservedCallFrame( 57 const MachineFunction &MF) const { 58 return !MF.getFrameInfo()->hasVarSizedObjects(); 59 } 60 61 62 /// Returns true if this function needs a local user-space stack pointer. 63 /// Unlike a machine stack pointer, the wasm user stack pointer is a global 64 /// variable, so it is loaded into a register in the prolog. 65 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF, 66 const MachineFrameInfo &MFI) const { 67 return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF); 68 } 69 70 /// Returns true if the local user-space stack pointer needs to be written back 71 /// to memory by this function (this is not meaningful if needsSP is false). If 72 /// false, the stack red zone can be used and only a local SP is needed. 73 bool WebAssemblyFrameLowering::needsSPWriteback( 74 const MachineFunction &MF, const MachineFrameInfo &MFI) const { 75 assert(needsSP(MF, MFI)); 76 return MFI.getStackSize() > RedZoneSize || MFI.hasCalls() || 77 MF.getFunction()->hasFnAttribute(Attribute::NoRedZone); 78 } 79 80 static void writeSPToMemory(unsigned SrcReg, MachineFunction &MF, 81 MachineBasicBlock &MBB, 82 MachineBasicBlock::iterator &InsertAddr, 83 MachineBasicBlock::iterator &InsertStore, 84 const DebugLoc &DL) { 85 const char *ES = "__stack_pointer"; 86 auto *SPSymbol = MF.createExternalSymbolName(ES); 87 MachineRegisterInfo &MRI = MF.getRegInfo(); 88 const TargetRegisterClass *PtrRC = 89 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 90 unsigned Zero = MRI.createVirtualRegister(PtrRC); 91 unsigned Drop = MRI.createVirtualRegister(PtrRC); 92 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 93 94 BuildMI(MBB, InsertAddr, DL, TII->get(WebAssembly::CONST_I32), Zero) 95 .addImm(0); 96 auto *MMO = new MachineMemOperand(MachinePointerInfo(MF.getPSVManager() 97 .getExternalSymbolCallEntry(ES)), 98 MachineMemOperand::MOStore, 4, 4); 99 BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::STORE_I32), Drop) 100 .addExternalSymbol(SPSymbol) 101 .addReg(Zero) 102 .addImm(2) // p2align 103 .addReg(SrcReg) 104 .addMemOperand(MMO); 105 } 106 107 MachineBasicBlock::iterator 108 WebAssemblyFrameLowering::eliminateCallFramePseudoInstr( 109 MachineFunction &MF, MachineBasicBlock &MBB, 110 MachineBasicBlock::iterator I) const { 111 assert(!I->getOperand(0).getImm() && hasFP(MF) && 112 "Call frame pseudos should only be used for dynamic stack adjustment"); 113 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 114 if (I->getOpcode() == TII->getCallFrameDestroyOpcode() && 115 needsSPWriteback(MF, *MF.getFrameInfo())) { 116 DebugLoc DL = I->getDebugLoc(); 117 writeSPToMemory(WebAssembly::SP32, MF, MBB, I, I, DL); 118 } 119 return MBB.erase(I); 120 } 121 122 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF, 123 MachineBasicBlock &MBB) const { 124 // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions 125 auto *MFI = MF.getFrameInfo(); 126 assert(MFI->getCalleeSavedInfo().empty() && 127 "WebAssembly should not have callee-saved registers"); 128 129 if (!needsSP(MF, *MFI)) return; 130 uint64_t StackSize = MFI->getStackSize(); 131 132 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 133 auto &MRI = MF.getRegInfo(); 134 135 auto InsertPt = MBB.begin(); 136 DebugLoc DL; 137 138 const TargetRegisterClass *PtrRC = 139 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 140 unsigned Zero = MRI.createVirtualRegister(PtrRC); 141 unsigned SPReg = MRI.createVirtualRegister(PtrRC); 142 const char *ES = "__stack_pointer"; 143 auto *SPSymbol = MF.createExternalSymbolName(ES); 144 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), Zero) 145 .addImm(0); 146 auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(MF.getPSVManager() 147 .getExternalSymbolCallEntry(ES)), 148 MachineMemOperand::MOLoad, 4, 4); 149 // Load the SP value. 150 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), 151 StackSize ? SPReg : (unsigned)WebAssembly::SP32) 152 .addExternalSymbol(SPSymbol) 153 .addReg(Zero) // addr 154 .addImm(2) // p2align 155 .addMemOperand(LoadMMO); 156 157 if (StackSize) { 158 // Subtract the frame size 159 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC); 160 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 161 .addImm(StackSize); 162 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32), 163 WebAssembly::SP32) 164 .addReg(SPReg) 165 .addReg(OffsetReg); 166 } 167 if (hasFP(MF)) { 168 // Unlike most conventional targets (where FP points to the saved FP), 169 // FP points to the bottom of the fixed-size locals, so we can use positive 170 // offsets in load/store instructions. 171 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), 172 WebAssembly::FP32) 173 .addReg(WebAssembly::SP32); 174 } 175 if (StackSize && needsSPWriteback(MF, *MFI)) { 176 writeSPToMemory(WebAssembly::SP32, MF, MBB, InsertPt, InsertPt, DL); 177 } 178 } 179 180 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF, 181 MachineBasicBlock &MBB) const { 182 auto *MFI = MF.getFrameInfo(); 183 uint64_t StackSize = MFI->getStackSize(); 184 if (!needsSP(MF, *MFI) || !needsSPWriteback(MF, *MFI)) return; 185 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 186 auto &MRI = MF.getRegInfo(); 187 auto InsertPt = MBB.getFirstTerminator(); 188 DebugLoc DL; 189 190 if (InsertPt != MBB.end()) 191 DL = InsertPt->getDebugLoc(); 192 193 // Restore the stack pointer. If we had fixed-size locals, add the offset 194 // subtracted in the prolog. 195 unsigned SPReg = 0; 196 MachineBasicBlock::iterator InsertAddr = InsertPt; 197 if (StackSize) { 198 const TargetRegisterClass *PtrRC = 199 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 200 unsigned OffsetReg = MRI.createVirtualRegister(PtrRC); 201 InsertAddr = 202 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 203 .addImm(StackSize); 204 // In the epilog we don't need to write the result back to the SP32 physreg 205 // because it won't be used again. We can use a stackified register instead. 206 SPReg = MRI.createVirtualRegister(PtrRC); 207 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg) 208 .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32) 209 .addReg(OffsetReg); 210 } else { 211 SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32; 212 } 213 214 writeSPToMemory(SPReg, MF, MBB, InsertAddr, InsertPt, DL); 215 } 216
