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: Implement a red zone? 38 // TODO: wasm64 39 // TODO: Prolog/epilog should be stackified too. This pass runs after register 40 // stackification, so we'll have to do it manually. 41 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions 42 43 /// Return true if the specified function should have a dedicated frame pointer 44 /// register. 45 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const { 46 const MachineFrameInfo *MFI = MF.getFrameInfo(); 47 const auto *RegInfo = 48 MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo(); 49 return MFI->hasVarSizedObjects() || MFI->isFrameAddressTaken() || 50 MFI->hasStackMap() || MFI->hasPatchPoint() || 51 RegInfo->needsStackRealignment(MF); 52 } 53 54 /// Under normal circumstances, when a frame pointer is not required, we reserve 55 /// argument space for call sites in the function immediately on entry to the 56 /// current function. This eliminates the need for add/sub sp brackets around 57 /// call sites. Returns true if the call frame is included as part of the stack 58 /// frame. 59 bool WebAssemblyFrameLowering::hasReservedCallFrame( 60 const MachineFunction &MF) const { 61 return !MF.getFrameInfo()->hasVarSizedObjects(); 62 } 63 64 65 /// Adjust the stack pointer by a constant amount. 66 static void adjustStackPointer(unsigned StackSize, 67 bool AdjustUp, 68 MachineFunction& MF, 69 MachineBasicBlock& MBB, 70 const TargetInstrInfo* TII, 71 MachineBasicBlock::iterator InsertPt, 72 const DebugLoc& DL) { 73 auto &MRI = MF.getRegInfo(); 74 unsigned SPReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass); 75 auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer"); 76 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), SPReg) 77 .addExternalSymbol(SPSymbol); 78 // This MachinePointerInfo should reference __stack_pointer as well but 79 // doesn't because MachinePointerInfo() takes a GV which we don't have for 80 // __stack_pointer. TODO: check if PseudoSourceValue::ExternalSymbolCallEntry 81 // is appropriate instead. (likewise for EmitEpologue below) 82 auto *LoadMMO = new MachineMemOperand(MachinePointerInfo(), 83 MachineMemOperand::MOLoad, 4, 4); 84 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::LOAD_I32), SPReg) 85 .addImm(0) 86 .addReg(SPReg) 87 .addImm(2) // p2align 88 .addMemOperand(LoadMMO); 89 // Add/Subtract the frame size 90 unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass); 91 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 92 .addImm(StackSize); 93 BuildMI(MBB, InsertPt, DL, 94 TII->get(AdjustUp ? WebAssembly::ADD_I32 : WebAssembly::SUB_I32), 95 WebAssembly::SP32) 96 .addReg(SPReg) 97 .addReg(OffsetReg); 98 // The SP32 register now has the new stacktop. Also write it back to memory. 99 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 100 .addExternalSymbol(SPSymbol); 101 auto *MMO = new MachineMemOperand(MachinePointerInfo(), 102 MachineMemOperand::MOStore, 4, 4); 103 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32) 104 .addImm(0) 105 .addReg(OffsetReg) 106 .addImm(2) // p2align 107 .addReg(WebAssembly::SP32) 108 .addMemOperand(MMO); 109 } 110 111 void WebAssemblyFrameLowering::eliminateCallFramePseudoInstr( 112 MachineFunction &MF, MachineBasicBlock &MBB, 113 MachineBasicBlock::iterator I) const { 114 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 115 DebugLoc DL = I->getDebugLoc(); 116 unsigned Opc = I->getOpcode(); 117 bool IsDestroy = Opc == TII->getCallFrameDestroyOpcode(); 118 unsigned Amount = I->getOperand(0).getImm(); 119 if (Amount) 120 adjustStackPointer(Amount, IsDestroy, MF, MBB, 121 TII, I, DL); 122 MBB.erase(I); 123 } 124 125 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF, 126 MachineBasicBlock &MBB) const { 127 // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions 128 auto *MFI = MF.getFrameInfo(); 129 assert(MFI->getCalleeSavedInfo().empty() && 130 "WebAssembly should not have callee-saved registers"); 131 assert(!hasFP(MF) && "Functions needing frame pointers not yet supported"); 132 uint64_t StackSize = MFI->getStackSize(); 133 if (!StackSize && (!MFI->adjustsStack() || MFI->getMaxCallFrameSize() == 0)) 134 return; 135 136 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 137 138 auto InsertPt = MBB.begin(); 139 DebugLoc DL; 140 141 adjustStackPointer(StackSize, false, MF, MBB, TII, InsertPt, DL); 142 } 143 144 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF, 145 MachineBasicBlock &MBB) const { 146 uint64_t StackSize = MF.getFrameInfo()->getStackSize(); 147 if (!StackSize) 148 return; 149 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 150 auto &MRI = MF.getRegInfo(); 151 unsigned OffsetReg = MRI.createVirtualRegister(&WebAssembly::I32RegClass); 152 auto InsertPt = MBB.getFirstTerminator(); 153 DebugLoc DL; 154 155 if (InsertPt != MBB.end()) { 156 DL = InsertPt->getDebugLoc(); 157 } 158 159 // Restore the stack pointer. Without FP its value is just SP32 - stacksize 160 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 161 .addImm(StackSize); 162 auto *SPSymbol = MF.createExternalSymbolName("__stack_pointer"); 163 // TODO: Fold this add into the const offset field of the store. 164 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), WebAssembly::SP32) 165 .addReg(WebAssembly::SP32) 166 .addReg(OffsetReg); 167 // Re-use OffsetReg to hold the address of the stacktop 168 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 169 .addExternalSymbol(SPSymbol); 170 auto *MMO = new MachineMemOperand(MachinePointerInfo(), 171 MachineMemOperand::MOStore, 4, 4); 172 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::STORE_I32), WebAssembly::SP32) 173 .addImm(0) 174 .addReg(OffsetReg) 175 .addImm(2) // p2align 176 .addReg(WebAssembly::SP32) 177 .addMemOperand(MMO); 178 } 179