1//===-- WebAssemblyFrameLowering.cpp - WebAssembly Frame Lowering ----------==// 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/// This file contains the WebAssembly implementation of 11/// TargetFrameLowering class. 12/// 13/// On WebAssembly, there aren't a lot of things to do here. There are no 14/// callee-saved registers to save, and no spill slots. 15/// 16/// The stack grows downward. 17/// 18//===----------------------------------------------------------------------===// 19 20#include "WebAssemblyFrameLowering.h" 21#include "MCTargetDesc/WebAssemblyMCTargetDesc.h" 22#include "WebAssemblyInstrInfo.h" 23#include "WebAssemblyMachineFunctionInfo.h" 24#include "WebAssemblySubtarget.h" 25#include "WebAssemblyTargetMachine.h" 26#include "WebAssemblyUtilities.h" 27#include "llvm/CodeGen/MachineFrameInfo.h" 28#include "llvm/CodeGen/MachineFunction.h" 29#include "llvm/CodeGen/MachineInstrBuilder.h" 30#include "llvm/CodeGen/MachineModuleInfoImpls.h" 31#include "llvm/CodeGen/MachineRegisterInfo.h" 32#include "llvm/MC/MCAsmInfo.h" 33#include "llvm/Support/Debug.h" 34using namespace llvm; 35 36#define DEBUG_TYPE "wasm-frame-info" 37 38// TODO: wasm64 39// TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions 40 41/// We need a base pointer in the case of having items on the stack that 42/// require stricter alignment than the stack pointer itself. Because we need 43/// to shift the stack pointer by some unknown amount to force the alignment, 44/// we need to record the value of the stack pointer on entry to the function. 45bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const { 46 const auto *RegInfo = 47 MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo(); 48 return RegInfo->needsStackRealignment(MF); 49} 50 51/// Return true if the specified function should have a dedicated frame pointer 52/// register. 53bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const { 54 const MachineFrameInfo &MFI = MF.getFrameInfo(); 55 56 // When we have var-sized objects, we move the stack pointer by an unknown 57 // amount, and need to emit a frame pointer to restore the stack to where we 58 // were on function entry. 59 // If we already need a base pointer, we use that to fix up the stack pointer. 60 // If there are no fixed-size objects, we would have no use of a frame 61 // pointer, and thus should not emit one. 62 bool HasFixedSizedObjects = MFI.getStackSize() > 0; 63 bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects; 64 65 return MFI.isFrameAddressTaken() || 66 (MFI.hasVarSizedObjects() && NeedsFixedReference) || 67 MFI.hasStackMap() || MFI.hasPatchPoint(); 68} 69 70/// Under normal circumstances, when a frame pointer is not required, we reserve 71/// argument space for call sites in the function immediately on entry to the 72/// current function. This eliminates the need for add/sub sp brackets around 73/// call sites. Returns true if the call frame is included as part of the stack 74/// frame. 75bool WebAssemblyFrameLowering::hasReservedCallFrame( 76 const MachineFunction &MF) const { 77 return !MF.getFrameInfo().hasVarSizedObjects(); 78} 79 80// Returns true if this function needs a local user-space stack pointer for its 81// local frame (not for exception handling). 82bool WebAssemblyFrameLowering::needsSPForLocalFrame( 83 const MachineFunction &MF) const { 84 auto &MFI = MF.getFrameInfo(); 85 return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF); 86} 87 88// In function with EH pads, we need to make a copy of the value of 89// __stack_pointer global in SP32 register, in order to use it when restoring 90// __stack_pointer after an exception is caught. 91bool WebAssemblyFrameLowering::needsPrologForEH( 92 const MachineFunction &MF) const { 93 auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType(); 94 return EHType == ExceptionHandling::Wasm && 95 MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls(); 96} 97 98/// Returns true if this function needs a local user-space stack pointer. 99/// Unlike a machine stack pointer, the wasm user stack pointer is a global 100/// variable, so it is loaded into a register in the prolog. 101bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const { 102 return needsSPForLocalFrame(MF) || needsPrologForEH(MF); 103} 104 105/// Returns true if the local user-space stack pointer needs to be written back 106/// to __stack_pointer global by this function (this is not meaningful if 107/// needsSP is false). If false, the stack red zone can be used and only a local 108/// SP is needed. 109bool WebAssemblyFrameLowering::needsSPWriteback( 110 const MachineFunction &MF) const { 111 auto &MFI = MF.getFrameInfo(); 112 assert(needsSP(MF)); 113 // When we don't need a local stack pointer for its local frame but only to 114 // support EH, we don't need to write SP back in the epilog, because we don't 115 // bump down the stack pointer in the prolog. We need to write SP back in the 116 // epilog only if 117 // 1. We need SP not only for EH support but also because we actually use 118 // stack or we have a frame address taken. 119 // 2. We cannot use the red zone. 120 bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() && 121 !MF.getFunction().hasFnAttribute(Attribute::NoRedZone); 122 return needsSPForLocalFrame(MF) && !CanUseRedZone; 123} 124 125void WebAssemblyFrameLowering::writeSPToGlobal( 126 unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB, 127 MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const { 128 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 129 130 const char *ES = "__stack_pointer"; 131 auto *SPSymbol = MF.createExternalSymbolName(ES); 132 BuildMI(MBB, InsertStore, DL, TII->get(WebAssembly::GLOBAL_SET_I32)) 133 .addExternalSymbol(SPSymbol) 134 .addReg(SrcReg); 135} 136 137MachineBasicBlock::iterator 138WebAssemblyFrameLowering::eliminateCallFramePseudoInstr( 139 MachineFunction &MF, MachineBasicBlock &MBB, 140 MachineBasicBlock::iterator I) const { 141 assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) && 142 "Call frame pseudos should only be used for dynamic stack adjustment"); 143 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 144 if (I->getOpcode() == TII->getCallFrameDestroyOpcode() && 145 needsSPWriteback(MF)) { 146 DebugLoc DL = I->getDebugLoc(); 147 writeSPToGlobal(WebAssembly::SP32, MF, MBB, I, DL); 148 } 149 return MBB.erase(I); 150} 151 152void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF, 153 MachineBasicBlock &MBB) const { 154 // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions 155 auto &MFI = MF.getFrameInfo(); 156 assert(MFI.getCalleeSavedInfo().empty() && 157 "WebAssembly should not have callee-saved registers"); 158 159 if (!needsSP(MF)) 160 return; 161 uint64_t StackSize = MFI.getStackSize(); 162 163 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 164 auto &MRI = MF.getRegInfo(); 165 166 auto InsertPt = MBB.begin(); 167 while (InsertPt != MBB.end() && 168 WebAssembly::isArgument(InsertPt->getOpcode())) 169 ++InsertPt; 170 DebugLoc DL; 171 172 const TargetRegisterClass *PtrRC = 173 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 174 unsigned SPReg = WebAssembly::SP32; 175 if (StackSize) 176 SPReg = MRI.createVirtualRegister(PtrRC); 177 178 const char *ES = "__stack_pointer"; 179 auto *SPSymbol = MF.createExternalSymbolName(ES); 180 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::GLOBAL_GET_I32), SPReg) 181 .addExternalSymbol(SPSymbol); 182 183 bool HasBP = hasBP(MF); 184 if (HasBP) { 185 auto FI = MF.getInfo<WebAssemblyFunctionInfo>(); 186 Register BasePtr = MRI.createVirtualRegister(PtrRC); 187 FI->setBasePointerVreg(BasePtr); 188 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr) 189 .addReg(SPReg); 190 } 191 if (StackSize) { 192 // Subtract the frame size 193 Register OffsetReg = MRI.createVirtualRegister(PtrRC); 194 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 195 .addImm(StackSize); 196 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::SUB_I32), 197 WebAssembly::SP32) 198 .addReg(SPReg) 199 .addReg(OffsetReg); 200 } 201 if (HasBP) { 202 Register BitmaskReg = MRI.createVirtualRegister(PtrRC); 203 unsigned Alignment = MFI.getMaxAlignment(); 204 assert((1u << countTrailingZeros(Alignment)) == Alignment && 205 "Alignment must be a power of 2"); 206 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), BitmaskReg) 207 .addImm((int)~(Alignment - 1)); 208 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::AND_I32), 209 WebAssembly::SP32) 210 .addReg(WebAssembly::SP32) 211 .addReg(BitmaskReg); 212 } 213 if (hasFP(MF)) { 214 // Unlike most conventional targets (where FP points to the saved FP), 215 // FP points to the bottom of the fixed-size locals, so we can use positive 216 // offsets in load/store instructions. 217 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), WebAssembly::FP32) 218 .addReg(WebAssembly::SP32); 219 } 220 if (StackSize && needsSPWriteback(MF)) { 221 writeSPToGlobal(WebAssembly::SP32, MF, MBB, InsertPt, DL); 222 } 223} 224 225void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF, 226 MachineBasicBlock &MBB) const { 227 uint64_t StackSize = MF.getFrameInfo().getStackSize(); 228 if (!needsSP(MF) || !needsSPWriteback(MF)) 229 return; 230 const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo(); 231 auto &MRI = MF.getRegInfo(); 232 auto InsertPt = MBB.getFirstTerminator(); 233 DebugLoc DL; 234 235 if (InsertPt != MBB.end()) 236 DL = InsertPt->getDebugLoc(); 237 238 // Restore the stack pointer. If we had fixed-size locals, add the offset 239 // subtracted in the prolog. 240 unsigned SPReg = 0; 241 if (hasBP(MF)) { 242 auto FI = MF.getInfo<WebAssemblyFunctionInfo>(); 243 SPReg = FI->getBasePointerVreg(); 244 } else if (StackSize) { 245 const TargetRegisterClass *PtrRC = 246 MRI.getTargetRegisterInfo()->getPointerRegClass(MF); 247 Register OffsetReg = MRI.createVirtualRegister(PtrRC); 248 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::CONST_I32), OffsetReg) 249 .addImm(StackSize); 250 // In the epilog we don't need to write the result back to the SP32 physreg 251 // because it won't be used again. We can use a stackified register instead. 252 SPReg = MRI.createVirtualRegister(PtrRC); 253 BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::ADD_I32), SPReg) 254 .addReg(hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32) 255 .addReg(OffsetReg); 256 } else { 257 SPReg = hasFP(MF) ? WebAssembly::FP32 : WebAssembly::SP32; 258 } 259 260 writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL); 261} 262