llvm/lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp

   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 "Utils/WebAssemblyTypeUtilities.h"
  23 #include "WebAssembly.h"
  24 #include "WebAssemblyInstrInfo.h"
  25 #include "WebAssemblyMachineFunctionInfo.h"
  26 #include "WebAssemblySubtarget.h"
  27 #include "WebAssemblyTargetMachine.h"
  28 #include "llvm/CodeGen/Analysis.h"
  29 #include "llvm/CodeGen/MachineFrameInfo.h"
  30 #include "llvm/CodeGen/MachineFunction.h"
  31 #include "llvm/CodeGen/MachineInstrBuilder.h"
  32 #include "llvm/CodeGen/MachineModuleInfoImpls.h"
  33 #include "llvm/CodeGen/MachineRegisterInfo.h"
  34 #include "llvm/IR/Instructions.h"
  35 #include "llvm/MC/MCAsmInfo.h"
  36 #include "llvm/Support/Debug.h"
  37 using namespace llvm;
  38
  39 #define DEBUG_TYPE "wasm-frame-info"
  40
  41 // TODO: wasm64
  42 // TODO: Emit TargetOpcode::CFI_INSTRUCTION instructions
  43
  44 // In an ideal world, when objects are added to the MachineFrameInfo by
  45 // FunctionLoweringInfo::set, we could somehow hook into target-specific code to
  46 // ensure they are assigned the right stack ID.  However there isn't a hook that
  47 // runs between then and DAG building time, though, so instead we hoist stack
  48 // objects lazily when they are first used, and comprehensively after the DAG is
  49 // built via the PreprocessISelDAG hook, called by the
  50 // SelectionDAGISel::runOnMachineFunction.  We have to do it in two places
  51 // because we want to do it while building the selection DAG for uses of alloca,
  52 // but not all alloca instructions are used so we have to follow up afterwards.
  53 std::optional<unsigned>
  54 WebAssemblyFrameLowering::getLocalForStackObject(MachineFunction &MF,
  55                                                  int FrameIndex) {
  56   MachineFrameInfo &MFI = MF.getFrameInfo();
  57
  58   // If already hoisted to a local, done.
  59   if (MFI.getStackID(FrameIndex) == TargetStackID::WasmLocal)
  60     return static_cast<unsigned>(MFI.getObjectOffset(FrameIndex));
  61
  62   // If not allocated in the object address space, this object will be in
  63   // linear memory.
  64   const AllocaInst *AI = MFI.getObjectAllocation(FrameIndex);
  65   if (!AI || !WebAssembly::isWasmVarAddressSpace(AI->getAddressSpace()))
  66     return std::nullopt;
  67
  68   // Otherwise, allocate this object in the named value stack, outside of linear
  69   // memory.
  70   SmallVector<EVT, 4> ValueVTs;
  71   const WebAssemblyTargetLowering &TLI =
  72       *MF.getSubtarget<WebAssemblySubtarget>().getTargetLowering();
  73   WebAssemblyFunctionInfo *FuncInfo = MF.getInfo<WebAssemblyFunctionInfo>();
  74   ComputeValueVTs(TLI, MF.getDataLayout(), AI->getAllocatedType(), ValueVTs);
  75   MFI.setStackID(FrameIndex, TargetStackID::WasmLocal);
  76   // Abuse SP offset to record the index of the first local in the object.
  77   unsigned Local = FuncInfo->getParams().size() + FuncInfo->getLocals().size();
  78   MFI.setObjectOffset(FrameIndex, Local);
  79   // Allocate WebAssembly locals for each non-aggregate component of the
  80   // allocation.
  81   for (EVT ValueVT : ValueVTs)
  82     FuncInfo->addLocal(ValueVT.getSimpleVT());
  83   // Abuse object size to record number of WebAssembly locals allocated to
  84   // this object.
  85   MFI.setObjectSize(FrameIndex, ValueVTs.size());
  86   return static_cast<unsigned>(Local);
  87 }
  88
  89 /// We need a base pointer in the case of having items on the stack that
  90 /// require stricter alignment than the stack pointer itself.  Because we need
  91 /// to shift the stack pointer by some unknown amount to force the alignment,
  92 /// we need to record the value of the stack pointer on entry to the function.
  93 bool WebAssemblyFrameLowering::hasBP(const MachineFunction &MF) const {
  94   const auto *RegInfo =
  95       MF.getSubtarget<WebAssemblySubtarget>().getRegisterInfo();
  96   return RegInfo->hasStackRealignment(MF);
  97 }
  98
  99 /// Return true if the specified function should have a dedicated frame pointer
 100 /// register.
 101 bool WebAssemblyFrameLowering::hasFP(const MachineFunction &MF) const {
 102   const MachineFrameInfo &MFI = MF.getFrameInfo();
 103
 104   // When we have var-sized objects, we move the stack pointer by an unknown
 105   // amount, and need to emit a frame pointer to restore the stack to where we
 106   // were on function entry.
 107   // If we already need a base pointer, we use that to fix up the stack pointer.
 108   // If there are no fixed-size objects, we would have no use of a frame
 109   // pointer, and thus should not emit one.
 110   bool HasFixedSizedObjects = MFI.getStackSize() > 0;
 111   bool NeedsFixedReference = !hasBP(MF) || HasFixedSizedObjects;
 112
 113   return MFI.isFrameAddressTaken() ||
 114          (MFI.hasVarSizedObjects() && NeedsFixedReference) ||
 115          MFI.hasStackMap() || MFI.hasPatchPoint();
 116 }
 117
 118 /// Under normal circumstances, when a frame pointer is not required, we reserve
 119 /// argument space for call sites in the function immediately on entry to the
 120 /// current function. This eliminates the need for add/sub sp brackets around
 121 /// call sites. Returns true if the call frame is included as part of the stack
 122 /// frame.
 123 bool WebAssemblyFrameLowering::hasReservedCallFrame(
 124     const MachineFunction &MF) const {
 125   return !MF.getFrameInfo().hasVarSizedObjects();
 126 }
 127
 128 // Returns true if this function needs a local user-space stack pointer for its
 129 // local frame (not for exception handling).
 130 bool WebAssemblyFrameLowering::needsSPForLocalFrame(
 131     const MachineFunction &MF) const {
 132   auto &MFI = MF.getFrameInfo();
 133   auto &MRI = MF.getRegInfo();
 134   // llvm.stacksave can explicitly read SP register and it can appear without
 135   // dynamic alloca.
 136   bool HasExplicitSPUse =
 137       any_of(MRI.use_operands(getSPReg(MF)),
 138              [](MachineOperand &MO) { return !MO.isImplicit(); });
 139
 140   return MFI.getStackSize() || MFI.adjustsStack() || hasFP(MF) ||
 141          HasExplicitSPUse;
 142 }
 143
 144 // In function with EH pads, we need to make a copy of the value of
 145 // __stack_pointer global in SP32/64 register, in order to use it when
 146 // restoring __stack_pointer after an exception is caught.
 147 bool WebAssemblyFrameLowering::needsPrologForEH(
 148     const MachineFunction &MF) const {
 149   auto EHType = MF.getTarget().getMCAsmInfo()->getExceptionHandlingType();
 150   return EHType == ExceptionHandling::Wasm &&
 151          MF.getFunction().hasPersonalityFn() && MF.getFrameInfo().hasCalls();
 152 }
 153
 154 /// Returns true if this function needs a local user-space stack pointer.
 155 /// Unlike a machine stack pointer, the wasm user stack pointer is a global
 156 /// variable, so it is loaded into a register in the prolog.
 157 bool WebAssemblyFrameLowering::needsSP(const MachineFunction &MF) const {
 158   return needsSPForLocalFrame(MF) || needsPrologForEH(MF);
 159 }
 160
 161 /// Returns true if the local user-space stack pointer needs to be written back
 162 /// to __stack_pointer global by this function (this is not meaningful if
 163 /// needsSP is false). If false, the stack red zone can be used and only a local
 164 /// SP is needed.
 165 bool WebAssemblyFrameLowering::needsSPWriteback(
 166     const MachineFunction &MF) const {
 167   auto &MFI = MF.getFrameInfo();
 168   assert(needsSP(MF));
 169   // When we don't need a local stack pointer for its local frame but only to
 170   // support EH, we don't need to write SP back in the epilog, because we don't
 171   // bump down the stack pointer in the prolog. We need to write SP back in the
 172   // epilog only if
 173   // 1. We need SP not only for EH support but also because we actually use
 174   // stack or we have a frame address taken.
 175   // 2. We cannot use the red zone.
 176   bool CanUseRedZone = MFI.getStackSize() <= RedZoneSize && !MFI.hasCalls() &&
 177                        !MF.getFunction().hasFnAttribute(Attribute::NoRedZone);
 178   return needsSPForLocalFrame(MF) && !CanUseRedZone;
 179 }
 180
 181 unsigned WebAssemblyFrameLowering::getSPReg(const MachineFunction &MF) {
 182   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 183              ? WebAssembly::SP64
 184              : WebAssembly::SP32;
 185 }
 186
 187 unsigned WebAssemblyFrameLowering::getFPReg(const MachineFunction &MF) {
 188   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 189              ? WebAssembly::FP64
 190              : WebAssembly::FP32;
 191 }
 192
 193 unsigned
 194 WebAssemblyFrameLowering::getOpcConst(const MachineFunction &MF) {
 195   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 196              ? WebAssembly::CONST_I64
 197              : WebAssembly::CONST_I32;
 198 }
 199
 200 unsigned WebAssemblyFrameLowering::getOpcAdd(const MachineFunction &MF) {
 201   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 202              ? WebAssembly::ADD_I64
 203              : WebAssembly::ADD_I32;
 204 }
 205
 206 unsigned WebAssemblyFrameLowering::getOpcSub(const MachineFunction &MF) {
 207   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 208              ? WebAssembly::SUB_I64
 209              : WebAssembly::SUB_I32;
 210 }
 211
 212 unsigned WebAssemblyFrameLowering::getOpcAnd(const MachineFunction &MF) {
 213   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 214              ? WebAssembly::AND_I64
 215              : WebAssembly::AND_I32;
 216 }
 217
 218 unsigned
 219 WebAssemblyFrameLowering::getOpcGlobGet(const MachineFunction &MF) {
 220   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 221              ? WebAssembly::GLOBAL_GET_I64
 222              : WebAssembly::GLOBAL_GET_I32;
 223 }
 224
 225 unsigned
 226 WebAssemblyFrameLowering::getOpcGlobSet(const MachineFunction &MF) {
 227   return MF.getSubtarget<WebAssemblySubtarget>().hasAddr64()
 228              ? WebAssembly::GLOBAL_SET_I64
 229              : WebAssembly::GLOBAL_SET_I32;
 230 }
 231
 232 void WebAssemblyFrameLowering::writeSPToGlobal(
 233     unsigned SrcReg, MachineFunction &MF, MachineBasicBlock &MBB,
 234     MachineBasicBlock::iterator &InsertStore, const DebugLoc &DL) const {
 235   const auto *TII = MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
 236
 237   const char *ES = "__stack_pointer";
 238   auto *SPSymbol = MF.createExternalSymbolName(ES);
 239
 240   BuildMI(MBB, InsertStore, DL, TII->get(getOpcGlobSet(MF)))
 241       .addExternalSymbol(SPSymbol)
 242       .addReg(SrcReg);
 243 }
 244
 245 MachineBasicBlock::iterator
 246 WebAssemblyFrameLowering::eliminateCallFramePseudoInstr(
 247     MachineFunction &MF, MachineBasicBlock &MBB,
 248     MachineBasicBlock::iterator I) const {
 249   assert(!I->getOperand(0).getImm() && (hasFP(MF) || hasBP(MF)) &&
 250          "Call frame pseudos should only be used for dynamic stack adjustment");
 251   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
 252   const auto *TII = ST.getInstrInfo();
 253   if (I->getOpcode() == TII->getCallFrameDestroyOpcode() &&
 254       needsSPWriteback(MF)) {
 255     DebugLoc DL = I->getDebugLoc();
 256     writeSPToGlobal(getSPReg(MF), MF, MBB, I, DL);
 257   }
 258   return MBB.erase(I);
 259 }
 260
 261 void WebAssemblyFrameLowering::emitPrologue(MachineFunction &MF,
 262                                             MachineBasicBlock &MBB) const {
 263   // TODO: Do ".setMIFlag(MachineInstr::FrameSetup)" on emitted instructions
 264   auto &MFI = MF.getFrameInfo();
 265   assert(MFI.getCalleeSavedInfo().empty() &&
 266          "WebAssembly should not have callee-saved registers");
 267
 268   if (!needsSP(MF))
 269     return;
 270   uint64_t StackSize = MFI.getStackSize();
 271
 272   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
 273   const auto *TII = ST.getInstrInfo();
 274   auto &MRI = MF.getRegInfo();
 275
 276   auto InsertPt = MBB.begin();
 277   while (InsertPt != MBB.end() &&
 278          WebAssembly::isArgument(InsertPt->getOpcode()))
 279     ++InsertPt;
 280   DebugLoc DL;
 281
 282   const TargetRegisterClass *PtrRC =
 283       MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
 284   unsigned SPReg = getSPReg(MF);
 285   if (StackSize)
 286     SPReg = MRI.createVirtualRegister(PtrRC);
 287
 288   const char *ES = "__stack_pointer";
 289   auto *SPSymbol = MF.createExternalSymbolName(ES);
 290   BuildMI(MBB, InsertPt, DL, TII->get(getOpcGlobGet(MF)), SPReg)
 291       .addExternalSymbol(SPSymbol);
 292
 293   bool HasBP = hasBP(MF);
 294   if (HasBP) {
 295     auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
 296     Register BasePtr = MRI.createVirtualRegister(PtrRC);
 297     FI->setBasePointerVreg(BasePtr);
 298     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), BasePtr)
 299         .addReg(SPReg);
 300   }
 301   if (StackSize) {
 302     // Subtract the frame size
 303     Register OffsetReg = MRI.createVirtualRegister(PtrRC);
 304     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
 305         .addImm(StackSize);
 306     BuildMI(MBB, InsertPt, DL, TII->get(getOpcSub(MF)), getSPReg(MF))
 307         .addReg(SPReg)
 308         .addReg(OffsetReg);
 309   }
 310   if (HasBP) {
 311     Register BitmaskReg = MRI.createVirtualRegister(PtrRC);
 312     Align Alignment = MFI.getMaxAlign();
 313     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), BitmaskReg)
 314         .addImm((int64_t) ~(Alignment.value() - 1));
 315     BuildMI(MBB, InsertPt, DL, TII->get(getOpcAnd(MF)), getSPReg(MF))
 316         .addReg(getSPReg(MF))
 317         .addReg(BitmaskReg);
 318   }
 319   if (hasFP(MF)) {
 320     // Unlike most conventional targets (where FP points to the saved FP),
 321     // FP points to the bottom of the fixed-size locals, so we can use positive
 322     // offsets in load/store instructions.
 323     BuildMI(MBB, InsertPt, DL, TII->get(WebAssembly::COPY), getFPReg(MF))
 324         .addReg(getSPReg(MF));
 325   }
 326   if (StackSize && needsSPWriteback(MF)) {
 327     writeSPToGlobal(getSPReg(MF), MF, MBB, InsertPt, DL);
 328   }
 329 }
 330
 331 void WebAssemblyFrameLowering::emitEpilogue(MachineFunction &MF,
 332                                             MachineBasicBlock &MBB) const {
 333   uint64_t StackSize = MF.getFrameInfo().getStackSize();
 334   if (!needsSP(MF) || !needsSPWriteback(MF))
 335     return;
 336   auto &ST = MF.getSubtarget<WebAssemblySubtarget>();
 337   const auto *TII = ST.getInstrInfo();
 338   auto &MRI = MF.getRegInfo();
 339   auto InsertPt = MBB.getFirstTerminator();
 340   DebugLoc DL;
 341
 342   if (InsertPt != MBB.end())
 343     DL = InsertPt->getDebugLoc();
 344
 345   // Restore the stack pointer. If we had fixed-size locals, add the offset
 346   // subtracted in the prolog.
 347   unsigned SPReg = 0;
 348   unsigned SPFPReg = hasFP(MF) ? getFPReg(MF) : getSPReg(MF);
 349   if (hasBP(MF)) {
 350     auto FI = MF.getInfo<WebAssemblyFunctionInfo>();
 351     SPReg = FI->getBasePointerVreg();
 352   } else if (StackSize) {
 353     const TargetRegisterClass *PtrRC =
 354         MRI.getTargetRegisterInfo()->getPointerRegClass(MF);
 355     Register OffsetReg = MRI.createVirtualRegister(PtrRC);
 356     BuildMI(MBB, InsertPt, DL, TII->get(getOpcConst(MF)), OffsetReg)
 357         .addImm(StackSize);
 358     // In the epilog we don't need to write the result back to the SP32/64
 359     // physreg because it won't be used again. We can use a stackified register
 360     // instead.
 361     SPReg = MRI.createVirtualRegister(PtrRC);
 362     BuildMI(MBB, InsertPt, DL, TII->get(getOpcAdd(MF)), SPReg)
 363         .addReg(SPFPReg)
 364         .addReg(OffsetReg);
 365   } else {
 366     SPReg = SPFPReg;
 367   }
 368
 369   writeSPToGlobal(SPReg, MF, MBB, InsertPt, DL);
 370 }
 371
 372 bool WebAssemblyFrameLowering::isSupportedStackID(
 373     TargetStackID::Value ID) const {
 374   // Use the Object stack for WebAssembly locals which can only be accessed
 375   // by name, not via an address in linear memory.
 376   if (ID == TargetStackID::WasmLocal)
 377     return true;
 378
 379   return TargetFrameLowering::isSupportedStackID(ID);
 380 }
 381
 382 TargetFrameLowering::DwarfFrameBase
 383 WebAssemblyFrameLowering::getDwarfFrameBase(const MachineFunction &MF) const {
 384   DwarfFrameBase Loc;
 385   Loc.Kind = DwarfFrameBase::WasmFrameBase;
 386   const WebAssemblyFunctionInfo &MFI = *MF.getInfo<WebAssemblyFunctionInfo>();
 387   if (needsSP(MF) && MFI.isFrameBaseVirtual()) {
 388     unsigned LocalNum = MFI.getFrameBaseLocal();
 389     Loc.Location.WasmLoc = {WebAssembly::TI_LOCAL, LocalNum};
 390   } else {
 391     // TODO: This should work on a breakpoint at a function with no frame,
 392     // but probably won't work for traversing up the stack.
 393     Loc.Location.WasmLoc = {WebAssembly::TI_GLOBAL_RELOC, 0};
 394   }
 395   return Loc;
 396 }