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