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 "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"
  34 using 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.
  45 bool 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.
  53 bool 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.
  75 bool 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).
  82 bool 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.
  91 bool 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.
 101 bool 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.
 109 bool 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
 125 void 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
 137 MachineBasicBlock::iterator
 138 WebAssemblyFrameLowering::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
 152 void 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
 225 void 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 }