WebAssemblyMCInstLower.cpp

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// WebAssemblyMCInstLower.cpp - Convert WebAssembly MachineInstr to an MCInst //
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file contains code to lower WebAssembly MachineInstrs to their
/// corresponding MCInst records.
///
//===----------------------------------------------------------------------===//

#include "WebAssemblyMCInstLower.h"
#include "WebAssemblyAsmPrinter.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblyRuntimeLibcallSignatures.h"
#include "WebAssemblyUtilities.h"
#include "llvm/CodeGen/AsmPrinter.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/IR/Constants.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/MC/MCContext.h"
#include "llvm/MC/MCExpr.h"
#include "llvm/MC/MCInst.h"
#include "llvm/MC/MCSymbolWasm.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;

// This disables the removal of registers when lowering into MC, as required
// by some current tests.
static cl::opt<bool>
    WasmKeepRegisters("wasm-keep-registers", cl::Hidden,
                      cl::desc("WebAssembly: output stack registers in"
                               " instruction output for test purposes only."),
                      cl::init(false));

static unsigned regInstructionToStackInstruction(unsigned OpCode);
static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI);

MCSymbol *
WebAssemblyMCInstLower::GetGlobalAddressSymbol(const MachineOperand &MO) const {
  const GlobalValue *Global = MO.getGlobal();
  MCSymbolWasm *WasmSym = cast<MCSymbolWasm>(Printer.getSymbol(Global));

  if (const auto *FuncTy = dyn_cast<FunctionType>(Global->getValueType())) {
    const MachineFunction &MF = *MO.getParent()->getParent()->getParent();
    const TargetMachine &TM = MF.getTarget();
    const Function &CurrentFunc = MF.getFunction();

    SmallVector<MVT, 1> ResultMVTs;
    SmallVector<MVT, 4> ParamMVTs;
    ComputeSignatureVTs(FuncTy, CurrentFunc, TM, ParamMVTs, ResultMVTs);

    auto Signature = SignatureFromMVTs(ResultMVTs, ParamMVTs);
    WasmSym->setSignature(Signature.get());
    Printer.addSignature(std::move(Signature));
    WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);
  }

  return WasmSym;
}

MCSymbol *WebAssemblyMCInstLower::GetExternalSymbolSymbol(
    const MachineOperand &MO) const {
  const char *Name = MO.getSymbolName();
  MCSymbolWasm *WasmSym =
      cast<MCSymbolWasm>(Printer.GetExternalSymbolSymbol(Name));
  const WebAssemblySubtarget &Subtarget = Printer.getSubtarget();

  // __stack_pointer is a global variable; all other external symbols used by
  // CodeGen are functions.  It's OK to hardcode knowledge of specific symbols
  // here; this method is precisely there for fetching the signatures of known
  // Clang-provided symbols.
  if (strcmp(Name, "__stack_pointer") == 0) {
    WasmSym->setType(wasm::WASM_SYMBOL_TYPE_GLOBAL);
    WasmSym->setGlobalType(wasm::WasmGlobalType{
        uint8_t(Subtarget.hasAddr64() ? wasm::WASM_TYPE_I64
                                      : wasm::WASM_TYPE_I32),
        true});
    return WasmSym;
  }

  SmallVector<wasm::ValType, 4> Returns;
  SmallVector<wasm::ValType, 4> Params;
  GetLibcallSignature(Subtarget, Name, Returns, Params);
  auto Signature =
      make_unique<wasm::WasmSignature>(std::move(Returns), std::move(Params));
  WasmSym->setSignature(Signature.get());
  Printer.addSignature(std::move(Signature));
  WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);

  return WasmSym;
}

MCOperand WebAssemblyMCInstLower::LowerSymbolOperand(MCSymbol *Sym,
                                                     int64_t Offset,
                                                     bool IsFunc,
                                                     bool IsGlob) const {
  MCSymbolRefExpr::VariantKind VK =
      IsFunc ? MCSymbolRefExpr::VK_WebAssembly_FUNCTION
             : IsGlob ? MCSymbolRefExpr::VK_WebAssembly_GLOBAL
                      : MCSymbolRefExpr::VK_None;

  const MCExpr *Expr = MCSymbolRefExpr::create(Sym, VK, Ctx);

  if (Offset != 0) {
    if (IsFunc)
      report_fatal_error("Function addresses with offsets not supported");
    if (IsGlob)
      report_fatal_error("Global indexes with offsets not supported");
    Expr =
        MCBinaryExpr::createAdd(Expr, MCConstantExpr::create(Offset, Ctx), Ctx);
  }

  return MCOperand::createExpr(Expr);
}

// Return the WebAssembly type associated with the given register class.
static wasm::ValType getType(const TargetRegisterClass *RC) {
  if (RC == &WebAssembly::I32RegClass)
    return wasm::ValType::I32;
  if (RC == &WebAssembly::I64RegClass)
    return wasm::ValType::I64;
  if (RC == &WebAssembly::F32RegClass)
    return wasm::ValType::F32;
  if (RC == &WebAssembly::F64RegClass)
    return wasm::ValType::F64;
  if (RC == &WebAssembly::V128RegClass)
    return wasm::ValType::V128;
  llvm_unreachable("Unexpected register class");
}

void WebAssemblyMCInstLower::Lower(const MachineInstr *MI,
                                   MCInst &OutMI) const {
  OutMI.setOpcode(MI->getOpcode());

  const MCInstrDesc &Desc = MI->getDesc();
  for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
    const MachineOperand &MO = MI->getOperand(i);

    MCOperand MCOp;
    switch (MO.getType()) {
    default:
      MI->print(errs());
      llvm_unreachable("unknown operand type");
    case MachineOperand::MO_MachineBasicBlock:
      MI->print(errs());
      llvm_unreachable("MachineBasicBlock operand should have been rewritten");
    case MachineOperand::MO_Register: {
      // Ignore all implicit register operands.
      if (MO.isImplicit())
        continue;
      const WebAssemblyFunctionInfo &MFI =
          *MI->getParent()->getParent()->getInfo<WebAssemblyFunctionInfo>();
      unsigned WAReg = MFI.getWAReg(MO.getReg());
      MCOp = MCOperand::createReg(WAReg);
      break;
    }
    case MachineOperand::MO_Immediate:
      if (i < Desc.NumOperands) {
        const MCOperandInfo &Info = Desc.OpInfo[i];
        if (Info.OperandType == WebAssembly::OPERAND_TYPEINDEX) {
          MCSymbol *Sym = Printer.createTempSymbol("typeindex");

          SmallVector<wasm::ValType, 4> Returns;
          SmallVector<wasm::ValType, 4> Params;

          const MachineRegisterInfo &MRI =
              MI->getParent()->getParent()->getRegInfo();
          for (const MachineOperand &MO : MI->defs())
            Returns.push_back(getType(MRI.getRegClass(MO.getReg())));
          for (const MachineOperand &MO : MI->explicit_uses())
            if (MO.isReg())
              Params.push_back(getType(MRI.getRegClass(MO.getReg())));

          // call_indirect instructions have a callee operand at the end which
          // doesn't count as a param.
          if (WebAssembly::isCallIndirect(*MI))
            Params.pop_back();

          MCSymbolWasm *WasmSym = cast<MCSymbolWasm>(Sym);
          auto Signature = make_unique<wasm::WasmSignature>(std::move(Returns),
                                                            std::move(Params));
          WasmSym->setSignature(Signature.get());
          Printer.addSignature(std::move(Signature));
          WasmSym->setType(wasm::WASM_SYMBOL_TYPE_FUNCTION);

          const MCExpr *Expr = MCSymbolRefExpr::create(
              WasmSym, MCSymbolRefExpr::VK_WebAssembly_TYPEINDEX, Ctx);
          MCOp = MCOperand::createExpr(Expr);
          break;
        }
      }
      MCOp = MCOperand::createImm(MO.getImm());
      break;
    case MachineOperand::MO_FPImmediate: {
      // TODO: MC converts all floating point immediate operands to double.
      // This is fine for numeric values, but may cause NaNs to change bits.
      const ConstantFP *Imm = MO.getFPImm();
      if (Imm->getType()->isFloatTy())
        MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToFloat());
      else if (Imm->getType()->isDoubleTy())
        MCOp = MCOperand::createFPImm(Imm->getValueAPF().convertToDouble());
      else
        llvm_unreachable("unknown floating point immediate type");
      break;
    }
    case MachineOperand::MO_GlobalAddress:
      assert(MO.getTargetFlags() == WebAssemblyII::MO_NO_FLAG &&
             "WebAssembly does not use target flags on GlobalAddresses");
      MCOp = LowerSymbolOperand(GetGlobalAddressSymbol(MO), MO.getOffset(),
                                MO.getGlobal()->getValueType()->isFunctionTy(),
                                false);
      break;
    case MachineOperand::MO_ExternalSymbol:
      // The target flag indicates whether this is a symbol for a
      // variable or a function.
      assert((MO.getTargetFlags() & ~WebAssemblyII::MO_SYMBOL_MASK) == 0 &&
             "WebAssembly uses only symbol flags on ExternalSymbols");
      MCOp = LowerSymbolOperand(
          GetExternalSymbolSymbol(MO), /*Offset=*/0,
          (MO.getTargetFlags() & WebAssemblyII::MO_SYMBOL_FUNCTION) != 0,
          (MO.getTargetFlags() & WebAssemblyII::MO_SYMBOL_GLOBAL) != 0);
      break;
    }

    OutMI.addOperand(MCOp);
  }

  if (!WasmKeepRegisters)
    removeRegisterOperands(MI, OutMI);
}

static void removeRegisterOperands(const MachineInstr *MI, MCInst &OutMI) {
  // Remove all uses of stackified registers to bring the instruction format
  // into its final stack form used thruout MC, and transition opcodes to
  // their _S variant.
  // We do this seperate from the above code that still may need these
  // registers for e.g. call_indirect signatures.
  // See comments in lib/Target/WebAssembly/WebAssemblyInstrFormats.td for
  // details.
  // TODO: the code above creates new registers which are then removed here.
  // That code could be slightly simplified by not doing that, though maybe
  // it is simpler conceptually to keep the code above in "register mode"
  // until this transition point.
  // FIXME: we are not processing inline assembly, which contains register
  // operands, because it is used by later target generic code.
  if (MI->isDebugInstr() || MI->isLabel() || MI->isInlineAsm())
    return;

  // Transform to _S instruction.
  auto RegOpcode = OutMI.getOpcode();
  auto StackOpcode = regInstructionToStackInstruction(RegOpcode);
  OutMI.setOpcode(StackOpcode);

  // Remove register operands.
  for (auto I = OutMI.getNumOperands(); I; --I) {
    auto &MO = OutMI.getOperand(I - 1);
    if (MO.isReg()) {
      OutMI.erase(&MO);
    }
  }
}

static unsigned regInstructionToStackInstruction(unsigned OpCode) {
  // For most opcodes, this function could have been implemented as "return
  // OpCode + 1", but since table-gen alphabetically sorts them, this cannot be
  // guaranteed (see e.g. BR and BR_IF). Instead we use a giant switch statement
  // generated by a custom TableGen backend (WebAssemblyStackifierEmitter.cpp)
  // that emits switch cases of the form
  //
  //   case WebAssembly::RegisterInstr: return WebAssembly::StackInstr;
  //
  // for every pair of equivalent register and stack instructions.
  switch (OpCode) {
  default:
    llvm_unreachable(
        "unknown WebAssembly instruction in WebAssemblyMCInstLower pass");
#include "WebAssemblyGenStackifier.inc"
  }
}