llvm-project/llvm/lib/Target/WebAssembly/Utils/WebAssemblyUtilities.cpp

//===-- WebAssemblyUtilities.cpp - WebAssembly Utility Functions ----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file implements several utility functions for WebAssembly.
///
//===----------------------------------------------------------------------===//

#include "WebAssemblyUtilities.h"
#include "WebAssemblyMachineFunctionInfo.h"
#include "WebAssemblySubtarget.h"
#include "llvm/CodeGen/Analysis.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineLoopInfo.h"
#include "llvm/IR/Instructions.h"
#include "llvm/MC/MCContext.h"
using namespace llvm;

const char *const WebAssembly::CxaBeginCatchFn = "__cxa_begin_catch";
const char *const WebAssembly::CxaRethrowFn = "__cxa_rethrow";
const char *const WebAssembly::StdTerminateFn = "_ZSt9terminatev";
const char *const WebAssembly::PersonalityWrapperFn =
    "_Unwind_Wasm_CallPersonality";

// In an ideal world, when objects are added to the MachineFrameInfo by
// FunctionLoweringInfo::set, we could somehow hook into target-specific code to
// ensure they are assigned the right stack ID.  However there isn't a hook that
// runs between then and DAG building time, though, so instead we hoist stack
// objects lazily when they are first used, and comprehensively after the DAG is
// built via the PreprocessISelDAG hook, called by the
// SelectionDAGISel::runOnMachineFunction.  We have to do it in two places
// because we want to do it while building the selection DAG for uses of alloca,
// but not all alloca instructions are used so we have to follow up afterwards.
Optional<unsigned> WebAssembly::getLocalForStackObject(MachineFunction &MF,
                                                       int FrameIndex) {
  auto &MFI = MF.getFrameInfo();

  // If already hoisted to a local, done.
  if (MFI.getStackID(FrameIndex) == TargetStackID::WasmLocal)
    return static_cast<unsigned>(MFI.getObjectOffset(FrameIndex));

  // If not allocated in the object address space, this object will be in
  // linear memory.
  const AllocaInst *AI = MFI.getObjectAllocation(FrameIndex);
  if (!AI || !isWasmVarAddressSpace(AI->getType()->getAddressSpace()))
    return None;

  // Otherwise, allocate this object in the named value stack, outside of linear
  // memory.
  SmallVector<EVT, 4> ValueVTs;
  const WebAssemblyTargetLowering &TLI =
      *MF.getSubtarget<WebAssemblySubtarget>().getTargetLowering();
  WebAssemblyFunctionInfo *FuncInfo = MF.getInfo<WebAssemblyFunctionInfo>();
  ComputeValueVTs(TLI, MF.getDataLayout(), AI->getAllocatedType(), ValueVTs);
  MFI.setStackID(FrameIndex, TargetStackID::WasmLocal);
  // Abuse SP offset to record the index of the first local in the object.
  unsigned Local = FuncInfo->getParams().size() + FuncInfo->getLocals().size();
  MFI.setObjectOffset(FrameIndex, Local);
  // Allocate WebAssembly locals for each non-aggregate component of the
  // allocation.
  for (EVT ValueVT : ValueVTs)
    FuncInfo->addLocal(ValueVT.getSimpleVT());
  // Abuse object size to record number of WebAssembly locals allocated to
  // this object.
  MFI.setObjectSize(FrameIndex, ValueVTs.size());
  return static_cast<unsigned>(Local);
}

/// Test whether MI is a child of some other node in an expression tree.
bool WebAssembly::isChild(const MachineInstr &MI,
                          const WebAssemblyFunctionInfo &MFI) {
  if (MI.getNumOperands() == 0)
    return false;
  const MachineOperand &MO = MI.getOperand(0);
  if (!MO.isReg() || MO.isImplicit() || !MO.isDef())
    return false;
  Register Reg = MO.getReg();
  return Register::isVirtualRegister(Reg) && MFI.isVRegStackified(Reg);
}

bool WebAssembly::mayThrow(const MachineInstr &MI) {
  switch (MI.getOpcode()) {
  case WebAssembly::THROW:
  case WebAssembly::THROW_S:
  case WebAssembly::RETHROW:
  case WebAssembly::RETHROW_S:
    return true;
  }
  if (isCallIndirect(MI.getOpcode()))
    return true;
  if (!MI.isCall())
    return false;

  const MachineOperand &MO = getCalleeOp(MI);
  assert(MO.isGlobal() || MO.isSymbol());

  if (MO.isSymbol()) {
    // Some intrinsics are lowered to calls to external symbols, which are then
    // lowered to calls to library functions. Most of libcalls don't throw, but
    // we only list some of them here now.
    // TODO Consider adding 'nounwind' info in TargetLowering::CallLoweringInfo
    // instead for more accurate info.
    const char *Name = MO.getSymbolName();
    if (strcmp(Name, "memcpy") == 0 || strcmp(Name, "memmove") == 0 ||
        strcmp(Name, "memset") == 0)
      return false;
    return true;
  }

  const auto *F = dyn_cast<Function>(MO.getGlobal());
  if (!F)
    return true;
  if (F->doesNotThrow())
    return false;
  // These functions never throw
  if (F->getName() == CxaBeginCatchFn || F->getName() == PersonalityWrapperFn ||
      F->getName() == StdTerminateFn)
    return false;

  // TODO Can we exclude call instructions that are marked as 'nounwind' in the
  // original LLVm IR? (Even when the callee may throw)
  return true;
}

const MachineOperand &WebAssembly::getCalleeOp(const MachineInstr &MI) {
  switch (MI.getOpcode()) {
  case WebAssembly::CALL:
  case WebAssembly::CALL_S:
  case WebAssembly::RET_CALL:
  case WebAssembly::RET_CALL_S:
    return MI.getOperand(MI.getNumExplicitDefs());
  case WebAssembly::CALL_INDIRECT:
  case WebAssembly::CALL_INDIRECT_S:
  case WebAssembly::RET_CALL_INDIRECT:
  case WebAssembly::RET_CALL_INDIRECT_S:
    return MI.getOperand(MI.getNumExplicitOperands() - 1);
  default:
    llvm_unreachable("Not a call instruction");
  }
}

MCSymbolWasm *WebAssembly::getOrCreateFunctionTableSymbol(
    MCContext &Ctx, const WebAssemblySubtarget *Subtarget) {
  StringRef Name = "__indirect_function_table";
  MCSymbolWasm *Sym = cast_or_null<MCSymbolWasm>(Ctx.lookupSymbol(Name));
  if (Sym) {
    if (!Sym->isFunctionTable())
      Ctx.reportError(SMLoc(), "symbol is not a wasm funcref table");
  } else {
    Sym = cast<MCSymbolWasm>(Ctx.getOrCreateSymbol(Name));
    Sym->setFunctionTable();
    // The default function table is synthesized by the linker.
    Sym->setUndefined();
  }
  // MVP object files can't have symtab entries for tables.
  if (!(Subtarget && Subtarget->hasReferenceTypes()))
    Sym->setOmitFromLinkingSection();
  return Sym;
}

// Find a catch instruction from an EH pad.
MachineInstr *WebAssembly::findCatch(MachineBasicBlock *EHPad) {
  assert(EHPad->isEHPad());
  auto Pos = EHPad->begin();
  // Skip any label or debug instructions. Also skip 'end' marker instructions
  // that may exist after marker placement in CFGStackify.
  while (Pos != EHPad->end() &&
         (Pos->isLabel() || Pos->isDebugInstr() || isMarker(Pos->getOpcode())))
    Pos++;
  if (Pos != EHPad->end() && WebAssembly::isCatch(Pos->getOpcode()))
    return &*Pos;
  return nullptr;
}