llvm/llvm/lib/Target/WebAssembly/WebAssemblyFixBrTableDefaults.cpp

//=- WebAssemblyFixBrTableDefaults.cpp - Fix br_table default branch targets -//
//
// 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 a pass that eliminates redundant range checks
/// guarding br_table instructions. Since jump tables on most targets cannot
/// handle out of range indices, LLVM emits these checks before most jump
/// tables. But br_table takes a default branch target as an argument, so it
/// does not need the range checks.
///
//===----------------------------------------------------------------------===//

#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssembly.h"
#include "WebAssemblySubtarget.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Pass.h"

usingnamespacellvm;

#define DEBUG_TYPE …

namespace {

class WebAssemblyFixBrTableDefaults final : public MachineFunctionPass { … };

char WebAssemblyFixBrTableDefaults::ID = …;

// Target independent selection dag assumes that it is ok to use PointerTy
// as the index for a "switch", whereas Wasm so far only has a 32-bit br_table.
// See e.g. SelectionDAGBuilder::visitJumpTableHeader
// We have a 64-bit br_table in the tablegen defs as a result, which does get
// selected, and thus we get incorrect truncates/extensions happening on
// wasm64. Here we fix that.
void fixBrTableIndex(MachineInstr &MI, MachineBasicBlock *MBB,
                     MachineFunction &MF) { … }

// `MI` is a br_table instruction with a dummy default target argument. This
// function finds and adds the default target argument and removes any redundant
// range check preceding the br_table. Returns the MBB that the br_table is
// moved into so it can be removed from further consideration, or nullptr if the
// br_table cannot be optimized.
MachineBasicBlock *fixBrTableDefault(MachineInstr &MI, MachineBasicBlock *MBB,
                                     MachineFunction &MF) { … }

bool WebAssemblyFixBrTableDefaults::runOnMachineFunction(MachineFunction &MF) { … }

} // end anonymous namespace

INITIALIZE_PASS(…)

FunctionPass *llvm::createWebAssemblyFixBrTableDefaults() { … }