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//=- 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 "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/Pass.h"
using namespace llvm;
#define DEBUG_TYPE "wasm-fix-br-table-defaults"
namespace {
class WebAssemblyFixBrTableDefaults final : public MachineFunctionPass {
StringRef getPassName() const override {
return "WebAssembly Fix br_table Defaults";
}
bool runOnMachineFunction(MachineFunction &MF) override;
public:
static char ID; // Pass identification, replacement for typeid
WebAssemblyFixBrTableDefaults() : MachineFunctionPass(ID) {}
};
char WebAssemblyFixBrTableDefaults::ID = 0;
// `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 *fixBrTable(MachineInstr &MI, MachineBasicBlock *MBB,
MachineFunction &MF) {
// Get the header block, which contains the redundant range check.
assert(MBB->pred_size() == 1 && "Expected a single guard predecessor");
auto *HeaderMBB = *MBB->pred_begin();
// Find the conditional jump to the default target. If it doesn't exist, the
// default target is unreachable anyway, so we can keep the existing dummy
// target.
MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
SmallVector<MachineOperand, 2> Cond;
const auto &TII = *MF.getSubtarget<WebAssemblySubtarget>().getInstrInfo();
bool Analyzed = !TII.analyzeBranch(*HeaderMBB, TBB, FBB, Cond);
assert(Analyzed && "Could not analyze jump header branches");
(void)Analyzed;
// Here are the possible outcomes. '_' is nullptr, `J` is the jump table block
// aka MBB, 'D' is the default block.
//
// TBB | FBB | Meaning
// _ | _ | No default block, header falls through to jump table
// J | _ | No default block, header jumps to the jump table
// D | _ | Header jumps to the default and falls through to the jump table
// D | J | Header jumps to the default and also to the jump table
if (TBB && TBB != MBB) {
assert((FBB == nullptr || FBB == MBB) &&
"Expected jump or fallthrough to br_table block");
assert(Cond.size() == 2 && Cond[1].isReg() && "Unexpected condition info");
// If the range check checks an i64 value, we cannot optimize it out because
// the i64 index is truncated to an i32, making values over 2^32
// indistinguishable from small numbers. There are also other strange edge
// cases that can arise in practice that we don't want to reason about, so
// conservatively only perform the optimization if the range check is the
// normal case of an i32.gt_u.
MachineRegisterInfo &MRI = MF.getRegInfo();
auto *RangeCheck = MRI.getVRegDef(Cond[1].getReg());
assert(RangeCheck != nullptr);
if (RangeCheck->getOpcode() != WebAssembly::GT_U_I32)
return nullptr;
// Remove the dummy default target and install the real one.
MI.RemoveOperand(MI.getNumExplicitOperands() - 1);
MI.addOperand(MF, MachineOperand::CreateMBB(TBB));
}
// Remove any branches from the header and splice in the jump table instead
TII.removeBranch(*HeaderMBB, nullptr);
HeaderMBB->splice(HeaderMBB->end(), MBB, MBB->begin(), MBB->end());
// Update CFG to skip the old jump table block. Remove shared successors
// before transferring to avoid duplicated successors.
HeaderMBB->removeSuccessor(MBB);
for (auto &Succ : MBB->successors())
if (HeaderMBB->isSuccessor(Succ))
HeaderMBB->removeSuccessor(Succ);
HeaderMBB->transferSuccessorsAndUpdatePHIs(MBB);
// Remove the old jump table block from the function
MF.erase(MBB);
return HeaderMBB;
}
bool WebAssemblyFixBrTableDefaults::runOnMachineFunction(MachineFunction &MF) {
LLVM_DEBUG(dbgs() << "********** Fixing br_table Default Targets **********\n"
"********** Function: "
<< MF.getName() << '\n');
bool Changed = false;
SmallPtrSet<MachineBasicBlock *, 16> MBBSet;
for (auto &MBB : MF)
MBBSet.insert(&MBB);
while (!MBBSet.empty()) {
MachineBasicBlock *MBB = *MBBSet.begin();
MBBSet.erase(MBB);
for (auto &MI : *MBB) {
if (WebAssembly::isBrTable(MI)) {
auto *Fixed = fixBrTable(MI, MBB, MF);
if (Fixed != nullptr) {
MBBSet.erase(Fixed);
Changed = true;
}
break;
}
}
}
if (Changed) {
// We rewrote part of the function; recompute relevant things.
MF.RenumberBlocks();
return true;
}
return false;
}
} // end anonymous namespace
INITIALIZE_PASS(WebAssemblyFixBrTableDefaults, DEBUG_TYPE,
"Removes range checks and sets br_table default targets", false,
false)
FunctionPass *llvm::createWebAssemblyFixBrTableDefaults() {
return new WebAssemblyFixBrTableDefaults();
}
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