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//===----- X86AvoidTrailingCall.cpp - Insert int3 after trailing calls ----===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// The Windows x64 unwinder has trouble unwinding the stack when a return
// address points to the end of the function. This pass maintains the invariant
// that every return address is inside the bounds of its parent function or
// funclet by inserting int3 if the last instruction would otherwise be a call.
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#define DEBUG_TYPE "x86-avoid-trailing-call"
using namespace llvm;
namespace {
class X86AvoidTrailingCallPass : public MachineFunctionPass {
X86AvoidTrailingCallPass() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
StringRef getPassName() const override {
return "X86 avoid trailing call pass";
static char ID;
char X86AvoidTrailingCallPass::ID = 0;
} // end anonymous namespace
FunctionPass *llvm::createX86AvoidTrailingCallPass() {
return new X86AvoidTrailingCallPass();
// A real instruction is a non-meta, non-pseudo instruction. Some pseudos
// expand to nothing, and some expand to code. This logic conservatively assumes
// they might expand to nothing.
static bool isRealInstruction(MachineInstr &MI) {
return !MI.isPseudo() && !MI.isMetaInstruction();
// Return true if this is a call instruction, but not a tail call.
static bool isCallInstruction(const MachineInstr &MI) {
return MI.isCall() && !MI.isReturn();
bool X86AvoidTrailingCallPass::runOnMachineFunction(MachineFunction &MF) {
const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
const X86InstrInfo &TII = *STI.getInstrInfo();
assert(STI.isTargetWin64() && "pass only runs on Win64");
// FIXME: Perhaps this pass should also replace SEH_Epilogue by inserting nops
// before epilogues.
bool Changed = false;
for (MachineBasicBlock &MBB : MF) {
// Look for basic blocks that precede funclet entries or are at the end of
// the function.
MachineBasicBlock *NextMBB = MBB.getNextNode();
if (NextMBB && !NextMBB->isEHFuncletEntry())
// Find the last real instruction in this block, or previous blocks if this
// block is empty.
MachineBasicBlock::reverse_iterator LastRealInstr;
for (MachineBasicBlock &RMBB :
make_range(MBB.getReverseIterator(), MF.rend())) {
LastRealInstr = llvm::find_if(reverse(RMBB), isRealInstruction);
if (LastRealInstr != RMBB.rend())
// Do nothing if this function or funclet has no instructions.
if (LastRealInstr == MF.begin()->rend())
// If this is a call instruction, insert int3 right after it with the same
// DebugLoc. Convert back to a forward iterator and advance the insertion
// position once.
if (isCallInstruction(*LastRealInstr)) {
dbgs() << "inserting int3 after trailing call instruction:\n";
dbgs() << '\n';
MachineBasicBlock::iterator MBBI = std::next(LastRealInstr.getReverse());
BuildMI(*LastRealInstr->getParent(), MBBI, LastRealInstr->getDebugLoc(),
Changed = true;
return Changed;