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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 decodes the instruction stream during unwinding.
// The unwinder decodes forward from the current PC to detect epilogue code
// patterns.
// First, this means that there must be an instruction after every
// call instruction for the unwinder to decode. LLVM must maintain the invariant
// that the last instruction of a function or funclet is not a call, or the
// unwinder may decode into the next function. Similarly, a call may not
// immediately precede an epilogue code pattern. As of this writing, the
// SEH_Epilogue pseudo instruction takes care of that.
// Second, all non-tail call jump targets must be within the *half-open*
// interval of the bounds of the function. The unwinder distinguishes between
// internal jump instructions and tail calls in an epilogue sequence by checking
// the jump target against the function bounds from the .pdata section. This
// means that the last regular MBB of an LLVM function must not be empty if
// there are regular jumps targeting it.
// This pass upholds these invariants by ensuring that blocks at the end of a
// function or funclet are a) not empty and b) do not end in a CALL instruction.
// Unwinder implementation for reference:
#include "X86.h"
#include "X86InstrInfo.h"
#include "X86Subtarget.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#define AVOIDCALL_DESC "X86 avoid trailing call pass"
#define AVOIDCALL_NAME "x86-avoid-trailing-call"
using namespace llvm;
namespace {
class X86AvoidTrailingCallPass : public MachineFunctionPass {
X86AvoidTrailingCallPass() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
static char ID;
StringRef getPassName() const override { return AVOIDCALL_DESC; }
} // end anonymous namespace
char X86AvoidTrailingCallPass::ID = 0;
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");
// We don't need to worry about any of the invariants described above if there
// is no unwind info (CFI).
if (!MF.hasWinCFI())
return false;
// 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.
auto LastRealInstr = llvm::find_if(reverse(MBB), isRealInstruction);
// If the block is empty or the last real instruction is a call instruction,
// insert an int3. If there is a call instruction, insert the int3 between
// the call and any labels or other meta instructions. If the block is
// empty, insert at block end.
bool IsEmpty = LastRealInstr == MBB.rend();
bool IsCall = !IsEmpty && isCallInstruction(*LastRealInstr);
if (IsEmpty || IsCall) {
if (IsCall) {
dbgs() << "inserting int3 after trailing call instruction:\n";
dbgs() << '\n';
} else {
dbgs() << "inserting int3 in trailing empty MBB:\n";
MachineBasicBlock::iterator MBBI = MBB.end();
DebugLoc DL;
if (IsCall) {
MBBI = std::next(LastRealInstr.getReverse());
DL = LastRealInstr->getDebugLoc();
BuildMI(MBB, MBBI, DL, TII.get(X86::INT3));
Changed = true;
return Changed;