1//===----- X86AvoidTrailingCall.cpp - Insert int3 after trailing calls ----===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// The Windows x64 unwinder decodes the instruction stream during unwinding.
10// The unwinder decodes forward from the current PC to detect epilogue code
11// patterns.
12//
13// First, this means that there must be an instruction after every
14// call instruction for the unwinder to decode. LLVM must maintain the invariant
15// that the last instruction of a function or funclet is not a call, or the
16// unwinder may decode into the next function. Similarly, a call may not
17// immediately precede an epilogue code pattern. As of this writing, the
18// SEH_Epilogue pseudo instruction takes care of that.
19//
20// Second, all non-tail call jump targets must be within the *half-open*
21// interval of the bounds of the function. The unwinder distinguishes between
22// internal jump instructions and tail calls in an epilogue sequence by checking
23// the jump target against the function bounds from the .pdata section. This
24// means that the last regular MBB of an LLVM function must not be empty if
25// there are regular jumps targeting it.
26//
27// This pass upholds these invariants by ensuring that blocks at the end of a
28// function or funclet are a) not empty and b) do not end in a CALL instruction.
29//
30// Unwinder implementation for reference:
31// https://github.com/dotnet/coreclr/blob/a9f3fc16483eecfc47fb79c362811d870be02249/src/unwinder/amd64/unwinder_amd64.cpp#L1015
32//
33//===----------------------------------------------------------------------===//
34
35#include "X86.h"
36#include "X86InstrInfo.h"
37#include "X86Subtarget.h"
38#include "llvm/CodeGen/MachineInstrBuilder.h"
39
40#define AVOIDCALL_DESC "X86 avoid trailing call pass"
41#define AVOIDCALL_NAME "x86-avoid-trailing-call"
42
43#define DEBUG_TYPE AVOIDCALL_NAME
44
45using namespace llvm;
46
47namespace {
48class X86AvoidTrailingCallPass : public MachineFunctionPass {
49public:
50  X86AvoidTrailingCallPass() : MachineFunctionPass(ID) {}
51
52  bool runOnMachineFunction(MachineFunction &MF) override;
53
54  static char ID;
55
56private:
57  StringRef getPassName() const override { return AVOIDCALL_DESC; }
58};
59} // end anonymous namespace
60
61char X86AvoidTrailingCallPass::ID = 0;
62
63FunctionPass *llvm::createX86AvoidTrailingCallPass() {
64  return new X86AvoidTrailingCallPass();
65}
66
67INITIALIZE_PASS(X86AvoidTrailingCallPass, AVOIDCALL_NAME, AVOIDCALL_DESC, false, false)
68
69// A real instruction is a non-meta, non-pseudo instruction.  Some pseudos
70// expand to nothing, and some expand to code. This logic conservatively assumes
71// they might expand to nothing.
72static bool isRealInstruction(MachineInstr &MI) {
73  return !MI.isPseudo() && !MI.isMetaInstruction();
74}
75
76// Return true if this is a call instruction, but not a tail call.
77static bool isCallInstruction(const MachineInstr &MI) {
78  return MI.isCall() && !MI.isReturn();
79}
80
81bool X86AvoidTrailingCallPass::runOnMachineFunction(MachineFunction &MF) {
82  const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
83  const X86InstrInfo &TII = *STI.getInstrInfo();
84  assert(STI.isTargetWin64() && "pass only runs on Win64");
85
86  // We don't need to worry about any of the invariants described above if there
87  // is no unwind info (CFI).
88  if (!MF.hasWinCFI())
89    return false;
90
91  // FIXME: Perhaps this pass should also replace SEH_Epilogue by inserting nops
92  // before epilogues.
93
94  bool Changed = false;
95  for (MachineBasicBlock &MBB : MF) {
96    // Look for basic blocks that precede funclet entries or are at the end of
97    // the function.
98    MachineBasicBlock *NextMBB = MBB.getNextNode();
99    if (NextMBB && !NextMBB->isEHFuncletEntry())
100      continue;
101
102    // Find the last real instruction in this block.
103    auto LastRealInstr = llvm::find_if(reverse(MBB), isRealInstruction);
104
105    // If the block is empty or the last real instruction is a call instruction,
106    // insert an int3. If there is a call instruction, insert the int3 between
107    // the call and any labels or other meta instructions. If the block is
108    // empty, insert at block end.
109    bool IsEmpty = LastRealInstr == MBB.rend();
110    bool IsCall = !IsEmpty && isCallInstruction(*LastRealInstr);
111    if (IsEmpty || IsCall) {
112      LLVM_DEBUG({
113        if (IsCall) {
114          dbgs() << "inserting int3 after trailing call instruction:\n";
115          LastRealInstr->dump();
116          dbgs() << '\n';
117        } else {
118          dbgs() << "inserting int3 in trailing empty MBB:\n";
119          MBB.dump();
120        }
121      });
122
123      MachineBasicBlock::iterator MBBI = MBB.end();
124      DebugLoc DL;
125      if (IsCall) {
126        MBBI = std::next(LastRealInstr.getReverse());
127        DL = LastRealInstr->getDebugLoc();
128      }
129      BuildMI(MBB, MBBI, DL, TII.get(X86::INT3));
130      Changed = true;
131    }
132  }
133
134  return Changed;
135}
136