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/MachineFunctionPass.h"
39 #include "llvm/CodeGen/MachineInstrBuilder.h"
40 
41 #define AVOIDCALL_DESC "X86 avoid trailing call pass"
42 #define AVOIDCALL_NAME "x86-avoid-trailing-call"
43 
44 #define DEBUG_TYPE AVOIDCALL_NAME
45 
46 using namespace llvm;
47 
48 namespace {
49 class X86AvoidTrailingCallPass : public MachineFunctionPass {
50 public:
51   X86AvoidTrailingCallPass() : MachineFunctionPass(ID) {}
52 
53   bool runOnMachineFunction(MachineFunction &MF) override;
54 
55   static char ID;
56 
57 private:
58   StringRef getPassName() const override { return AVOIDCALL_DESC; }
59 };
60 } // end anonymous namespace
61 
62 char X86AvoidTrailingCallPass::ID = 0;
63 
64 FunctionPass *llvm::createX86AvoidTrailingCallPass() {
65   return new X86AvoidTrailingCallPass();
66 }
67 
68 INITIALIZE_PASS(X86AvoidTrailingCallPass, AVOIDCALL_NAME, AVOIDCALL_DESC, false, false)
69 
70 // A real instruction is a non-meta, non-pseudo instruction.  Some pseudos
71 // expand to nothing, and some expand to code. This logic conservatively assumes
72 // they might expand to nothing.
73 static bool isCallOrRealInstruction(MachineInstr &MI) {
74   return MI.isCall() || (!MI.isPseudo() && !MI.isMetaInstruction());
75 }
76 
77 // Return true if this is a call instruction, but not a tail call.
78 static bool isCallInstruction(const MachineInstr &MI) {
79   return MI.isCall() && !MI.isReturn();
80 }
81 
82 bool X86AvoidTrailingCallPass::runOnMachineFunction(MachineFunction &MF) {
83   const X86Subtarget &STI = MF.getSubtarget<X86Subtarget>();
84   const X86InstrInfo &TII = *STI.getInstrInfo();
85   assert(STI.isTargetWin64() && "pass only runs on Win64");
86 
87   // We don't need to worry about any of the invariants described above if there
88   // is no unwind info (CFI).
89   if (!MF.hasWinCFI())
90     return false;
91 
92   // FIXME: Perhaps this pass should also replace SEH_Epilogue by inserting nops
93   // before epilogues.
94 
95   bool Changed = false;
96   for (MachineBasicBlock &MBB : MF) {
97     // Look for basic blocks that precede funclet entries or are at the end of
98     // the function.
99     MachineBasicBlock *NextMBB = MBB.getNextNode();
100     if (NextMBB && !NextMBB->isEHFuncletEntry())
101       continue;
102 
103     // Find the last real instruction in this block.
104     auto LastRealInstr = llvm::find_if(reverse(MBB), isCallOrRealInstruction);
105 
106     // If the block is empty or the last real instruction is a call instruction,
107     // insert an int3. If there is a call instruction, insert the int3 between
108     // the call and any labels or other meta instructions. If the block is
109     // empty, insert at block end.
110     bool IsEmpty = LastRealInstr == MBB.rend();
111     bool IsCall = !IsEmpty && isCallInstruction(*LastRealInstr);
112     if (IsEmpty || IsCall) {
113       LLVM_DEBUG({
114         if (IsCall) {
115           dbgs() << "inserting int3 after trailing call instruction:\n";
116           LastRealInstr->dump();
117           dbgs() << '\n';
118         } else {
119           dbgs() << "inserting int3 in trailing empty MBB:\n";
120           MBB.dump();
121         }
122       });
123 
124       MachineBasicBlock::iterator MBBI = MBB.end();
125       DebugLoc DL;
126       if (IsCall) {
127         MBBI = std::next(LastRealInstr.getReverse());
128         DL = LastRealInstr->getDebugLoc();
129       }
130       BuildMI(MBB, MBBI, DL, TII.get(X86::INT3));
131       Changed = true;
132     }
133   }
134 
135   return Changed;
136 }
137