1 //===-- PatchableFunction.cpp - Patchable prologues for LLVM -------------===//
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 // This file implements edits function bodies in place to support the
10 // "patchable-function" attribute.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "llvm/CodeGen/MachineFunction.h"
15 #include "llvm/CodeGen/MachineFunctionPass.h"
16 #include "llvm/CodeGen/MachineInstrBuilder.h"
17 #include "llvm/CodeGen/TargetInstrInfo.h"
18 #include "llvm/CodeGen/TargetSubtargetInfo.h"
19 #include "llvm/InitializePasses.h"
20 #include "llvm/Pass.h"
21 #include "llvm/PassRegistry.h"
22 
23 using namespace llvm;
24 
25 namespace {
26 struct PatchableFunction : public MachineFunctionPass {
27   static char ID; // Pass identification, replacement for typeid
28   PatchableFunction() : MachineFunctionPass(ID) {
29     initializePatchableFunctionPass(*PassRegistry::getPassRegistry());
30   }
31 
32   bool runOnMachineFunction(MachineFunction &F) override;
33    MachineFunctionProperties getRequiredProperties() const override {
34     return MachineFunctionProperties().set(
35         MachineFunctionProperties::Property::NoVRegs);
36   }
37 };
38 }
39 
40 bool PatchableFunction::runOnMachineFunction(MachineFunction &MF) {
41   if (MF.getFunction().hasFnAttribute("patchable-function-entry")) {
42     MachineBasicBlock &FirstMBB = *MF.begin();
43     const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
44     // The initial .loc covers PATCHABLE_FUNCTION_ENTER.
45     BuildMI(FirstMBB, FirstMBB.begin(), DebugLoc(),
46             TII->get(TargetOpcode::PATCHABLE_FUNCTION_ENTER));
47     return true;
48   }
49 
50   if (!MF.getFunction().hasFnAttribute("patchable-function"))
51     return false;
52 
53 #ifndef NDEBUG
54   Attribute PatchAttr = MF.getFunction().getFnAttribute("patchable-function");
55   StringRef PatchType = PatchAttr.getValueAsString();
56   assert(PatchType == "prologue-short-redirect" && "Only possibility today!");
57 #endif
58 
59   auto &FirstMBB = *MF.begin();
60   auto *TII = MF.getSubtarget().getInstrInfo();
61 
62   MachineBasicBlock::iterator FirstActualI = llvm::find_if(
63       FirstMBB, [](const MachineInstr &MI) { return !MI.isMetaInstruction(); });
64 
65   if (FirstActualI == FirstMBB.end()) {
66     // As of Microsoft documentation on /hotpatch feature, we must ensure that
67     // "the first instruction of each function is at least two bytes, and no
68     // jump within the function goes to the first instruction"
69 
70     // When the first MBB is empty, insert a patchable no-op. This ensures the
71     // first instruction is patchable in two special cases:
72     // - the function is empty (e.g. unreachable)
73     // - the function jumps back to the first instruction, which is in a
74     // successor MBB.
75     BuildMI(&FirstMBB, DebugLoc(), TII->get(TargetOpcode::PATCHABLE_OP))
76         .addImm(2)
77         .addImm(TargetOpcode::PATCHABLE_OP);
78     MF.ensureAlignment(Align(16));
79     return true;
80   }
81 
82   auto MIB = BuildMI(FirstMBB, FirstActualI, FirstActualI->getDebugLoc(),
83                      TII->get(TargetOpcode::PATCHABLE_OP))
84                  .addImm(2)
85                  .addImm(FirstActualI->getOpcode());
86 
87   for (auto &MO : FirstActualI->operands())
88     MIB.add(MO);
89 
90   FirstActualI->eraseFromParent();
91   MF.ensureAlignment(Align(16));
92   return true;
93 }
94 
95 char PatchableFunction::ID = 0;
96 char &llvm::PatchableFunctionID = PatchableFunction::ID;
97 INITIALIZE_PASS(PatchableFunction, "patchable-function",
98                 "Implement the 'patchable-function' attribute", false, false)
99