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