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//===-- PatchableFunction.cpp - Patchable prologues for LLVM -------------===//
// 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
// This file implements edits function bodies in place to support the
// "patchable-function" attribute.
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/TargetInstrInfo.h"
#include "llvm/CodeGen/TargetSubtargetInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/PassRegistry.h"
using namespace llvm;
namespace {
struct PatchableFunction : public MachineFunctionPass {
static char ID; // Pass identification, replacement for typeid
PatchableFunction() : MachineFunctionPass(ID) {
bool runOnMachineFunction(MachineFunction &F) override;
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
bool PatchableFunction::runOnMachineFunction(MachineFunction &MF) {
if (MF.getFunction().hasFnAttribute("patchable-function-entry")) {
MachineBasicBlock &FirstMBB = *MF.begin();
const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
// The initial .loc covers PATCHABLE_FUNCTION_ENTER.
BuildMI(FirstMBB, FirstMBB.begin(), DebugLoc(),
return true;
if (!MF.getFunction().hasFnAttribute("patchable-function"))
return false;
#ifndef NDEBUG
Attribute PatchAttr = MF.getFunction().getFnAttribute("patchable-function");
StringRef PatchType = PatchAttr.getValueAsString();
assert(PatchType == "prologue-short-redirect" && "Only possibility today!");
auto &FirstMBB = *MF.begin();
auto *TII = MF.getSubtarget().getInstrInfo();
MachineBasicBlock::iterator FirstActualI = llvm::find_if(
FirstMBB, [](const MachineInstr &MI) { return !MI.isMetaInstruction(); });
if (FirstActualI == FirstMBB.end()) {
// As of Microsoft documentation on /hotpatch feature, we must ensure that
// "the first instruction of each function is at least two bytes, and no
// jump within the function goes to the first instruction"
// When the first MBB is empty, insert a patchable no-op. This ensures the
// first instruction is patchable in two special cases:
// - the function is empty (e.g. unreachable)
// - the function jumps back to the first instruction, which is in a
// successor MBB.
BuildMI(&FirstMBB, DebugLoc(), TII->get(TargetOpcode::PATCHABLE_OP))
return true;
auto MIB = BuildMI(FirstMBB, FirstActualI, FirstActualI->getDebugLoc(),
for (auto &MO : FirstActualI->operands())
return true;
char PatchableFunction::ID = 0;
char &llvm::PatchableFunctionID = PatchableFunction::ID;
INITIALIZE_PASS(PatchableFunction, "patchable-function",
"Implement the 'patchable-function' attribute", false, false)