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//===---- IndirectThunks.h - Indirect thunk insertion helpers ---*- C++ -*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
///
/// \file
/// Contains a base ThunkInserter class that simplifies injection of MI thunks
/// as well as a default implementation of MachineFunctionPass wrapping
/// several `ThunkInserter`s for targets to extend.
///
//===----------------------------------------------------------------------===//
#ifndef LLVM_CODEGEN_INDIRECTTHUNKS_H
#define LLVM_CODEGEN_INDIRECTTHUNKS_H
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineModuleInfo.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
namespace llvm {
/// This class assists in inserting MI thunk functions into the module and
/// rewriting the existing machine functions to call these thunks.
///
/// One of the common cases is implementing security mitigations that involve
/// replacing some machine code patterns with calls to special thunk functions.
///
/// Inserting a module pass late in the codegen pipeline may increase memory
/// usage, as it serializes the transformations and forces preceding passes to
/// produce machine code for all functions before running the module pass.
/// For that reason, ThunkInserter can be driven by a MachineFunctionPass by
/// passing one MachineFunction at a time to its `run(MMI, MF)` method.
/// Then, the derived class should
/// * call createThunkFunction from its insertThunks method exactly once for
/// each of the thunk functions to be inserted
/// * populate the thunk in its populateThunk method
///
/// Note that if some other pass is responsible for rewriting the functions,
/// the insertThunks method may simply create all possible thunks at once,
/// probably postponed until the first occurrence of possibly affected MF.
///
/// Alternatively, insertThunks method can rewrite MF by itself and only insert
/// the thunks being called. In that case InsertedThunks variable can be used
/// to track which thunks were already inserted.
///
/// In any case, the thunk function has to be inserted on behalf of some other
/// function and then populated on its own "iteration" later - this is because
/// MachineFunctionPass will see the newly created functions, but they first
/// have to go through the preceding passes from the same pass manager,
/// possibly even through the instruction selector.
//
// FIXME Maybe implement a documented and less surprising way of modifying
// the module from a MachineFunctionPass that is restricted to inserting
// completely new functions to the module.
template <typename Derived, typename InsertedThunksTy = bool>
class ThunkInserter {
Derived &getDerived() { return *static_cast<Derived *>(this); }
// A variable used to track whether (and possible which) thunks have been
// inserted so far. InsertedThunksTy is usually a bool, but can be other types
// to represent more than one type of thunk. Requires an |= operator to
// accumulate results.
InsertedThunksTy InsertedThunks;
protected:
// Interface for subclasses to use.
/// Create an empty thunk function.
///
/// The new function will eventually be passed to populateThunk. If multiple
/// thunks are created, populateThunk can distinguish them by their names.
void createThunkFunction(MachineModuleInfo &MMI, StringRef Name,
bool Comdat = true, StringRef TargetAttrs = "");
protected:
// Interface for subclasses to implement.
//
// Note: all functions are non-virtual and are called via getDerived().
// Note: only doInitialization() has an implementation.
/// Initializes thunk inserter.
void doInitialization(Module &M) {}
/// Returns common prefix for thunk function's names.
const char *getThunkPrefix(); // undefined
/// Checks if MF may use thunks (true - maybe, false - definitely not).
bool mayUseThunk(const MachineFunction &MF); // undefined
/// Rewrites the function if necessary, returns the set of thunks added.
InsertedThunksTy insertThunks(MachineModuleInfo &MMI, MachineFunction &MF,
InsertedThunksTy ExistingThunks); // undefined
/// Populate the thunk function with instructions.
///
/// If multiple thunks are created, the content that must be inserted in the
/// thunk function body should be derived from the MF's name.
///
/// Depending on the preceding passes in the pass manager, by the time
/// populateThunk is called, MF may have a few target-specific instructions
/// (such as a single MBB containing the return instruction).
void populateThunk(MachineFunction &MF); // undefined
public:
void init(Module &M) {
InsertedThunks = InsertedThunksTy{};
getDerived().doInitialization(M);
}
// return `true` if `MMI` or `MF` was modified
bool run(MachineModuleInfo &MMI, MachineFunction &MF);
};
template <typename Derived, typename InsertedThunksTy>
void ThunkInserter<Derived, InsertedThunksTy>::createThunkFunction(
MachineModuleInfo &MMI, StringRef Name, bool Comdat,
StringRef TargetAttrs) {
assert(Name.starts_with(getDerived().getThunkPrefix()) &&
"Created a thunk with an unexpected prefix!");
Module &M = const_cast<Module &>(*MMI.getModule());
LLVMContext &Ctx = M.getContext();
auto *Type = FunctionType::get(Type::getVoidTy(Ctx), false);
Function *F = Function::Create(Type,
Comdat ? GlobalValue::LinkOnceODRLinkage
: GlobalValue::InternalLinkage,
Name, &M);
if (Comdat) {
F->setVisibility(GlobalValue::HiddenVisibility);
F->setComdat(M.getOrInsertComdat(Name));
}
// Add Attributes so that we don't create a frame, unwind information, or
// inline.
AttrBuilder B(Ctx);
B.addAttribute(llvm::Attribute::NoUnwind);
B.addAttribute(llvm::Attribute::Naked);
if (TargetAttrs != "")
B.addAttribute("target-features", TargetAttrs);
F->addFnAttrs(B);
// Populate our function a bit so that we can verify.
BasicBlock *Entry = BasicBlock::Create(Ctx, "entry", F);
IRBuilder<> Builder(Entry);
Builder.CreateRetVoid();
// MachineFunctions aren't created automatically for the IR-level constructs
// we already made. Create them and insert them into the module.
MachineFunction &MF = MMI.getOrCreateMachineFunction(*F);
// A MachineBasicBlock must not be created for the Entry block; code
// generation from an empty naked function in C source code also does not
// generate one. At least GlobalISel asserts if this invariant isn't
// respected.
// Set MF properties. We never use vregs...
MF.getProperties().set(MachineFunctionProperties::Property::NoVRegs);
}
template <typename Derived, typename InsertedThunksTy>
bool ThunkInserter<Derived, InsertedThunksTy>::run(MachineModuleInfo &MMI,
MachineFunction &MF) {
// If MF is not a thunk, check to see if we need to insert a thunk.
if (!MF.getName().starts_with(getDerived().getThunkPrefix())) {
// Only add thunks if one of the functions may use them.
if (!getDerived().mayUseThunk(MF))
return false;
// The target can use InsertedThunks to detect whether relevant thunks
// have already been inserted.
// FIXME: Provide the way for insertThunks to notify us whether it changed
// the MF, instead of conservatively assuming it did.
InsertedThunks |= getDerived().insertThunks(MMI, MF, InsertedThunks);
return true;
}
// If this *is* a thunk function, we need to populate it with the correct MI.
getDerived().populateThunk(MF);
return true;
}
/// Basic implementation of MachineFunctionPass wrapping one or more
/// `ThunkInserter`s passed as type parameters.
template <typename... Inserters>
class ThunkInserterPass : public MachineFunctionPass {
protected:
std::tuple<Inserters...> TIs;
ThunkInserterPass(char &ID) : MachineFunctionPass(ID) {}
public:
bool doInitialization(Module &M) override {
initTIs(M, TIs);
return false;
}
bool runOnMachineFunction(MachineFunction &MF) override {
auto &MMI = getAnalysis<MachineModuleInfoWrapperPass>().getMMI();
return runTIs(MMI, MF, TIs);
}
private:
template <typename... ThunkInserterT>
static void initTIs(Module &M,
std::tuple<ThunkInserterT...> &ThunkInserters) {
(..., std::get<ThunkInserterT>(ThunkInserters).init(M));
}
template <typename... ThunkInserterT>
static bool runTIs(MachineModuleInfo &MMI, MachineFunction &MF,
std::tuple<ThunkInserterT...> &ThunkInserters) {
return (0 | ... | std::get<ThunkInserterT>(ThunkInserters).run(MMI, MF));
}
};
} // namespace llvm
#endif