================ @@ -0,0 +1,1037 @@ +//===-- ExpandVariadicsPass.cpp --------------------------------*- 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 +// +//===----------------------------------------------------------------------===// +// +// This is an optimization pass for variadic functions. If called from codegen, +// it can serve as the implementation of variadic functions for a given target. +// +// The strategy is to turn the ... part of a variadic function into a va_list +// and fix up the call sites. The majority of the pass is target independent. +// The exceptions are the va_list type itself and the rules for where to store +// variables in memory such that va_arg can iterate over them given a va_list. +// +// The majority of the plumbing is splitting the variadic function into a +// single basic block that packs the variadic arguments into a va_list and +// a second function that does the work of the original. That packing is +// exactly what is done by va_start. Further, the transform from ... to va_list +// replaced va_start with an operation to copy a va_list from the new argument, +// which is exactly a va_copy. This is useful for reducing target-dependence. +// +// A va_list instance is a forward iterator, where the primary operation va_arg +// is dereference-then-increment. This interface forces significant convergent +// evolution between target specific implementations. The variation in runtime +// data layout is limited to that representable by the iterator, parameterised +// by the type passed to the va_arg instruction. +// +// Therefore the majority of the target specific subtlety is packing arguments +// into a stack allocated buffer such that a va_list can be initialised with it +// and the va_arg expansion for the target will find the arguments at runtime. +// +// The aggregate effect is to unblock other transforms, most critically the +// general purpose inliner. Known calls to variadic functions become zero cost. +// +// Consistency with clang is primarily tested by emitting va_arg using clang +// then expanding the variadic functions using this pass, followed by trying +// to constant fold the functions to no-ops. +// +// Target specific behaviour is tested in IR - mainly checking that values are +// put into positions in call frames that make sense for that particular target. +// +// There is one "clever" invariant in use. va_start intrinsics that are not +// within a varidic functions are an error in the IR verifier. When this +// transform moves blocks from a variadic function into a fixed arity one, it +// moves va_start intrinsics along with everything else. That means that the +// va_start intrinsics that need to be rewritten to use the trailing argument +// are exactly those that are in non-variadic functions so no further state +// is needed to distinguish those that need to be rewritten. +// +//===----------------------------------------------------------------------===// + +#include "llvm/Transforms/IPO/ExpandVariadics.h" +#include "llvm/ADT/SmallVector.h" +#include "llvm/CodeGen/Passes.h" +#include "llvm/IR/Constants.h" +#include "llvm/IR/IRBuilder.h" +#include "llvm/IR/IntrinsicInst.h" +#include "llvm/IR/Module.h" +#include "llvm/IR/PassManager.h" +#include "llvm/InitializePasses.h" +#include "llvm/Pass.h" +#include "llvm/Passes/OptimizationLevel.h" +#include "llvm/Support/CommandLine.h" +#include "llvm/TargetParser/Triple.h" +#include "llvm/Transforms/Utils/ModuleUtils.h" + +#define DEBUG_TYPE "expand-variadics" + +using namespace llvm; + +namespace { + +cl::opt<ExpandVariadicsMode> ExpandVariadicsModeOption( + DEBUG_TYPE "-override", cl::desc("Override the behaviour of " DEBUG_TYPE), + cl::init(ExpandVariadicsMode::Unspecified), + cl::values(clEnumValN(ExpandVariadicsMode::Unspecified, "unspecified", + "Use the implementation defaults"), + clEnumValN(ExpandVariadicsMode::Disable, "disable", + "Disable the pass entirely"), + clEnumValN(ExpandVariadicsMode::Optimize, "optimize", + "Optimise without changing ABI"), + clEnumValN(ExpandVariadicsMode::Lowering, "lowering", + "Change variadic calling convention"))); + +bool commandLineOverride() { + return ExpandVariadicsModeOption != ExpandVariadicsMode::Unspecified; +} + +// Instances of this class encapsulate the target-dependant behaviour as a +// function of triple. Implementing a new ABI is adding a case to the switch +// in create(llvm::Triple) at the end of this file. +class VariadicABIInfo { +protected: + VariadicABIInfo() {} + +public: + static std::unique_ptr<VariadicABIInfo> create(llvm::Triple const &Triple); + + // Allow overriding whether the pass runs on a per-target basis + virtual bool enableForTarget() = 0; + + // Whether a valist instance is passed by value or by address + // I.e. does it need to be alloca'ed and stored into, or can + // it be passed directly in a SSA register + virtual bool vaListPassedInSSARegister() = 0; + + // The type of a va_list iterator object + virtual Type *vaListType(LLVMContext &Ctx) = 0; + + // The type of a va_list as a function argument as lowered by C + virtual Type *vaListParameterType(Module &M) = 0; + + // Initialize an allocated va_list object to point to an already + // initialized contiguous memory region. + // Return the value to pass as the va_list argument + virtual Value *initializeVaList(Module &M, LLVMContext &Ctx, + IRBuilder<> &Builder, AllocaInst *VaList, + Value *Buffer) = 0; + + struct VAArgSlotInfo { + Align DataAlign; // With respect to the call frame + bool Indirect; // Passed via a pointer + }; + virtual VAArgSlotInfo slotInfo(const DataLayout &DL, Type *Parameter) = 0; + + // Targets implemented so far all have the same trivial lowering for these + bool vaEndIsNop() { return true; } + bool vaCopyIsMemcpy() { return true; } + + virtual ~VariadicABIInfo() {} +}; + +// Module implements getFunction() which returns nullptr on missing declaration +// and getOrInsertFunction which creates one when absent. Intrinsics.h only +// implements getDeclaration which creates one when missing. Checking whether +// an intrinsic exists thus inserts it in the module and it then needs to be +// deleted again to clean up. +// The right name for the two functions on intrinsics would match Module::, +// but doing that in a single change would introduce nullptr dereferences +// where currently there are none. The minimal collateral damage approach +// would split the change over a release to help downstream branches. As it +// is unclear what approach will be preferred, implementing the trivial +// function here in the meantime to decouple from that discussion. +Function *getPreexistingDeclaration(Module *M, Intrinsic::ID Id, + ArrayRef<Type *> Tys = std::nullopt) { + auto *FT = Intrinsic::getType(M->getContext(), Id, Tys); + return M->getFunction(Tys.empty() ? Intrinsic::getName(Id) + : Intrinsic::getName(Id, Tys, M, FT)); +} + +class ExpandVariadics : public ModulePass { + + // The pass construction sets the default to optimize when called from middle + // end and lowering when called from the backend. The command line variable + // overrides that. This is useful for testing and debugging. It also allows + // building an applications with variadic functions wholly removed if one + // has sufficient control over the dependencies, e.g. a statically linked + // clang that has no variadic function calls remaining in the binary. + +public: + static char ID; + const ExpandVariadicsMode Mode; + std::unique_ptr<VariadicABIInfo> ABI; + + ExpandVariadics(ExpandVariadicsMode Mode) + : ModulePass(ID), + Mode(commandLineOverride() ? ExpandVariadicsModeOption : Mode) {} + + StringRef getPassName() const override { return "Expand variadic functions"; } + + bool rewriteABI() { return Mode == ExpandVariadicsMode::Lowering; } + + bool runOnModule(Module &M) override; + + bool runOnFunction(Module &M, IRBuilder<> &Builder, Function *F); + + Function *replaceAllUsesWithNewDeclaration(Module &M, + Function *OriginalFunction); + + Function *deriveFixedArityReplacement(Module &M, IRBuilder<> &Builder, + Function *OriginalFunction); + + Function *defineVariadicWrapper(Module &M, IRBuilder<> &Builder, + Function *VariadicWrapper, + Function *FixedArityReplacement); + + bool expandCall(Module &M, IRBuilder<> &Builder, CallBase *CB, FunctionType *, + Function *NF); + + template <Intrinsic::ID ID, typename InstructionType> + bool expandIntrinsicUsers(Module &M, IRBuilder<> &Builder, + PointerType *IntrinsicArgType) { + bool Changed = false; + const DataLayout &DL = M.getDataLayout(); + if (Function *Intrinsic = + getPreexistingDeclaration(&M, ID, {IntrinsicArgType})) { + for (User *U : llvm::make_early_inc_range(Intrinsic->users())) { + if (auto *I = dyn_cast<InstructionType>(U)) { + Changed |= expandVAIntrinsicCall(Builder, DL, I); + } + } + if (Intrinsic->use_empty()) + Intrinsic->eraseFromParent(); + } + return Changed; + } + + bool expandVAIntrinsicUsersWithAddrspace(Module &M, IRBuilder<> &Builder, + unsigned Addrspace) { + auto &Ctx = M.getContext(); + PointerType *IntrinsicArgType = PointerType::get(Ctx, Addrspace); + bool Changed = false; + + // expand vastart before vacopy as vastart may introduce a vacopy + Changed |= expandIntrinsicUsers<Intrinsic::vastart, VAStartInst>( + M, Builder, IntrinsicArgType); + Changed |= expandIntrinsicUsers<Intrinsic::vaend, VAEndInst>( + M, Builder, IntrinsicArgType); + Changed |= expandIntrinsicUsers<Intrinsic::vacopy, VACopyInst>( + M, Builder, IntrinsicArgType); + return Changed; + } + + bool expandVAIntrinsicCall(IRBuilder<> &Builder, const DataLayout &DL, + VAStartInst *Inst); + + bool expandVAIntrinsicCall(IRBuilder<> &, const DataLayout &, + VAEndInst *Inst); + + bool expandVAIntrinsicCall(IRBuilder<> &Builder, const DataLayout &DL, + VACopyInst *Inst); + + FunctionType *inlinableVariadicFunctionType(Module &M, FunctionType *FTy) { + // The type of "FTy" with the ... removed and a va_list appended + SmallVector<Type *> ArgTypes(FTy->param_begin(), FTy->param_end()); + ArgTypes.push_back(ABI->vaListParameterType(M)); + return FunctionType::get(FTy->getReturnType(), ArgTypes, + /*IsVarArgs=*/false); + } + + static ConstantInt *sizeOfAlloca(LLVMContext &Ctx, const DataLayout &DL, + AllocaInst *Alloced) { + Type *AllocaType = Alloced->getAllocatedType(); + TypeSize AllocaTypeSize = DL.getTypeAllocSize(AllocaType); + uint64_t AsInt = AllocaTypeSize.getFixedValue(); + return ConstantInt::get(Type::getInt64Ty(Ctx), AsInt); + } + + bool expansionApplicableToFunction(Module &M, Function *F) { + if (F->isIntrinsic() || !F->isVarArg() || + F->hasFnAttribute(Attribute::Naked)) { + return false; + } + + if (F->getCallingConv() != CallingConv::C) + return false; + + if (rewriteABI()) + return true; + + if (!F->hasExactDefinition()) + return false; + + return true; + } + + bool expansionApplicableToFunctionCall(CallBase *CB) { + if (CallInst *CI = dyn_cast<CallInst>(CB)) { + if (CI->isMustTailCall()) { + // Cannot expand musttail calls + return false; + } + + if (CI->getCallingConv() != CallingConv::C) + return false; + + return true; + } + + if (isa<InvokeInst>(CB)) { + // Invoke not implemented in initial implementation of pass + return false; + } + + // Other unimplemented derivative of CallBase + return false; + } + + class ExpandedCallFrame { + // Helper for constructing an alloca instance containing the arguments bound + // to the variadic ... parameter, rearranged to allow indexing through a + // va_list iterator + enum { N = 4 }; + SmallVector<Type *, N> FieldTypes; + enum Tag { Store, Memcpy, Padding }; + SmallVector<std::tuple<Value *, uint64_t, Tag>, N> Source; + + template <Tag tag> void append(Type *FieldType, Value *V, uint64_t Bytes) { + FieldTypes.push_back(FieldType); + Source.push_back({V, Bytes, tag}); + } + + public: + void store(LLVMContext &Ctx, Type *T, Value *V) { append<Store>(T, V, 0); } + + void memcpy(LLVMContext &Ctx, Type *T, Value *V, uint64_t Bytes) { + append<Memcpy>(T, V, Bytes); + } + + void padding(LLVMContext &Ctx, uint64_t By) { + append<Padding>(ArrayType::get(Type::getInt8Ty(Ctx), By), nullptr, 0); + } + + size_t size() const { return FieldTypes.size(); } + bool empty() const { return FieldTypes.empty(); } + + StructType *asStruct(LLVMContext &Ctx, StringRef Name) { + const bool IsPacked = true; + return StructType::create(Ctx, FieldTypes, + (Twine(Name) + ".vararg").str(), IsPacked); + } + + void initializeStructAlloca(const DataLayout &DL, IRBuilder<> &Builder, + AllocaInst *Alloced) { + + StructType *VarargsTy = cast<StructType>(Alloced->getAllocatedType()); + + for (size_t I = 0; I < size(); I++) { + + auto [V, bytes, tag] = Source[I]; + + if (tag == Padding) { + assert(V == nullptr); + continue; + } + + auto Dst = Builder.CreateStructGEP(VarargsTy, Alloced, I); + + assert(V != nullptr); + + if (tag == Store) { + Builder.CreateStore(V, Dst); + } + + if (tag == Memcpy) { + Builder.CreateMemCpy(Dst, {}, V, {}, bytes); + } + } + } + }; +}; + +bool ExpandVariadics::runOnModule(Module &M) { + bool Changed = false; + if (Mode == ExpandVariadicsMode::Disable) + return Changed; + + llvm::Triple Triple(M.getTargetTriple()); + + ABI = VariadicABIInfo::create(Triple); + if (!ABI) { + return Changed; + } + + if (!ABI->enableForTarget()) { + return Changed; + } + + auto &Ctx = M.getContext(); + const DataLayout &DL = M.getDataLayout(); + IRBuilder<> Builder(Ctx); + + // Lowering needs to run on all functions exactly once. + // Optimize could run on functions containing va_start exactly once. + for (Function &F : llvm::make_early_inc_range(M)) + Changed |= runOnFunction(M, Builder, &F); + + // After runOnFunction, all known calls to known variadic functions have been + // replaced. va_start intrinsics are presently (and invalidly!) only present + // in functions that used to be variadic and have now been replaced to take a + // va_list instead. If lowering as opposed to optimising, calls to unknown + // variadic functions have also been replaced. + + { + // 0 and AllocaAddrSpace are sufficient for the targets implemented so far ---------------- efriedma-quic wrote:
If you wanted to make this more general, you could scan the module for function declarations starting with llvm.va_start etc. Not sure it's necessary. https://github.com/llvm/llvm-project/pull/93362 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
