[clang] [libc] [llvm] [AMDGPU] Implement variadic functions by IR lowering (PR #93362)
Jon Chesterfield via llvm-commits
llvm-commits at lists.llvm.org
Tue Jun 4 04:36:24 PDT 2024
================
@@ -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
----------------
JonChesterfield wrote:
The addrspace qualifier on these intrinsics might be worth rolling back on the basis that their lowering is independent of addrspace on amdgpu and other targets don't use the addrspace forms. Even on amdgpu clang still emits the intrinsics using addrspace(0). I'll ping Alex about that sometime after this patch goes through.
https://github.com/llvm/llvm-project/pull/93362
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