ExpandVariadics.cpp | Explore in Territory

//===-- 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/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/Support/CommandLine.h"
#include "llvm/TargetParser/Triple.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"

#define DEBUG_TYPE …

usingnamespacellvm;

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() { … }

// 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.
// This class may end up instantiated in TargetMachine instances, keeping it
// here for now until enough targets are implemented for the API to evolve.
class 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 = { … }

class ExpandVariadics : public ModulePass { … };

bool ExpandVariadics::runOnModule(Module &M) { … }

bool ExpandVariadics::runOnFunction(Module &M, IRBuilder<> &Builder,
                                    Function *OriginalFunction) { … }

Function *
ExpandVariadics::replaceAllUsesWithNewDeclaration(Module &M,
                                                  Function *OriginalFunction) { … }

Function *
ExpandVariadics::deriveFixedArityReplacement(Module &M, IRBuilder<> &Builder,
                                             Function *OriginalFunction) { … }

Function *
ExpandVariadics::defineVariadicWrapper(Module &M, IRBuilder<> &Builder,
                                       Function *VariadicWrapper,
                                       Function *FixedArityReplacement) { … }

bool ExpandVariadics::expandCall(Module &M, IRBuilder<> &Builder, CallBase *CB,
                                 FunctionType *VarargFunctionType,
                                 Function *NF) { … }

bool ExpandVariadics::expandVAIntrinsicCall(IRBuilder<> &Builder,
                                            const DataLayout &DL,
                                            VAStartInst *Inst) { … }

bool ExpandVariadics::expandVAIntrinsicCall(IRBuilder<> &, const DataLayout &,
                                            VAEndInst *Inst) { … }

bool ExpandVariadics::expandVAIntrinsicCall(IRBuilder<> &Builder,
                                            const DataLayout &DL,
                                            VACopyInst *Inst) { … }

struct Amdgpu final : public VariadicABIInfo { … };

struct NVPTX final : public VariadicABIInfo { … };

struct Wasm final : public VariadicABIInfo { … };

std::unique_ptr<VariadicABIInfo> VariadicABIInfo::create(const Triple &T) { … }

} // namespace

char ExpandVariadics::ID = …;

INITIALIZE_PASS(…)

ModulePass *llvm::createExpandVariadicsPass(ExpandVariadicsMode M) { … }

PreservedAnalyses ExpandVariadicsPass::run(Module &M, ModuleAnalysisManager &) { … }

ExpandVariadicsPass::ExpandVariadicsPass(ExpandVariadicsMode M) : … { … }
llvm/llvm/lib/Transforms/IPO/ExpandVariadics.cpp
