ArgumentPromotion.cpp | Explore in Territory

//===- ArgumentPromotion.cpp - Promote by-reference arguments -------------===//
//
// 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 pass promotes "by reference" arguments to be "by value" arguments.  In
// practice, this means looking for internal functions that have pointer
// arguments.  If it can prove, through the use of alias analysis, that an
// argument is *only* loaded, then it can pass the value into the function
// instead of the address of the value.  This can cause recursive simplification
// of code and lead to the elimination of allocas (especially in C++ template
// code like the STL).
//
// This pass also handles aggregate arguments that are passed into a function,
// scalarizing them if the elements of the aggregate are only loaded.  Note that
// by default it refuses to scalarize aggregates which would require passing in
// more than three operands to the function, because passing thousands of
// operands for a large array or structure is unprofitable! This limit can be
// configured or disabled, however.
//
// Note that this transformation could also be done for arguments that are only
// stored to (returning the value instead), but does not currently.  This case
// would be best handled when and if LLVM begins supporting multiple return
// values from functions.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO/ArgumentPromotion.h"

#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Argument.h"
#include "llvm/IR/Attributes.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/NoFolder.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <utility>
#include <vector>

usingnamespacellvm;

#define DEBUG_TYPE …

STATISTIC(NumArgumentsPromoted, "Number of pointer arguments promoted");
STATISTIC(NumArgumentsDead, "Number of dead pointer args eliminated");

namespace {

struct ArgPart { … };

OffsetAndArgPart;

} // end anonymous namespace

static Value *createByteGEP(IRBuilderBase &IRB, const DataLayout &DL,
                            Value *Ptr, Type *ResElemTy, int64_t Offset) { … }

/// DoPromotion - This method actually performs the promotion of the specified
/// arguments, and returns the new function.  At this point, we know that it's
/// safe to do so.
static Function *
doPromotion(Function *F, FunctionAnalysisManager &FAM,
            const DenseMap<Argument *, SmallVector<OffsetAndArgPart, 4>>
                &ArgsToPromote) { … }

/// Return true if we can prove that all callees pass in a valid pointer for the
/// specified function argument.
static bool allCallersPassValidPointerForArgument(
    Argument *Arg, SmallPtrSetImpl<CallBase *> &RecursiveCalls,
    Align NeededAlign, uint64_t NeededDerefBytes) { … }

// Try to prove that all Calls to F do not modify the memory pointed to by Arg,
// using alias analysis local to each caller of F.
static bool isArgUnmodifiedByAllCalls(Argument *Arg,
                                      FunctionAnalysisManager &FAM) { … }

/// Determine that this argument is safe to promote, and find the argument
/// parts it can be promoted into.
static bool findArgParts(Argument *Arg, const DataLayout &DL, AAResults &AAR,
                         unsigned MaxElements, bool IsRecursive,
                         SmallVectorImpl<OffsetAndArgPart> &ArgPartsVec,
                         FunctionAnalysisManager &FAM) { … }

/// Check if callers and callee agree on how promoted arguments would be
/// passed.
static bool areTypesABICompatible(ArrayRef<Type *> Types, const Function &F,
                                  const TargetTransformInfo &TTI) { … }

/// PromoteArguments - This method checks the specified function to see if there
/// are any promotable arguments and if it is safe to promote the function (for
/// example, all callers are direct).  If safe to promote some arguments, it
/// calls the DoPromotion method.
static Function *promoteArguments(Function *F, FunctionAnalysisManager &FAM,
                                  unsigned MaxElements, bool IsRecursive) { … }

PreservedAnalyses ArgumentPromotionPass::run(LazyCallGraph::SCC &C,
                                             CGSCCAnalysisManager &AM,
                                             LazyCallGraph &CG,
                                             CGSCCUpdateResult &UR) { … }
llvm/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp
