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//===- ArgumentPromotion.cpp - Promote by-reference arguments -------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See 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/None.h"
#include "llvm/ADT/Optional.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/CGSCCPassManager.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CallGraphSCCPass.h"
#include "llvm/Analysis/LazyCallGraph.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/MemoryLocation.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.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/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/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/FormatVariadic.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <functional>
#include <iterator>
#include <map>
#include <set>
#include <string>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "argpromotion"
STATISTIC(NumArgumentsPromoted, "Number of pointer arguments promoted");
STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted");
STATISTIC(NumByValArgsPromoted, "Number of byval arguments promoted");
STATISTIC(NumArgumentsDead, "Number of dead pointer args eliminated");
/// A vector used to hold the indices of a single GEP instruction
using IndicesVector = std::vector<uint64_t>;
/// 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, SmallPtrSetImpl<Argument *> &ArgsToPromote,
SmallPtrSetImpl<Argument *> &ByValArgsToTransform,
Optional<function_ref<void(CallBase &OldCS, CallBase &NewCS)>>
ReplaceCallSite) {
// Start by computing a new prototype for the function, which is the same as
// the old function, but has modified arguments.
FunctionType *FTy = F->getFunctionType();
std::vector<Type *> Params;
using ScalarizeTable = std::set<std::pair<Type *, IndicesVector>>;
// ScalarizedElements - If we are promoting a pointer that has elements
// accessed out of it, keep track of which elements are accessed so that we
// can add one argument for each.
// Arguments that are directly loaded will have a zero element value here, to
// handle cases where there are both a direct load and GEP accesses.
std::map<Argument *, ScalarizeTable> ScalarizedElements;
// OriginalLoads - Keep track of a representative load instruction from the
// original function so that we can tell the alias analysis implementation
// what the new GEP/Load instructions we are inserting look like.
// We need to keep the original loads for each argument and the elements
// of the argument that are accessed.
std::map<std::pair<Argument *, IndicesVector>, LoadInst *> OriginalLoads;
// Attribute - Keep track of the parameter attributes for the arguments
// that we are *not* promoting. For the ones that we do promote, the parameter
// attributes are lost
SmallVector<AttributeSet, 8> ArgAttrVec;
AttributeList PAL = F->getAttributes();
// First, determine the new argument list
unsigned ArgNo = 0;
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
++I, ++ArgNo) {
if (ByValArgsToTransform.count(&*I)) {
// Simple byval argument? Just add all the struct element types.
Type *AgTy = cast<PointerType>(I->getType())->getElementType();
StructType *STy = cast<StructType>(AgTy);
llvm::append_range(Params, STy->elements());
ArgAttrVec.insert(ArgAttrVec.end(), STy->getNumElements(),
} else if (!ArgsToPromote.count(&*I)) {
// Unchanged argument
} else if (I->use_empty()) {
// Dead argument (which are always marked as promotable)
} else {
// Okay, this is being promoted. This means that the only uses are loads
// or GEPs which are only used by loads
// In this table, we will track which indices are loaded from the argument
// (where direct loads are tracked as no indices).
ScalarizeTable &ArgIndices = ScalarizedElements[&*I];
for (User *U : make_early_inc_range(I->users())) {
Instruction *UI = cast<Instruction>(U);
Type *SrcTy;
if (LoadInst *L = dyn_cast<LoadInst>(UI))
SrcTy = L->getType();
SrcTy = cast<GetElementPtrInst>(UI)->getSourceElementType();
// Skip dead GEPs and remove them.
if (isa<GetElementPtrInst>(UI) && UI->use_empty()) {
IndicesVector Indices;
Indices.reserve(UI->getNumOperands() - 1);
// Since loads will only have a single operand, and GEPs only a single
// non-index operand, this will record direct loads without any indices,
// and gep+loads with the GEP indices.
for (User::op_iterator II = UI->op_begin() + 1, IE = UI->op_end();
II != IE; ++II)
// GEPs with a single 0 index can be merged with direct loads
if (Indices.size() == 1 && Indices.front() == 0)
ArgIndices.insert(std::make_pair(SrcTy, Indices));
LoadInst *OrigLoad;
if (LoadInst *L = dyn_cast<LoadInst>(UI))
OrigLoad = L;
// Take any load, we will use it only to update Alias Analysis
OrigLoad = cast<LoadInst>(UI->user_back());
OriginalLoads[std::make_pair(&*I, Indices)] = OrigLoad;
// Add a parameter to the function for each element passed in.
for (const auto &ArgIndex : ArgIndices) {
// not allowed to dereference ->begin() if size() is 0
if (ArgIndices.size() == 1 && ArgIndices.begin()->second.empty())
Type *RetTy = FTy->getReturnType();
// Construct the new function type using the new arguments.
FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());
// Create the new function body and insert it into the module.
Function *NF = Function::Create(NFTy, F->getLinkage(), F->getAddressSpace(),
NF->copyMetadata(F, 0);
// The new function will have the !dbg metadata copied from the original
// function. The original function may not be deleted, and dbg metadata need
// to be unique so we need to drop it.
LLVM_DEBUG(dbgs() << "ARG PROMOTION: Promoting to:" << *NF << "\n"
<< "From: " << *F);
// Recompute the parameter attributes list based on the new arguments for
// the function.
NF->setAttributes(AttributeList::get(F->getContext(), PAL.getFnAttributes(),
PAL.getRetAttributes(), ArgAttrVec));
F->getParent()->getFunctionList().insert(F->getIterator(), NF);
// Loop over all of the callers of the function, transforming the call sites
// to pass in the loaded pointers.
SmallVector<Value *, 16> Args;
while (!F->use_empty()) {
CallBase &CB = cast<CallBase>(*F->user_back());
assert(CB.getCalledFunction() == F);
const AttributeList &CallPAL = CB.getAttributes();
IRBuilder<NoFolder> IRB(&CB);
// Loop over the operands, inserting GEP and loads in the caller as
// appropriate.
auto AI = CB.arg_begin();
ArgNo = 0;
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
++I, ++AI, ++ArgNo)
if (!ArgsToPromote.count(&*I) && !ByValArgsToTransform.count(&*I)) {
Args.push_back(*AI); // Unmodified argument
} else if (ByValArgsToTransform.count(&*I)) {
// Emit a GEP and load for each element of the struct.
Type *AgTy = cast<PointerType>(I->getType())->getElementType();
StructType *STy = cast<StructType>(AgTy);
Value *Idxs[2] = {
ConstantInt::get(Type::getInt32Ty(F->getContext()), 0), nullptr};
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
Idxs[1] = ConstantInt::get(Type::getInt32Ty(F->getContext()), i);
auto *Idx =
IRB.CreateGEP(STy, *AI, Idxs, (*AI)->getName() + "." + Twine(i));
// TODO: Tell AA about the new values?
Args.push_back(IRB.CreateLoad(STy->getElementType(i), Idx,
Idx->getName() + ".val"));
} else if (!I->use_empty()) {
// Non-dead argument: insert GEPs and loads as appropriate.
ScalarizeTable &ArgIndices = ScalarizedElements[&*I];
// Store the Value* version of the indices in here, but declare it now
// for reuse.
std::vector<Value *> Ops;
for (const auto &ArgIndex : ArgIndices) {
Value *V = *AI;
LoadInst *OrigLoad =
OriginalLoads[std::make_pair(&*I, ArgIndex.second)];
if (!ArgIndex.second.empty()) {
Type *ElTy = V->getType();
for (auto II : ArgIndex.second) {
// Use i32 to index structs, and i64 for others (pointers/arrays).
// This satisfies GEP constraints.
Type *IdxTy =
(ElTy->isStructTy() ? Type::getInt32Ty(F->getContext())
: Type::getInt64Ty(F->getContext()));
Ops.push_back(ConstantInt::get(IdxTy, II));
// Keep track of the type we're currently indexing.
if (auto *ElPTy = dyn_cast<PointerType>(ElTy))
ElTy = ElPTy->getElementType();
ElTy = GetElementPtrInst::getTypeAtIndex(ElTy, II);
// And create a GEP to extract those indices.
V = IRB.CreateGEP(ArgIndex.first, V, Ops, V->getName() + ".idx");
// Since we're replacing a load make sure we take the alignment
// of the previous load.
LoadInst *newLoad =
IRB.CreateLoad(OrigLoad->getType(), V, V->getName() + ".val");
// Transfer the AA info too.
AAMDNodes AAInfo;
// Push any varargs arguments on the list.
for (; AI != CB.arg_end(); ++AI, ++ArgNo) {
SmallVector<OperandBundleDef, 1> OpBundles;
CallBase *NewCS = nullptr;
if (InvokeInst *II = dyn_cast<InvokeInst>(&CB)) {
NewCS = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
Args, OpBundles, "", &CB);
} else {
auto *NewCall = CallInst::Create(NF, Args, OpBundles, "", &CB);
NewCS = NewCall;
AttributeList::get(F->getContext(), CallPAL.getFnAttributes(),
CallPAL.getRetAttributes(), ArgAttrVec));
NewCS->copyMetadata(CB, {LLVMContext::MD_prof, LLVMContext::MD_dbg});
// Update the callgraph to know that the callsite has been transformed.
if (ReplaceCallSite)
(*ReplaceCallSite)(CB, *NewCS);
if (!CB.use_empty()) {
// Finally, remove the old call from the program, reducing the use-count of
// F.
const DataLayout &DL = F->getParent()->getDataLayout();
// Since we have now created the new function, splice the body of the old
// function right into the new function, leaving the old rotting hulk of the
// function empty.
NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());
// Loop over the argument list, transferring uses of the old arguments over to
// the new arguments, also transferring over the names as well.
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
I2 = NF->arg_begin();
I != E; ++I) {
if (!ArgsToPromote.count(&*I) && !ByValArgsToTransform.count(&*I)) {
// If this is an unmodified argument, move the name and users over to the
// new version.
if (ByValArgsToTransform.count(&*I)) {
// In the callee, we create an alloca, and store each of the new incoming
// arguments into the alloca.
Instruction *InsertPt = &NF->begin()->front();
// Just add all the struct element types.
Type *AgTy = cast<PointerType>(I->getType())->getElementType();
Value *TheAlloca = new AllocaInst(
AgTy, DL.getAllocaAddrSpace(), nullptr,
I->getParamAlign().getValueOr(DL.getPrefTypeAlign(AgTy)), "",
StructType *STy = cast<StructType>(AgTy);
Value *Idxs[2] = {ConstantInt::get(Type::getInt32Ty(F->getContext()), 0),
for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
Idxs[1] = ConstantInt::get(Type::getInt32Ty(F->getContext()), i);
Value *Idx = GetElementPtrInst::Create(
AgTy, TheAlloca, Idxs, TheAlloca->getName() + "." + Twine(i),
I2->setName(I->getName() + "." + Twine(i));
new StoreInst(&*I2++, Idx, InsertPt);
// Anything that used the arg should now use the alloca.
// If the alloca is used in a call, we must clear the tail flag since
// the callee now uses an alloca from the caller.
for (User *U : TheAlloca->users()) {
CallInst *Call = dyn_cast<CallInst>(U);
if (!Call)
// There potentially are metadata uses for things like llvm.dbg.value.
// Replace them with undef, after handling the other regular uses.
auto RauwUndefMetadata = make_scope_exit(
[&]() { I->replaceAllUsesWith(UndefValue::get(I->getType())); });
if (I->use_empty())
// Otherwise, if we promoted this argument, then all users are load
// instructions (or GEPs with only load users), and all loads should be
// using the new argument that we added.
ScalarizeTable &ArgIndices = ScalarizedElements[&*I];
while (!I->use_empty()) {
if (LoadInst *LI = dyn_cast<LoadInst>(I->user_back())) {
assert(ArgIndices.begin()->second.empty() &&
"Load element should sort to front!");
I2->setName(I->getName() + ".val");
LLVM_DEBUG(dbgs() << "*** Promoted load of argument '" << I->getName()
<< "' in function '" << F->getName() << "'\n");
} else {
GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->user_back());
assert(!GEP->use_empty() &&
"GEPs without uses should be cleaned up already");
IndicesVector Operands;
for (const Use &Idx : GEP->indices())
// GEPs with a single 0 index can be merged with direct loads
if (Operands.size() == 1 && Operands.front() == 0)
Function::arg_iterator TheArg = I2;
for (ScalarizeTable::iterator It = ArgIndices.begin();
It->second != Operands; ++It, ++TheArg) {
assert(It != ArgIndices.end() && "GEP not handled??");
TheArg->setName(formatv("{0}.{1:$[.]}.val", I->getName(),
make_range(Operands.begin(), Operands.end())));
LLVM_DEBUG(dbgs() << "*** Promoted agg argument '" << TheArg->getName()
<< "' of function '" << NF->getName() << "'\n");
// All of the uses must be load instructions. Replace them all with
// the argument specified by ArgNo.
while (!GEP->use_empty()) {
LoadInst *L = cast<LoadInst>(GEP->user_back());
// Increment I2 past all of the arguments added for this promoted pointer.
std::advance(I2, ArgIndices.size());
return NF;
/// Return true if we can prove that all callees pass in a valid pointer for the
/// specified function argument.
static bool allCallersPassValidPointerForArgument(Argument *Arg, Type *Ty) {
Function *Callee = Arg->getParent();
const DataLayout &DL = Callee->getParent()->getDataLayout();
unsigned ArgNo = Arg->getArgNo();
// Look at all call sites of the function. At this point we know we only have
// direct callees.
for (User *U : Callee->users()) {
CallBase &CB = cast<CallBase>(*U);
if (!isDereferenceablePointer(CB.getArgOperand(ArgNo), Ty, DL))
return false;
return true;
/// Returns true if Prefix is a prefix of longer. That means, Longer has a size
/// that is greater than or equal to the size of prefix, and each of the
/// elements in Prefix is the same as the corresponding elements in Longer.
/// This means it also returns true when Prefix and Longer are equal!
static bool isPrefix(const IndicesVector &Prefix, const IndicesVector &Longer) {
if (Prefix.size() > Longer.size())
return false;
return std::equal(Prefix.begin(), Prefix.end(), Longer.begin());
/// Checks if Indices, or a prefix of Indices, is in Set.
static bool prefixIn(const IndicesVector &Indices,
std::set<IndicesVector> &Set) {
std::set<IndicesVector>::iterator Low;
Low = Set.upper_bound(Indices);
if (Low != Set.begin())
// Low is now the last element smaller than or equal to Indices. This means
// it points to a prefix of Indices (possibly Indices itself), if such
// prefix exists.
// This load is safe if any prefix of its operands is safe to load.
return Low != Set.end() && isPrefix(*Low, Indices);
/// Mark the given indices (ToMark) as safe in the given set of indices
/// (Safe). Marking safe usually means adding ToMark to Safe. However, if there
/// is already a prefix of Indices in Safe, Indices are implicitely marked safe
/// already. Furthermore, any indices that Indices is itself a prefix of, are
/// removed from Safe (since they are implicitely safe because of Indices now).
static void markIndicesSafe(const IndicesVector &ToMark,
std::set<IndicesVector> &Safe) {
std::set<IndicesVector>::iterator Low;
Low = Safe.upper_bound(ToMark);
// Guard against the case where Safe is empty
if (Low != Safe.begin())
// Low is now the last element smaller than or equal to Indices. This
// means it points to a prefix of Indices (possibly Indices itself), if
// such prefix exists.
if (Low != Safe.end()) {
if (isPrefix(*Low, ToMark))
// If there is already a prefix of these indices (or exactly these
// indices) marked a safe, don't bother adding these indices
// Increment Low, so we can use it as a "insert before" hint
// Insert
Low = Safe.insert(Low, ToMark);
// If there we're a prefix of longer index list(s), remove those
std::set<IndicesVector>::iterator End = Safe.end();
while (Low != End && isPrefix(ToMark, *Low)) {
std::set<IndicesVector>::iterator Remove = Low;
/// isSafeToPromoteArgument - As you might guess from the name of this method,
/// it checks to see if it is both safe and useful to promote the argument.
/// This method limits promotion of aggregates to only promote up to three
/// elements of the aggregate in order to avoid exploding the number of
/// arguments passed in.
static bool isSafeToPromoteArgument(Argument *Arg, Type *ByValTy, AAResults &AAR,
unsigned MaxElements) {
using GEPIndicesSet = std::set<IndicesVector>;
// Quick exit for unused arguments
if (Arg->use_empty())
return true;
// We can only promote this argument if all of the uses are loads, or are GEP
// instructions (with constant indices) that are subsequently loaded.
// Promoting the argument causes it to be loaded in the caller
// unconditionally. This is only safe if we can prove that either the load
// would have happened in the callee anyway (ie, there is a load in the entry
// block) or the pointer passed in at every call site is guaranteed to be
// valid.
// In the former case, invalid loads can happen, but would have happened
// anyway, in the latter case, invalid loads won't happen. This prevents us
// from introducing an invalid load that wouldn't have happened in the
// original code.
// This set will contain all sets of indices that are loaded in the entry
// block, and thus are safe to unconditionally load in the caller.
GEPIndicesSet SafeToUnconditionallyLoad;
// This set contains all the sets of indices that we are planning to promote.
// This makes it possible to limit the number of arguments added.
GEPIndicesSet ToPromote;
// If the pointer is always valid, any load with first index 0 is valid.
if (ByValTy)
SafeToUnconditionallyLoad.insert(IndicesVector(1, 0));
// Whenever a new underlying type for the operand is found, make sure it's
// consistent with the GEPs and loads we've already seen and, if necessary,
// use it to see if all incoming pointers are valid (which implies the 0-index
// is safe).
Type *BaseTy = ByValTy;
auto UpdateBaseTy = [&](Type *NewBaseTy) {
if (BaseTy)
return BaseTy == NewBaseTy;
BaseTy = NewBaseTy;
if (allCallersPassValidPointerForArgument(Arg, BaseTy)) {
SafeToUnconditionallyLoad.insert(IndicesVector(1, 0));
return true;
// First, iterate the entry block and mark loads of (geps of) arguments as
// safe.
BasicBlock &EntryBlock = Arg->getParent()->front();
// Declare this here so we can reuse it
IndicesVector Indices;
for (Instruction &I : EntryBlock)
if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
Value *V = LI->getPointerOperand();
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V)) {
V = GEP->getPointerOperand();
if (V == Arg) {
// This load actually loads (part of) Arg? Check the indices then.
for (Use &Idx : GEP->indices())
if (ConstantInt *CI = dyn_cast<ConstantInt>(Idx))
// We found a non-constant GEP index for this argument? Bail out
// right away, can't promote this argument at all.
return false;
if (!UpdateBaseTy(GEP->getSourceElementType()))
return false;
// Indices checked out, mark them as safe
markIndicesSafe(Indices, SafeToUnconditionallyLoad);
} else if (V == Arg) {
// Direct loads are equivalent to a GEP with a single 0 index.
markIndicesSafe(IndicesVector(1, 0), SafeToUnconditionallyLoad);
if (BaseTy && LI->getType() != BaseTy)
return false;
BaseTy = LI->getType();
// Now, iterate all uses of the argument to see if there are any uses that are
// not (GEP+)loads, or any (GEP+)loads that are not safe to promote.
SmallVector<LoadInst *, 16> Loads;
IndicesVector Operands;
for (Use &U : Arg->uses()) {
User *UR = U.getUser();
if (LoadInst *LI = dyn_cast<LoadInst>(UR)) {
// Don't hack volatile/atomic loads
if (!LI->isSimple())
return false;
// Direct loads are equivalent to a GEP with a zero index and then a load.
if (!UpdateBaseTy(LI->getType()))
return false;
} else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(UR)) {
if (GEP->use_empty()) {
// Dead GEP's cause trouble later. Just remove them if we run into
// them.
if (!UpdateBaseTy(GEP->getSourceElementType()))
return false;
// Ensure that all of the indices are constants.
for (Use &Idx : GEP->indices())
if (ConstantInt *C = dyn_cast<ConstantInt>(Idx))
return false; // Not a constant operand GEP!
// Ensure that the only users of the GEP are load instructions.
for (User *GEPU : GEP->users())
if (LoadInst *LI = dyn_cast<LoadInst>(GEPU)) {
// Don't hack volatile/atomic loads
if (!LI->isSimple())
return false;
} else {
// Other uses than load?
return false;
} else {
return false; // Not a load or a GEP.
// Now, see if it is safe to promote this load / loads of this GEP. Loading
// is safe if Operands, or a prefix of Operands, is marked as safe.
if (!prefixIn(Operands, SafeToUnconditionallyLoad))
return false;
// See if we are already promoting a load with these indices. If not, check
// to make sure that we aren't promoting too many elements. If so, nothing
// to do.
if (ToPromote.find(Operands) == ToPromote.end()) {
if (MaxElements > 0 && ToPromote.size() == MaxElements) {
LLVM_DEBUG(dbgs() << "argpromotion not promoting argument '"
<< Arg->getName()
<< "' because it would require adding more "
<< "than " << MaxElements
<< " arguments to the function.\n");
// We limit aggregate promotion to only promoting up to a fixed number
// of elements of the aggregate.
return false;
if (Loads.empty())
return true; // No users, this is a dead argument.
// Okay, now we know that the argument is only used by load instructions and
// it is safe to unconditionally perform all of them. Use alias analysis to
// check to see if the pointer is guaranteed to not be modified from entry of
// the function to each of the load instructions.
// Because there could be several/many load instructions, remember which
// blocks we know to be transparent to the load.
df_iterator_default_set<BasicBlock *, 16> TranspBlocks;
for (LoadInst *Load : Loads) {
// Check to see if the load is invalidated from the start of the block to
// the load itself.
BasicBlock *BB = Load->getParent();
MemoryLocation Loc = MemoryLocation::get(Load);
if (AAR.canInstructionRangeModRef(BB->front(), *Load, Loc, ModRefInfo::Mod))
return false; // Pointer is invalidated!
// Now check every path from the entry block to the load for transparency.
// To do this, we perform a depth first search on the inverse CFG from the
// loading block.
for (BasicBlock *P : predecessors(BB)) {
for (BasicBlock *TranspBB : inverse_depth_first_ext(P, TranspBlocks))
if (AAR.canBasicBlockModify(*TranspBB, Loc))
return false;
// If the path from the entry of the function to each load is free of
// instructions that potentially invalidate the load, we can make the
// transformation!
return true;
bool ArgumentPromotionPass::isDenselyPacked(Type *type, const DataLayout &DL) {
// There is no size information, so be conservative.
if (!type->isSized())
return false;
// If the alloc size is not equal to the storage size, then there are padding
// bytes. For x86_fp80 on x86-64, size: 80 alloc size: 128.
if (DL.getTypeSizeInBits(type) != DL.getTypeAllocSizeInBits(type))
return false;
// FIXME: This isn't the right way to check for padding in vectors with
// non-byte-size elements.
if (VectorType *seqTy = dyn_cast<VectorType>(type))
return isDenselyPacked(seqTy->getElementType(), DL);
// For array types, check for padding within members.
if (ArrayType *seqTy = dyn_cast<ArrayType>(type))
return isDenselyPacked(seqTy->getElementType(), DL);
if (!isa<StructType>(type))
return true;
// Check for padding within and between elements of a struct.
StructType *StructTy = cast<StructType>(type);
const StructLayout *Layout = DL.getStructLayout(StructTy);
uint64_t StartPos = 0;
for (unsigned i = 0, E = StructTy->getNumElements(); i < E; ++i) {
Type *ElTy = StructTy->getElementType(i);
if (!isDenselyPacked(ElTy, DL))
return false;
if (StartPos != Layout->getElementOffsetInBits(i))
return false;
StartPos += DL.getTypeAllocSizeInBits(ElTy);
return true;
/// Checks if the padding bytes of an argument could be accessed.
static bool canPaddingBeAccessed(Argument *arg) {
// Track all the pointers to the argument to make sure they are not captured.
SmallPtrSet<Value *, 16> PtrValues;
// Track all of the stores.
SmallVector<StoreInst *, 16> Stores;
// Scan through the uses recursively to make sure the pointer is always used
// sanely.
SmallVector<Value *, 16> WorkList(arg->users());
while (!WorkList.empty()) {
Value *V = WorkList.pop_back_val();
if (isa<GetElementPtrInst>(V) || isa<PHINode>(V)) {
if (PtrValues.insert(V).second)
llvm::append_range(WorkList, V->users());
} else if (StoreInst *Store = dyn_cast<StoreInst>(V)) {
} else if (!isa<LoadInst>(V)) {
return true;
// Check to make sure the pointers aren't captured
for (StoreInst *Store : Stores)
if (PtrValues.count(Store->getValueOperand()))
return true;
return false;
bool ArgumentPromotionPass::areFunctionArgsABICompatible(
const Function &F, const TargetTransformInfo &TTI,
SmallPtrSetImpl<Argument *> &ArgsToPromote,
SmallPtrSetImpl<Argument *> &ByValArgsToTransform) {
for (const Use &U : F.uses()) {
CallBase *CB = dyn_cast<CallBase>(U.getUser());
if (!CB)
return false;
const Function *Caller = CB->getCaller();
const Function *Callee = CB->getCalledFunction();
if (!TTI.areFunctionArgsABICompatible(Caller, Callee, ArgsToPromote) ||
!TTI.areFunctionArgsABICompatible(Caller, Callee, ByValArgsToTransform))
return false;
return true;
/// 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, function_ref<AAResults &(Function &F)> AARGetter,
unsigned MaxElements,
Optional<function_ref<void(CallBase &OldCS, CallBase &NewCS)>>
const TargetTransformInfo &TTI) {
// Don't perform argument promotion for naked functions; otherwise we can end
// up removing parameters that are seemingly 'not used' as they are referred
// to in the assembly.
return nullptr;
// Make sure that it is local to this module.
if (!F->hasLocalLinkage())
return nullptr;
// Don't promote arguments for variadic functions. Adding, removing, or
// changing non-pack parameters can change the classification of pack
// parameters. Frontends encode that classification at the call site in the
// IR, while in the callee the classification is determined dynamically based
// on the number of registers consumed so far.
if (F->isVarArg())
return nullptr;
// Don't transform functions that receive inallocas, as the transformation may
// not be safe depending on calling convention.
if (F->getAttributes().hasAttrSomewhere(Attribute::InAlloca))
return nullptr;
// First check: see if there are any pointer arguments! If not, quick exit.
SmallVector<Argument *, 16> PointerArgs;
for (Argument &I : F->args())
if (I.getType()->isPointerTy())
if (PointerArgs.empty())
return nullptr;
// Second check: make sure that all callers are direct callers. We can't
// transform functions that have indirect callers. Also see if the function
// is self-recursive and check that target features are compatible.
bool isSelfRecursive = false;
for (Use &U : F->uses()) {
CallBase *CB = dyn_cast<CallBase>(U.getUser());
// Must be a direct call.
if (CB == nullptr || !CB->isCallee(&U))
return nullptr;
// Can't change signature of musttail callee
if (CB->isMustTailCall())
return nullptr;
if (CB->getParent()->getParent() == F)
isSelfRecursive = true;
// Can't change signature of musttail caller
// FIXME: Support promoting whole chain of musttail functions
for (BasicBlock &BB : *F)
if (BB.getTerminatingMustTailCall())
return nullptr;
const DataLayout &DL = F->getParent()->getDataLayout();
AAResults &AAR = AARGetter(*F);
// Check to see which arguments are promotable. If an argument is promotable,
// add it to ArgsToPromote.
SmallPtrSet<Argument *, 8> ArgsToPromote;
SmallPtrSet<Argument *, 8> ByValArgsToTransform;
for (Argument *PtrArg : PointerArgs) {
Type *AgTy = cast<PointerType>(PtrArg->getType())->getElementType();
// Replace sret attribute with noalias. This reduces register pressure by
// avoiding a register copy.
if (PtrArg->hasStructRetAttr()) {
unsigned ArgNo = PtrArg->getArgNo();
F->removeParamAttr(ArgNo, Attribute::StructRet);
F->addParamAttr(ArgNo, Attribute::NoAlias);
for (Use &U : F->uses()) {
CallBase &CB = cast<CallBase>(*U.getUser());
CB.removeParamAttr(ArgNo, Attribute::StructRet);
CB.addParamAttr(ArgNo, Attribute::NoAlias);
// If this is a byval argument, and if the aggregate type is small, just
// pass the elements, which is always safe, if the passed value is densely
// packed or if we can prove the padding bytes are never accessed.
bool isSafeToPromote = PtrArg->hasByValAttr() &&
(ArgumentPromotionPass::isDenselyPacked(AgTy, DL) ||
if (isSafeToPromote) {
if (StructType *STy = dyn_cast<StructType>(AgTy)) {
if (MaxElements > 0 && STy->getNumElements() > MaxElements) {
LLVM_DEBUG(dbgs() << "argpromotion disable promoting argument '"
<< PtrArg->getName()
<< "' because it would require adding more"
<< " than " << MaxElements
<< " arguments to the function.\n");
// If all the elements are single-value types, we can promote it.
bool AllSimple = true;
for (const auto *EltTy : STy->elements()) {
if (!EltTy->isSingleValueType()) {
AllSimple = false;
// Safe to transform, don't even bother trying to "promote" it.
// Passing the elements as a scalar will allow sroa to hack on
// the new alloca we introduce.
if (AllSimple) {
// If the argument is a recursive type and we're in a recursive
// function, we could end up infinitely peeling the function argument.
if (isSelfRecursive) {
if (StructType *STy = dyn_cast<StructType>(AgTy)) {
bool RecursiveType =
llvm::is_contained(STy->elements(), PtrArg->getType());
if (RecursiveType)
// Otherwise, see if we can promote the pointer to its value.
Type *ByValTy =
PtrArg->hasByValAttr() ? PtrArg->getParamByValType() : nullptr;
if (isSafeToPromoteArgument(PtrArg, ByValTy, AAR, MaxElements))
// No promotable pointer arguments.
if (ArgsToPromote.empty() && ByValArgsToTransform.empty())
return nullptr;
if (!ArgumentPromotionPass::areFunctionArgsABICompatible(
*F, TTI, ArgsToPromote, ByValArgsToTransform))
return nullptr;
return doPromotion(F, ArgsToPromote, ByValArgsToTransform, ReplaceCallSite);
PreservedAnalyses ArgumentPromotionPass::run(LazyCallGraph::SCC &C,
CGSCCAnalysisManager &AM,
LazyCallGraph &CG,
CGSCCUpdateResult &UR) {
bool Changed = false, LocalChange;
// Iterate until we stop promoting from this SCC.
do {
LocalChange = false;
for (LazyCallGraph::Node &N : C) {
Function &OldF = N.getFunction();
FunctionAnalysisManager &FAM =
AM.getResult<FunctionAnalysisManagerCGSCCProxy>(C, CG).getManager();
// FIXME: This lambda must only be used with this function. We should
// skip the lambda and just get the AA results directly.
auto AARGetter = [&](Function &F) -> AAResults & {
assert(&F == &OldF && "Called with an unexpected function!");
return FAM.getResult<AAManager>(F);
const TargetTransformInfo &TTI = FAM.getResult<TargetIRAnalysis>(OldF);
Function *NewF =
promoteArguments(&OldF, AARGetter, MaxElements, None, TTI);
if (!NewF)
LocalChange = true;
// Directly substitute the functions in the call graph. Note that this
// requires the old function to be completely dead and completely
// replaced by the new function. It does no call graph updates, it merely
// swaps out the particular function mapped to a particular node in the
// graph.
C.getOuterRefSCC().replaceNodeFunction(N, *NewF);
FAM.clear(OldF, OldF.getName());
Changed |= LocalChange;
} while (LocalChange);
if (!Changed)
return PreservedAnalyses::all();
return PreservedAnalyses::none();
namespace {
/// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
struct ArgPromotion : public CallGraphSCCPass {
// Pass identification, replacement for typeid
static char ID;
explicit ArgPromotion(unsigned MaxElements = 3)
: CallGraphSCCPass(ID), MaxElements(MaxElements) {
void getAnalysisUsage(AnalysisUsage &AU) const override {
bool runOnSCC(CallGraphSCC &SCC) override;
using llvm::Pass::doInitialization;
bool doInitialization(CallGraph &CG) override;
/// The maximum number of elements to expand, or 0 for unlimited.
unsigned MaxElements;
} // end anonymous namespace
char ArgPromotion::ID = 0;
INITIALIZE_PASS_BEGIN(ArgPromotion, "argpromotion",
"Promote 'by reference' arguments to scalars", false,
INITIALIZE_PASS_END(ArgPromotion, "argpromotion",
"Promote 'by reference' arguments to scalars", false, false)
Pass *llvm::createArgumentPromotionPass(unsigned MaxElements) {
return new ArgPromotion(MaxElements);
bool ArgPromotion::runOnSCC(CallGraphSCC &SCC) {
if (skipSCC(SCC))
return false;
// Get the callgraph information that we need to update to reflect our
// changes.
CallGraph &CG = getAnalysis<CallGraphWrapperPass>().getCallGraph();
LegacyAARGetter AARGetter(*this);
bool Changed = false, LocalChange;
// Iterate until we stop promoting from this SCC.
do {
LocalChange = false;
// Attempt to promote arguments from all functions in this SCC.
for (CallGraphNode *OldNode : SCC) {
Function *OldF = OldNode->getFunction();
if (!OldF)
auto ReplaceCallSite = [&](CallBase &OldCS, CallBase &NewCS) {
Function *Caller = OldCS.getParent()->getParent();
CallGraphNode *NewCalleeNode =
CallGraphNode *CallerNode = CG[Caller];
cast<CallBase>(NewCS), NewCalleeNode);
const TargetTransformInfo &TTI =
if (Function *NewF = promoteArguments(OldF, AARGetter, MaxElements,
{ReplaceCallSite}, TTI)) {
LocalChange = true;
// Update the call graph for the newly promoted function.
CallGraphNode *NewNode = CG.getOrInsertFunction(NewF);
if (OldNode->getNumReferences() == 0)
delete CG.removeFunctionFromModule(OldNode);
// And updat ethe SCC we're iterating as well.
SCC.ReplaceNode(OldNode, NewNode);
// Remember that we changed something.
Changed |= LocalChange;
} while (LocalChange);
return Changed;
bool ArgPromotion::doInitialization(CallGraph &CG) {
return CallGraphSCCPass::doInitialization(CG);