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llvm-project/llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

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//===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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/Optional.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/CallGraphSCCPass.h"
#include "llvm/Analysis/LazyCallGraph.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/DebugInfo.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/IPO.h"
#include <set>
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
typedef std::vector<uint64_t> IndicesVector;
/// 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(CallSite OldCS, CallSite 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;
typedef std::set<std::pair<Type *, IndicesVector>> ScalarizeTable;
// 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> AttributesVec;
const AttributeSet &PAL = F->getAttributes();
// Add any return attributes.
if (PAL.hasAttributes(AttributeSet::ReturnIndex))
AttributesVec.push_back(
AttributeSet::get(F->getContext(), PAL.getRetAttributes()));
// First, determine the new argument list
unsigned ArgIndex = 1;
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
++I, ++ArgIndex) {
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);
Params.insert(Params.end(), STy->element_begin(), STy->element_end());
++NumByValArgsPromoted;
} else if (!ArgsToPromote.count(&*I)) {
// Unchanged argument
Params.push_back(I->getType());
AttributeSet attrs = PAL.getParamAttributes(ArgIndex);
if (attrs.hasAttributes(ArgIndex)) {
AttrBuilder B(attrs, ArgIndex);
AttributesVec.push_back(
AttributeSet::get(F->getContext(), Params.size(), B));
}
} else if (I->use_empty()) {
// Dead argument (which are always marked as promotable)
++NumArgumentsDead;
} 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 : I->users()) {
Instruction *UI = cast<Instruction>(U);
Type *SrcTy;
if (LoadInst *L = dyn_cast<LoadInst>(UI))
SrcTy = L->getType();
else
SrcTy = cast<GetElementPtrInst>(UI)->getSourceElementType();
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)
Indices.push_back(cast<ConstantInt>(*II)->getSExtValue());
// GEPs with a single 0 index can be merged with direct loads
if (Indices.size() == 1 && Indices.front() == 0)
Indices.clear();
ArgIndices.insert(std::make_pair(SrcTy, Indices));
LoadInst *OrigLoad;
if (LoadInst *L = dyn_cast<LoadInst>(UI))
OrigLoad = L;
else
// 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
Params.push_back(GetElementPtrInst::getIndexedType(
cast<PointerType>(I->getType()->getScalarType())->getElementType(),
ArgIndex.second));
assert(Params.back());
}
if (ArgIndices.size() == 1 && ArgIndices.begin()->second.empty())
++NumArgumentsPromoted;
else
++NumAggregatesPromoted;
}
}
// Add any function attributes.
if (PAL.hasAttributes(AttributeSet::FunctionIndex))
AttributesVec.push_back(
AttributeSet::get(FTy->getContext(), PAL.getFnAttributes()));
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->getName());
NF->copyAttributesFrom(F);
// Patch the pointer to LLVM function in debug info descriptor.
NF->setSubprogram(F->getSubprogram());
F->setSubprogram(nullptr);
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(AttributeSet::get(F->getContext(), AttributesVec));
AttributesVec.clear();
F->getParent()->getFunctionList().insert(F->getIterator(), NF);
NF->takeName(F);
// 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()) {
CallSite CS(F->user_back());
assert(CS.getCalledFunction() == F);
Instruction *Call = CS.getInstruction();
const AttributeSet &CallPAL = CS.getAttributes();
// Add any return attributes.
if (CallPAL.hasAttributes(AttributeSet::ReturnIndex))
AttributesVec.push_back(
AttributeSet::get(F->getContext(), CallPAL.getRetAttributes()));
// Loop over the operands, inserting GEP and loads in the caller as
// appropriate.
CallSite::arg_iterator AI = CS.arg_begin();
ArgIndex = 1;
for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
++I, ++AI, ++ArgIndex)
if (!ArgsToPromote.count(&*I) && !ByValArgsToTransform.count(&*I)) {
Args.push_back(*AI); // Unmodified argument
if (CallPAL.hasAttributes(ArgIndex)) {
AttrBuilder B(CallPAL, ArgIndex);
AttributesVec.push_back(
AttributeSet::get(F->getContext(), Args.size(), B));
}
} 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);
Value *Idx = GetElementPtrInst::Create(
STy, *AI, Idxs, (*AI)->getName() + "." + Twine(i), Call);
// TODO: Tell AA about the new values?
Args.push_back(new LoadInst(Idx, Idx->getName() + ".val", Call));
}
} 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()) {
Ops.reserve(ArgIndex.second.size());
Type *ElTy = V->getType();
for (unsigned long 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();
else
ElTy = cast<CompositeType>(ElTy)->getTypeAtIndex(II);
}
// And create a GEP to extract those indices.
V = GetElementPtrInst::Create(ArgIndex.first, V, Ops,
V->getName() + ".idx", Call);
Ops.clear();
}
// Since we're replacing a load make sure we take the alignment
// of the previous load.
LoadInst *newLoad = new LoadInst(V, V->getName() + ".val", Call);
newLoad->setAlignment(OrigLoad->getAlignment());
// Transfer the AA info too.
AAMDNodes AAInfo;
OrigLoad->getAAMetadata(AAInfo);
newLoad->setAAMetadata(AAInfo);
Args.push_back(newLoad);
}
}
// Push any varargs arguments on the list.
for (; AI != CS.arg_end(); ++AI, ++ArgIndex) {
Args.push_back(*AI);
if (CallPAL.hasAttributes(ArgIndex)) {
AttrBuilder B(CallPAL, ArgIndex);
AttributesVec.push_back(
AttributeSet::get(F->getContext(), Args.size(), B));
}
}
// Add any function attributes.
if (CallPAL.hasAttributes(AttributeSet::FunctionIndex))
AttributesVec.push_back(
AttributeSet::get(Call->getContext(), CallPAL.getFnAttributes()));
SmallVector<OperandBundleDef, 1> OpBundles;
CS.getOperandBundlesAsDefs(OpBundles);
Instruction *New;
if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
Args, OpBundles, "", Call);
cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
cast<InvokeInst>(New)->setAttributes(
AttributeSet::get(II->getContext(), AttributesVec));
} else {
New = CallInst::Create(NF, Args, OpBundles, "", Call);
cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
cast<CallInst>(New)->setAttributes(
AttributeSet::get(New->getContext(), AttributesVec));
cast<CallInst>(New)->setTailCallKind(
cast<CallInst>(Call)->getTailCallKind());
}
New->setDebugLoc(Call->getDebugLoc());
Args.clear();
AttributesVec.clear();
// Update the callgraph to know that the callsite has been transformed.
if (ReplaceCallSite)
(*ReplaceCallSite)(CS, CallSite(New));
if (!Call->use_empty()) {
Call->replaceAllUsesWith(New);
New->takeName(Call);
}
// Finally, remove the old call from the program, reducing the use-count of
// F.
Call->eraseFromParent();
}
// 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.
I->replaceAllUsesWith(&*I2);
I2->takeName(&*I);
++I2;
continue;
}
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, nullptr, "", InsertPt);
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);
Value *Idx = GetElementPtrInst::Create(
AgTy, TheAlloca, Idxs, TheAlloca->getName() + "." + Twine(i),
InsertPt);
I2->setName(I->getName() + "." + Twine(i));
new StoreInst(&*I2++, Idx, InsertPt);
}
// Anything that used the arg should now use the alloca.
I->replaceAllUsesWith(TheAlloca);
TheAlloca->takeName(&*I);
// 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)
continue;
Call->setTailCall(false);
}
continue;
}
if (I->use_empty())
continue;
// 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");
LI->replaceAllUsesWith(&*I2);
LI->eraseFromParent();
DEBUG(dbgs() << "*** Promoted load of argument '" << I->getName()
<< "' in function '" << F->getName() << "'\n");
} else {
GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->user_back());
IndicesVector Operands;
Operands.reserve(GEP->getNumIndices());
for (User::op_iterator II = GEP->idx_begin(), IE = GEP->idx_end();
II != IE; ++II)
Operands.push_back(cast<ConstantInt>(*II)->getSExtValue());
// GEPs with a single 0 index can be merged with direct loads
if (Operands.size() == 1 && Operands.front() == 0)
Operands.clear();
Function::arg_iterator TheArg = I2;
for (ScalarizeTable::iterator It = ArgIndices.begin();
It->second != Operands; ++It, ++TheArg) {
assert(It != ArgIndices.end() && "GEP not handled??");
}
std::string NewName = I->getName();
for (unsigned i = 0, e = Operands.size(); i != e; ++i) {
NewName += "." + utostr(Operands[i]);
}
NewName += ".val";
TheArg->setName(NewName);
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());
L->replaceAllUsesWith(&*TheArg);
L->eraseFromParent();
}
GEP->eraseFromParent();
}
}
// Increment I2 past all of the arguments added for this promoted pointer.
std::advance(I2, ArgIndices.size());
}
return NF;
}
/// AllCallersPassInValidPointerForArgument - Return true if we can prove that
/// all callees pass in a valid pointer for the specified function argument.
static bool allCallersPassInValidPointerForArgument(Argument *Arg) {
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()) {
CallSite CS(U);
assert(CS && "Should only have direct calls!");
if (!isDereferenceablePointer(CS.getArgument(ArgNo), 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--;
// 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--;
// 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
return;
// Increment Low, so we can use it as a "insert before" hint
++Low;
}
// Insert
Low = Safe.insert(Low, ToMark);
++Low;
// 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;
++Low;
Safe.erase(Remove);
}
}
/// 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, bool isByValOrInAlloca,
AAResults &AAR, unsigned MaxElements) {
typedef std::set<IndicesVector> GEPIndicesSet;
// 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.
//
// This optimization is also safe for InAlloca parameters, because it verifies
// that the address isn't captured.
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 (isByValOrInAlloca || allCallersPassInValidPointerForArgument(Arg))
SafeToUnconditionallyLoad.insert(IndicesVector(1, 0));
// 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.
Indices.reserve(GEP->getNumIndices());
for (User::op_iterator II = GEP->idx_begin(), IE = GEP->idx_end();
II != IE; ++II)
if (ConstantInt *CI = dyn_cast<ConstantInt>(*II))
Indices.push_back(CI->getSExtValue());
else
// We found a non-constant GEP index for this argument? Bail out
// right away, can't promote this argument at all.
return false;
// Indices checked out, mark them as safe
markIndicesSafe(Indices, SafeToUnconditionallyLoad);
Indices.clear();
}
} else if (V == Arg) {
// Direct loads are equivalent to a GEP with a single 0 index.
markIndicesSafe(IndicesVector(1, 0), SafeToUnconditionallyLoad);
}
}
// 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();
Operands.clear();
if (LoadInst *LI = dyn_cast<LoadInst>(UR)) {
// Don't hack volatile/atomic loads
if (!LI->isSimple())
return false;
Loads.push_back(LI);
// Direct loads are equivalent to a GEP with a zero index and then a load.
Operands.push_back(0);
} 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.
GEP->eraseFromParent();
// TODO: This runs the above loop over and over again for dead GEPs
// Couldn't we just do increment the UI iterator earlier and erase the
// use?
return isSafeToPromoteArgument(Arg, isByValOrInAlloca, AAR,
MaxElements);
}
// Ensure that all of the indices are constants.
for (User::op_iterator i = GEP->idx_begin(), e = GEP->idx_end(); i != e;
++i)
if (ConstantInt *C = dyn_cast<ConstantInt>(*i))
Operands.push_back(C->getSExtValue());
else
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;
Loads.push_back(LI);
} 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) {
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;
}
ToPromote.insert(std::move(Operands));
}
}
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, MRI_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;
}
/// \brief Checks if a type could have padding bytes.
static bool 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;
if (!isa<CompositeType>(type))
return true;
// For homogenous sequential types, check for padding within members.
if (SequentialType *seqTy = dyn_cast<SequentialType>(type))
return isDenselyPacked(seqTy->getElementType(), DL);
// 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;
}
/// \brief Checks if the padding bytes of an argument could be accessed.
static bool canPaddingBeAccessed(Argument *arg) {
assert(arg->hasByValAttr());
// Track all the pointers to the argument to make sure they are not captured.
SmallPtrSet<Value *, 16> PtrValues;
PtrValues.insert(arg);
// 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;
WorkList.insert(WorkList.end(), arg->user_begin(), arg->user_end());
while (!WorkList.empty()) {
Value *V = WorkList.back();
WorkList.pop_back();
if (isa<GetElementPtrInst>(V) || isa<PHINode>(V)) {
if (PtrValues.insert(V).second)
WorkList.insert(WorkList.end(), V->user_begin(), V->user_end());
} else if (StoreInst *Store = dyn_cast<StoreInst>(V)) {
Stores.push_back(Store);
} 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;
}
/// 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(CallSite OldCS, CallSite NewCS)>>
ReplaceCallSite) {
// 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;
// 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())
PointerArgs.push_back(&I);
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.
bool isSelfRecursive = false;
for (Use &U : F->uses()) {
CallSite CS(U.getUser());
// Must be a direct call.
if (CS.getInstruction() == nullptr || !CS.isCallee(&U))
return nullptr;
if (CS.getInstruction()->getParent()->getParent() == F)
isSelfRecursive = true;
}
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->setAttributes(
F->getAttributes()
.removeAttribute(F->getContext(), ArgNo + 1, Attribute::StructRet)
.addAttribute(F->getContext(), ArgNo + 1, Attribute::NoAlias));
for (Use &U : F->uses()) {
CallSite CS(U.getUser());
CS.setAttributes(
CS.getAttributes()
.removeAttribute(F->getContext(), ArgNo + 1,
Attribute::StructRet)
.addAttribute(F->getContext(), ArgNo + 1, 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. This does
// not apply to inalloca.
bool isSafeToPromote =
PtrArg->hasByValAttr() &&
(isDenselyPacked(AgTy, DL) || !canPaddingBeAccessed(PtrArg));
if (isSafeToPromote) {
if (StructType *STy = dyn_cast<StructType>(AgTy)) {
if (MaxElements > 0 && STy->getNumElements() > MaxElements) {
DEBUG(dbgs() << "argpromotion disable promoting argument '"
<< PtrArg->getName()
<< "' because it would require adding more"
<< " than " << MaxElements
<< " arguments to the function.\n");
continue;
}
// 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;
break;
}
}
// 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) {
ByValArgsToTransform.insert(PtrArg);
continue;
}
}
}
// 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 = false;
for (const auto *EltTy : STy->elements()) {
if (EltTy == PtrArg->getType()) {
RecursiveType = true;
break;
}
}
if (RecursiveType)
continue;
}
}
// Otherwise, see if we can promote the pointer to its value.
if (isSafeToPromoteArgument(PtrArg, PtrArg->hasByValOrInAllocaAttr(), AAR,
MaxElements))
ArgsToPromote.insert(PtrArg);
}
// No promotable pointer arguments.
if (ArgsToPromote.empty() && ByValArgsToTransform.empty())
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);
};
Function *NewF = promoteArguments(&OldF, AARGetter, 3u, None);
if (!NewF)
continue;
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);
OldF.eraseFromParent();
}
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 {
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
getAAResultsAnalysisUsage(AU);
CallGraphSCCPass::getAnalysisUsage(AU);
}
bool runOnSCC(CallGraphSCC &SCC) override;
static char ID; // Pass identification, replacement for typeid
explicit ArgPromotion(unsigned MaxElements = 3)
: CallGraphSCCPass(ID), MaxElements(MaxElements) {
initializeArgPromotionPass(*PassRegistry::getPassRegistry());
}
private:
using llvm::Pass::doInitialization;
bool doInitialization(CallGraph &CG) override;
/// The maximum number of elements to expand, or 0 for unlimited.
unsigned MaxElements;
};
}
char ArgPromotion::ID = 0;
INITIALIZE_PASS_BEGIN(ArgPromotion, "argpromotion",
"Promote 'by reference' arguments to scalars", false,
false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(CallGraphWrapperPass)
INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
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)
continue;
auto ReplaceCallSite = [&](CallSite OldCS, CallSite NewCS) {
Function *Caller = OldCS.getInstruction()->getParent()->getParent();
CallGraphNode *NewCalleeNode =
CG.getOrInsertFunction(NewCS.getCalledFunction());
CallGraphNode *CallerNode = CG[Caller];
CallerNode->replaceCallEdge(OldCS, NewCS, NewCalleeNode);
};
if (Function *NewF = promoteArguments(OldF, AARGetter, MaxElements,
{ReplaceCallSite})) {
LocalChange = true;
// Update the call graph for the newly promoted function.
CallGraphNode *NewNode = CG.getOrInsertFunction(NewF);
NewNode->stealCalledFunctionsFrom(OldNode);
if (OldNode->getNumReferences() == 0)
delete CG.removeFunctionFromModule(OldNode);
else
OldF->setLinkage(Function::ExternalLinkage);
// 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);
}
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