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//=- LiveVariables.cpp - Live Variable Analysis for Source CFGs ----------*-==//
//
// 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 file implements Live Variables analysis for source-level CFGs.
//
//===----------------------------------------------------------------------===//
#include "clang/Analysis/Analyses/LiveVariables.h"
#include "clang/AST/Stmt.h"
#include "clang/AST/StmtVisitor.h"
#include "clang/Analysis/AnalysisDeclContext.h"
#include "clang/Analysis/CFG.h"
#include "clang/Analysis/FlowSensitive/DataflowWorklist.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <vector>
using namespace clang;
namespace {
class LiveVariablesImpl {
public:
AnalysisDeclContext &analysisContext;
llvm::ImmutableSet<const Expr *>::Factory ESetFact;
llvm::ImmutableSet<const VarDecl *>::Factory DSetFact;
llvm::ImmutableSet<const BindingDecl *>::Factory BSetFact;
llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues> blocksEndToLiveness;
llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues> blocksBeginToLiveness;
llvm::DenseMap<const Stmt *, LiveVariables::LivenessValues> stmtsToLiveness;
llvm::DenseMap<const DeclRefExpr *, unsigned> inAssignment;
const bool killAtAssign;
LiveVariables::LivenessValues
merge(LiveVariables::LivenessValues valsA,
LiveVariables::LivenessValues valsB);
LiveVariables::LivenessValues
runOnBlock(const CFGBlock *block, LiveVariables::LivenessValues val,
LiveVariables::Observer *obs = nullptr);
void dumpBlockLiveness(const SourceManager& M);
void dumpExprLiveness(const SourceManager& M);
LiveVariablesImpl(AnalysisDeclContext &ac, bool KillAtAssign)
: analysisContext(ac),
ESetFact(false), // Do not canonicalize ImmutableSets by default.
DSetFact(false), // This is a *major* performance win.
BSetFact(false), killAtAssign(KillAtAssign) {}
};
} // namespace
static LiveVariablesImpl &getImpl(void *x) {
return *((LiveVariablesImpl *) x);
}
//===----------------------------------------------------------------------===//
// Operations and queries on LivenessValues.
//===----------------------------------------------------------------------===//
bool LiveVariables::LivenessValues::isLive(const Expr *E) const {
return liveExprs.contains(E);
}
bool LiveVariables::LivenessValues::isLive(const VarDecl *D) const {
if (const auto *DD = dyn_cast<DecompositionDecl>(D)) {
bool alive = false;
for (const BindingDecl *BD : DD->bindings())
alive |= liveBindings.contains(BD);
return alive;
}
return liveDecls.contains(D);
}
namespace {
template <typename SET>
SET mergeSets(SET A, SET B) {
if (A.isEmpty())
return B;
for (typename SET::iterator it = B.begin(), ei = B.end(); it != ei; ++it) {
A = A.add(*it);
}
return A;
}
} // namespace
void LiveVariables::Observer::anchor() { }
LiveVariables::LivenessValues
LiveVariablesImpl::merge(LiveVariables::LivenessValues valsA,
LiveVariables::LivenessValues valsB) {
llvm::ImmutableSetRef<const Expr *> SSetRefA(
valsA.liveExprs.getRootWithoutRetain(), ESetFact.getTreeFactory()),
SSetRefB(valsB.liveExprs.getRootWithoutRetain(),
ESetFact.getTreeFactory());
llvm::ImmutableSetRef<const VarDecl *>
DSetRefA(valsA.liveDecls.getRootWithoutRetain(), DSetFact.getTreeFactory()),
DSetRefB(valsB.liveDecls.getRootWithoutRetain(), DSetFact.getTreeFactory());
llvm::ImmutableSetRef<const BindingDecl *>
BSetRefA(valsA.liveBindings.getRootWithoutRetain(), BSetFact.getTreeFactory()),
BSetRefB(valsB.liveBindings.getRootWithoutRetain(), BSetFact.getTreeFactory());
SSetRefA = mergeSets(SSetRefA, SSetRefB);
DSetRefA = mergeSets(DSetRefA, DSetRefB);
BSetRefA = mergeSets(BSetRefA, BSetRefB);
// asImmutableSet() canonicalizes the tree, allowing us to do an easy
// comparison afterwards.
return LiveVariables::LivenessValues(SSetRefA.asImmutableSet(),
DSetRefA.asImmutableSet(),
BSetRefA.asImmutableSet());
}
bool LiveVariables::LivenessValues::equals(const LivenessValues &V) const {
return liveExprs == V.liveExprs && liveDecls == V.liveDecls;
}
//===----------------------------------------------------------------------===//
// Query methods.
//===----------------------------------------------------------------------===//
static bool isAlwaysAlive(const VarDecl *D) {
return D->hasGlobalStorage();
}
bool LiveVariables::isLive(const CFGBlock *B, const VarDecl *D) {
return isAlwaysAlive(D) || getImpl(impl).blocksEndToLiveness[B].isLive(D);
}
bool LiveVariables::isLive(const Stmt *S, const VarDecl *D) {
return isAlwaysAlive(D) || getImpl(impl).stmtsToLiveness[S].isLive(D);
}
bool LiveVariables::isLive(const Stmt *Loc, const Expr *Val) {
return getImpl(impl).stmtsToLiveness[Loc].isLive(Val);
}
//===----------------------------------------------------------------------===//
// Dataflow computation.
//===----------------------------------------------------------------------===//
namespace {
class TransferFunctions : public StmtVisitor<TransferFunctions> {
LiveVariablesImpl &LV;
LiveVariables::LivenessValues &val;
LiveVariables::Observer *observer;
const CFGBlock *currentBlock;
public:
TransferFunctions(LiveVariablesImpl &im,
LiveVariables::LivenessValues &Val,
LiveVariables::Observer *Observer,
const CFGBlock *CurrentBlock)
: LV(im), val(Val), observer(Observer), currentBlock(CurrentBlock) {}
void VisitBinaryOperator(BinaryOperator *BO);
void VisitBlockExpr(BlockExpr *BE);
void VisitDeclRefExpr(DeclRefExpr *DR);
void VisitDeclStmt(DeclStmt *DS);
void VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS);
void VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE);
void VisitUnaryOperator(UnaryOperator *UO);
void Visit(Stmt *S);
};
} // namespace
static const VariableArrayType *FindVA(QualType Ty) {
const Type *ty = Ty.getTypePtr();
while (const ArrayType *VT = dyn_cast<ArrayType>(ty)) {
if (const VariableArrayType *VAT = dyn_cast<VariableArrayType>(VT))
if (VAT->getSizeExpr())
return VAT;
ty = VT->getElementType().getTypePtr();
}
return nullptr;
}
static const Expr *LookThroughExpr(const Expr *E) {
while (E) {
if (const Expr *Ex = dyn_cast<Expr>(E))
E = Ex->IgnoreParens();
if (const FullExpr *FE = dyn_cast<FullExpr>(E)) {
E = FE->getSubExpr();
continue;
}
if (const OpaqueValueExpr *OVE = dyn_cast<OpaqueValueExpr>(E)) {
E = OVE->getSourceExpr();
continue;
}
break;
}
return E;
}
static void AddLiveExpr(llvm::ImmutableSet<const Expr *> &Set,
llvm::ImmutableSet<const Expr *>::Factory &F,
const Expr *E) {
Set = F.add(Set, LookThroughExpr(E));
}
void TransferFunctions::Visit(Stmt *S) {
if (observer)
observer->observeStmt(S, currentBlock, val);
StmtVisitor<TransferFunctions>::Visit(S);
if (const auto *E = dyn_cast<Expr>(S)) {
val.liveExprs = LV.ESetFact.remove(val.liveExprs, E);
}
// Mark all children expressions live.
switch (S->getStmtClass()) {
default:
break;
case Stmt::StmtExprClass: {
// For statement expressions, look through the compound statement.
S = cast<StmtExpr>(S)->getSubStmt();
break;
}
case Stmt::CXXMemberCallExprClass: {
// Include the implicit "this" pointer as being live.
CXXMemberCallExpr *CE = cast<CXXMemberCallExpr>(S);
if (Expr *ImplicitObj = CE->getImplicitObjectArgument()) {
AddLiveExpr(val.liveExprs, LV.ESetFact, ImplicitObj);
}
break;
}
case Stmt::ObjCMessageExprClass: {
// In calls to super, include the implicit "self" pointer as being live.
ObjCMessageExpr *CE = cast<ObjCMessageExpr>(S);
if (CE->getReceiverKind() == ObjCMessageExpr::SuperInstance)
val.liveDecls = LV.DSetFact.add(val.liveDecls,
LV.analysisContext.getSelfDecl());
break;
}
case Stmt::DeclStmtClass: {
const DeclStmt *DS = cast<DeclStmt>(S);
if (const VarDecl *VD = dyn_cast<VarDecl>(DS->getSingleDecl())) {
for (const VariableArrayType* VA = FindVA(VD->getType());
VA != nullptr; VA = FindVA(VA->getElementType())) {
AddLiveExpr(val.liveExprs, LV.ESetFact, VA->getSizeExpr());
}
}
break;
}
case Stmt::PseudoObjectExprClass: {
// A pseudo-object operation only directly consumes its result
// expression.
Expr *child = cast<PseudoObjectExpr>(S)->getResultExpr();
if (!child) return;
if (OpaqueValueExpr *OV = dyn_cast<OpaqueValueExpr>(child))
child = OV->getSourceExpr();
child = child->IgnoreParens();
val.liveExprs = LV.ESetFact.add(val.liveExprs, child);
return;
}
// FIXME: These cases eventually shouldn't be needed.
case Stmt::ExprWithCleanupsClass: {
S = cast<ExprWithCleanups>(S)->getSubExpr();
break;
}
case Stmt::CXXBindTemporaryExprClass: {
S = cast<CXXBindTemporaryExpr>(S)->getSubExpr();
break;
}
case Stmt::UnaryExprOrTypeTraitExprClass: {
// No need to unconditionally visit subexpressions.
return;
}
case Stmt::IfStmtClass: {
// If one of the branches is an expression rather than a compound
// statement, it will be bad if we mark it as live at the terminator
// of the if-statement (i.e., immediately after the condition expression).
AddLiveExpr(val.liveExprs, LV.ESetFact, cast<IfStmt>(S)->getCond());
return;
}
case Stmt::WhileStmtClass: {
// If the loop body is an expression rather than a compound statement,
// it will be bad if we mark it as live at the terminator of the loop
// (i.e., immediately after the condition expression).
AddLiveExpr(val.liveExprs, LV.ESetFact, cast<WhileStmt>(S)->getCond());
return;
}
case Stmt::DoStmtClass: {
// If the loop body is an expression rather than a compound statement,
// it will be bad if we mark it as live at the terminator of the loop
// (i.e., immediately after the condition expression).
AddLiveExpr(val.liveExprs, LV.ESetFact, cast<DoStmt>(S)->getCond());
return;
}
case Stmt::ForStmtClass: {
// If the loop body is an expression rather than a compound statement,
// it will be bad if we mark it as live at the terminator of the loop
// (i.e., immediately after the condition expression).
AddLiveExpr(val.liveExprs, LV.ESetFact, cast<ForStmt>(S)->getCond());
return;
}
}
// HACK + FIXME: What is this? One could only guess that this is an attempt to
// fish for live values, for example, arguments from a call expression.
// Maybe we could take inspiration from UninitializedVariable analysis?
for (Stmt *Child : S->children()) {
if (const auto *E = dyn_cast_or_null<Expr>(Child))
AddLiveExpr(val.liveExprs, LV.ESetFact, E);
}
}
static bool writeShouldKill(const VarDecl *VD) {
return VD && !VD->getType()->isReferenceType() &&
!isAlwaysAlive(VD);
}
void TransferFunctions::VisitBinaryOperator(BinaryOperator *B) {
if (LV.killAtAssign && B->getOpcode() == BO_Assign) {
if (const auto *DR = dyn_cast<DeclRefExpr>(B->getLHS()->IgnoreParens())) {
LV.inAssignment[DR] = 1;
}
}
if (B->isAssignmentOp()) {
if (!LV.killAtAssign)
return;
// Assigning to a variable?
Expr *LHS = B->getLHS()->IgnoreParens();
if (DeclRefExpr *DR = dyn_cast<DeclRefExpr>(LHS)) {
const Decl* D = DR->getDecl();
bool Killed = false;
if (const BindingDecl* BD = dyn_cast<BindingDecl>(D)) {
Killed = !BD->getType()->isReferenceType();
if (Killed)
val.liveBindings = LV.BSetFact.remove(val.liveBindings, BD);
} else if (const auto *VD = dyn_cast<VarDecl>(D)) {
Killed = writeShouldKill(VD);
if (Killed)
val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
}
if (Killed && observer)
observer->observerKill(DR);
}
}
}
void TransferFunctions::VisitBlockExpr(BlockExpr *BE) {
for (const VarDecl *VD :
LV.analysisContext.getReferencedBlockVars(BE->getBlockDecl())) {
if (isAlwaysAlive(VD))
continue;
val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
}
}
void TransferFunctions::VisitDeclRefExpr(DeclRefExpr *DR) {
const Decl* D = DR->getDecl();
bool InAssignment = LV.inAssignment[DR];
if (const auto *BD = dyn_cast<BindingDecl>(D)) {
if (!InAssignment)
val.liveBindings = LV.BSetFact.add(val.liveBindings, BD);
} else if (const auto *VD = dyn_cast<VarDecl>(D)) {
if (!InAssignment && !isAlwaysAlive(VD))
val.liveDecls = LV.DSetFact.add(val.liveDecls, VD);
}
}
void TransferFunctions::VisitDeclStmt(DeclStmt *DS) {
for (const auto *DI : DS->decls()) {
if (const auto *DD = dyn_cast<DecompositionDecl>(DI)) {
for (const auto *BD : DD->bindings())
val.liveBindings = LV.BSetFact.remove(val.liveBindings, BD);
} else if (const auto *VD = dyn_cast<VarDecl>(DI)) {
if (!isAlwaysAlive(VD))
val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
}
}
}
void TransferFunctions::VisitObjCForCollectionStmt(ObjCForCollectionStmt *OS) {
// Kill the iteration variable.
DeclRefExpr *DR = nullptr;
const VarDecl *VD = nullptr;
Stmt *element = OS->getElement();
if (DeclStmt *DS = dyn_cast<DeclStmt>(element)) {
VD = cast<VarDecl>(DS->getSingleDecl());
}
else if ((DR = dyn_cast<DeclRefExpr>(cast<Expr>(element)->IgnoreParens()))) {
VD = cast<VarDecl>(DR->getDecl());
}
if (VD) {
val.liveDecls = LV.DSetFact.remove(val.liveDecls, VD);
if (observer && DR)
observer->observerKill(DR);
}
}
void TransferFunctions::
VisitUnaryExprOrTypeTraitExpr(UnaryExprOrTypeTraitExpr *UE)
{
// While sizeof(var) doesn't technically extend the liveness of 'var', it
// does extent the liveness of metadata if 'var' is a VariableArrayType.
// We handle that special case here.
if (UE->getKind() != UETT_SizeOf || UE->isArgumentType())
return;
const Expr *subEx = UE->getArgumentExpr();
if (subEx->getType()->isVariableArrayType()) {
assert(subEx->isLValue());
val.liveExprs = LV.ESetFact.add(val.liveExprs, subEx->IgnoreParens());
}
}
void TransferFunctions::VisitUnaryOperator(UnaryOperator *UO) {
// Treat ++/-- as a kill.
// Note we don't actually have to do anything if we don't have an observer,
// since a ++/-- acts as both a kill and a "use".
if (!observer)
return;
switch (UO->getOpcode()) {
default:
return;
case UO_PostInc:
case UO_PostDec:
case UO_PreInc:
case UO_PreDec:
break;
}
if (auto *DR = dyn_cast<DeclRefExpr>(UO->getSubExpr()->IgnoreParens())) {
const Decl *D = DR->getDecl();
if (isa<VarDecl>(D) || isa<BindingDecl>(D)) {
// Treat ++/-- as a kill.
observer->observerKill(DR);
}
}
}
LiveVariables::LivenessValues
LiveVariablesImpl::runOnBlock(const CFGBlock *block,
LiveVariables::LivenessValues val,
LiveVariables::Observer *obs) {
TransferFunctions TF(*this, val, obs, block);
// Visit the terminator (if any).
if (const Stmt *term = block->getTerminatorStmt())
TF.Visit(const_cast<Stmt*>(term));
// Apply the transfer function for all Stmts in the block.
for (CFGBlock::const_reverse_iterator it = block->rbegin(),
ei = block->rend(); it != ei; ++it) {
const CFGElement &elem = *it;
if (Optional<CFGAutomaticObjDtor> Dtor =
elem.getAs<CFGAutomaticObjDtor>()) {
val.liveDecls = DSetFact.add(val.liveDecls, Dtor->getVarDecl());
continue;
}
if (!elem.getAs<CFGStmt>())
continue;
const Stmt *S = elem.castAs<CFGStmt>().getStmt();
TF.Visit(const_cast<Stmt*>(S));
stmtsToLiveness[S] = val;
}
return val;
}
void LiveVariables::runOnAllBlocks(LiveVariables::Observer &obs) {
const CFG *cfg = getImpl(impl).analysisContext.getCFG();
for (CFG::const_iterator it = cfg->begin(), ei = cfg->end(); it != ei; ++it)
getImpl(impl).runOnBlock(*it, getImpl(impl).blocksEndToLiveness[*it], &obs);
}
LiveVariables::LiveVariables(void *im) : impl(im) {}
LiveVariables::~LiveVariables() {
delete (LiveVariablesImpl*) impl;
}
std::unique_ptr<LiveVariables>
LiveVariables::computeLiveness(AnalysisDeclContext &AC, bool killAtAssign) {
// No CFG? Bail out.
CFG *cfg = AC.getCFG();
if (!cfg)
return nullptr;
// The analysis currently has scalability issues for very large CFGs.
// Bail out if it looks too large.
if (cfg->getNumBlockIDs() > 300000)
return nullptr;
LiveVariablesImpl *LV = new LiveVariablesImpl(AC, killAtAssign);
// Construct the dataflow worklist. Enqueue the exit block as the
// start of the analysis.
BackwardDataflowWorklist worklist(*cfg, AC);
llvm::BitVector everAnalyzedBlock(cfg->getNumBlockIDs());
// FIXME: we should enqueue using post order.
for (const CFGBlock *B : cfg->nodes()) {
worklist.enqueueBlock(B);
}
while (const CFGBlock *block = worklist.dequeue()) {
// Determine if the block's end value has changed. If not, we
// have nothing left to do for this block.
LivenessValues &prevVal = LV->blocksEndToLiveness[block];
// Merge the values of all successor blocks.
LivenessValues val;
for (CFGBlock::const_succ_iterator it = block->succ_begin(),
ei = block->succ_end(); it != ei; ++it) {
if (const CFGBlock *succ = *it) {
val = LV->merge(val, LV->blocksBeginToLiveness[succ]);
}
}
if (!everAnalyzedBlock[block->getBlockID()])
everAnalyzedBlock[block->getBlockID()] = true;
else if (prevVal.equals(val))
continue;
prevVal = val;
// Update the dataflow value for the start of this block.
LV->blocksBeginToLiveness[block] = LV->runOnBlock(block, val);
// Enqueue the value to the predecessors.
worklist.enqueuePredecessors(block);
}
return std::unique_ptr<LiveVariables>(new LiveVariables(LV));
}
void LiveVariables::dumpBlockLiveness(const SourceManager &M) {
getImpl(impl).dumpBlockLiveness(M);
}
void LiveVariablesImpl::dumpBlockLiveness(const SourceManager &M) {
std::vector<const CFGBlock *> vec;
for (llvm::DenseMap<const CFGBlock *, LiveVariables::LivenessValues>::iterator
it = blocksEndToLiveness.begin(), ei = blocksEndToLiveness.end();
it != ei; ++it) {
vec.push_back(it->first);
}
llvm::sort(vec, [](const CFGBlock *A, const CFGBlock *B) {
return A->getBlockID() < B->getBlockID();
});
std::vector<const VarDecl*> declVec;
for (std::vector<const CFGBlock *>::iterator
it = vec.begin(), ei = vec.end(); it != ei; ++it) {
llvm::errs() << "\n[ B" << (*it)->getBlockID()
<< " (live variables at block exit) ]\n";
LiveVariables::LivenessValues vals = blocksEndToLiveness[*it];
declVec.clear();
for (llvm::ImmutableSet<const VarDecl *>::iterator si =
vals.liveDecls.begin(),
se = vals.liveDecls.end(); si != se; ++si) {
declVec.push_back(*si);
}
llvm::sort(declVec, [](const Decl *A, const Decl *B) {
return A->getBeginLoc() < B->getBeginLoc();
});
for (std::vector<const VarDecl*>::iterator di = declVec.begin(),
de = declVec.end(); di != de; ++di) {
llvm::errs() << " " << (*di)->getDeclName().getAsString()
<< " <";
(*di)->getLocation().print(llvm::errs(), M);
llvm::errs() << ">\n";
}
}
llvm::errs() << "\n";
}
void LiveVariables::dumpExprLiveness(const SourceManager &M) {
getImpl(impl).dumpExprLiveness(M);
}
void LiveVariablesImpl::dumpExprLiveness(const SourceManager &M) {
// Don't iterate over blockEndsToLiveness directly because it's not sorted.
for (const CFGBlock *B : *analysisContext.getCFG()) {
llvm::errs() << "\n[ B" << B->getBlockID()
<< " (live expressions at block exit) ]\n";
for (const Expr *E : blocksEndToLiveness[B].liveExprs) {
llvm::errs() << "\n";
E->dump();
}
llvm::errs() << "\n";
}
}
const void *LiveVariables::getTag() { static int x; return &x; }
const void *RelaxedLiveVariables::getTag() { static int x; return &x; }