[clang] 8fca65c - [LifetimeSafety] Optimize loan propagation by separating persistent and block-local origins (#165789)
via cfe-commits
cfe-commits at lists.llvm.org
Thu Nov 6 19:02:44 PST 2025
Author: Utkarsh Saxena
Date: 2025-11-06T22:02:39-05:00
New Revision: 8fca65c65e3aab9bfdb1a3252335a880689a3f61
URL: https://github.com/llvm/llvm-project/commit/8fca65c65e3aab9bfdb1a3252335a880689a3f61
DIFF: https://github.com/llvm/llvm-project/commit/8fca65c65e3aab9bfdb1a3252335a880689a3f61.diff
LOG: [LifetimeSafety] Optimize loan propagation by separating persistent and block-local origins (#165789)
## Summary
Optimizes the lifetime analysis loan propagation by preventing block-local origins from participating in expensive join operations at block boundaries.
## Problem
The lifetime analysis currently performs join operations on all origins at every block boundary. However, many origins are block-local: they exist only to propagate loans between persistent origins across temporary declarations and expressions within a single block. Including them in joins is unnecessary and expensive.
## Solution
This PR classifies origins into two categories:
- **Persistent origins**: referenced in multiple basic blocks, must participate in joins
- **Block-local origins**: confined to a single block, can be discarded at block boundaries
### Implementation
1. **Pre-pass** (`computePersistentOrigins`): Analyzes all facts in the CFG to identify which origins appear in multiple blocks
2. **Split lattice**: Maintains two separate `OriginLoanMap`s:
- `PersistentOrigins`: participates in join operations
- `BlockLocalOrigins`: used during block execution, discarded at boundaries
3. **Optimized join**: Only merges persistent origins; block-local map is reset to empty
### Benefits
- Significantly reduces join complexity, especially in functions with many temporary locals
- Performance scales with the ratio of temporary to persistent origins
- Correctness preserved: block-local loan propagation still works within blocks
Fixes: https://github.com/llvm/llvm-project/issues/165780
Fixes: https://github.com/llvm/llvm-project/issues/164625. It brings down regression from **150% to 2%**.
---
Added:
Modified:
clang/include/clang/Analysis/Analyses/LifetimeSafety/Facts.h
clang/include/clang/Analysis/Analyses/LifetimeSafety/Origins.h
clang/lib/Analysis/LifetimeSafety/Facts.cpp
clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
clang/lib/Analysis/LifetimeSafety/LiveOrigins.cpp
clang/lib/Analysis/LifetimeSafety/LoanPropagation.cpp
clang/unittests/Analysis/LifetimeSafetyTest.cpp
Removed:
################################################################################
diff --git a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Facts.h b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Facts.h
index 6a90aeb01e638..063cb5c2d42ab 100644
--- a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Facts.h
+++ b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Facts.h
@@ -144,6 +144,7 @@ class ReturnOfOriginFact : public Fact {
class UseFact : public Fact {
const Expr *UseExpr;
+ OriginID OID;
// True if this use is a write operation (e.g., left-hand side of assignment).
// Write operations are exempted from use-after-free checks.
bool IsWritten = false;
@@ -151,12 +152,10 @@ class UseFact : public Fact {
public:
static bool classof(const Fact *F) { return F->getKind() == Kind::Use; }
- UseFact(const Expr *UseExpr) : Fact(Kind::Use), UseExpr(UseExpr) {}
+ UseFact(const Expr *UseExpr, OriginManager &OM)
+ : Fact(Kind::Use), UseExpr(UseExpr), OID(OM.get(*UseExpr)) {}
- OriginID getUsedOrigin(const OriginManager &OM) const {
- // TODO: Remove const cast and make OriginManager::get as const.
- return const_cast<OriginManager &>(OM).get(*UseExpr);
- }
+ OriginID getUsedOrigin() const { return OID; }
const Expr *getUseExpr() const { return UseExpr; }
void markAsWritten() { IsWritten = true; }
bool isWritten() const { return IsWritten; }
diff --git a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Origins.h b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Origins.h
index ba138b078b379..56b9010f41fa2 100644
--- a/clang/include/clang/Analysis/Analyses/LifetimeSafety/Origins.h
+++ b/clang/include/clang/Analysis/Analyses/LifetimeSafety/Origins.h
@@ -74,6 +74,8 @@ class OriginManager {
OriginID getOrCreate(const ValueDecl &D);
+ unsigned getNumOrigins() const { return NextOriginID.Value; }
+
void dump(OriginID OID, llvm::raw_ostream &OS) const;
private:
diff --git a/clang/lib/Analysis/LifetimeSafety/Facts.cpp b/clang/lib/Analysis/LifetimeSafety/Facts.cpp
index 1aea64f864367..ecde390cd6ca3 100644
--- a/clang/lib/Analysis/LifetimeSafety/Facts.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/Facts.cpp
@@ -53,7 +53,7 @@ void ReturnOfOriginFact::dump(llvm::raw_ostream &OS, const LoanManager &,
void UseFact::dump(llvm::raw_ostream &OS, const LoanManager &,
const OriginManager &OM) const {
OS << "Use (";
- OM.dump(getUsedOrigin(OM), OS);
+ OM.dump(getUsedOrigin(), OS);
OS << ", " << (isWritten() ? "Write" : "Read") << ")\n";
}
diff --git a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
index 9b68de107e314..bec8e1dabb0b5 100644
--- a/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/FactsGenerator.cpp
@@ -333,7 +333,7 @@ void FactsGenerator::handleAssignment(const Expr *LHSExpr,
// (e.g. on the left-hand side of an assignment).
void FactsGenerator::handleUse(const DeclRefExpr *DRE) {
if (isPointerType(DRE->getType())) {
- UseFact *UF = FactMgr.createFact<UseFact>(DRE);
+ UseFact *UF = FactMgr.createFact<UseFact>(DRE, FactMgr.getOriginMgr());
CurrentBlockFacts.push_back(UF);
assert(!UseFacts.contains(DRE));
UseFacts[DRE] = UF;
diff --git a/clang/lib/Analysis/LifetimeSafety/LiveOrigins.cpp b/clang/lib/Analysis/LifetimeSafety/LiveOrigins.cpp
index cddb3f3ce4c1c..59f594e50fb46 100644
--- a/clang/lib/Analysis/LifetimeSafety/LiveOrigins.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/LiveOrigins.cpp
@@ -111,7 +111,7 @@ class AnalysisImpl
/// dominates this program point. A write operation kills the liveness of
/// the origin since it overwrites the value.
Lattice transfer(Lattice In, const UseFact &UF) {
- OriginID OID = UF.getUsedOrigin(FactMgr.getOriginMgr());
+ OriginID OID = UF.getUsedOrigin();
// Write kills liveness.
if (UF.isWritten())
return Lattice(Factory.remove(In.LiveOrigins, OID));
diff --git a/clang/lib/Analysis/LifetimeSafety/LoanPropagation.cpp b/clang/lib/Analysis/LifetimeSafety/LoanPropagation.cpp
index 387097e705f94..0e6c194123df8 100644
--- a/clang/lib/Analysis/LifetimeSafety/LoanPropagation.cpp
+++ b/clang/lib/Analysis/LifetimeSafety/LoanPropagation.cpp
@@ -5,36 +5,114 @@
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
-#include "clang/Analysis/Analyses/LifetimeSafety/LoanPropagation.h"
-#include "Dataflow.h"
+#include <cassert>
#include <memory>
+#include "Dataflow.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/Facts.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/LoanPropagation.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/Loans.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/Origins.h"
+#include "clang/Analysis/Analyses/LifetimeSafety/Utils.h"
+#include "clang/Analysis/AnalysisDeclContext.h"
+#include "clang/Analysis/CFG.h"
+#include "clang/Basic/LLVM.h"
+#include "llvm/ADT/BitVector.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Support/TimeProfiler.h"
+#include "llvm/Support/raw_ostream.h"
+
namespace clang::lifetimes::internal {
+
+// Prepass to find persistent origins. An origin is persistent if it is
+// referenced in more than one basic block.
+static llvm::BitVector computePersistentOrigins(const FactManager &FactMgr,
+ const CFG &C) {
+ llvm::TimeTraceScope("ComputePersistentOrigins");
+ unsigned NumOrigins = FactMgr.getOriginMgr().getNumOrigins();
+ llvm::BitVector PersistentOrigins(NumOrigins);
+
+ llvm::SmallVector<const CFGBlock *> OriginToFirstSeenBlock(NumOrigins,
+ nullptr);
+ for (const CFGBlock *B : C) {
+ for (const Fact *F : FactMgr.getFacts(B)) {
+ auto CheckOrigin = [&](OriginID OID) {
+ if (PersistentOrigins.test(OID.Value))
+ return;
+ auto &FirstSeenBlock = OriginToFirstSeenBlock[OID.Value];
+ if (FirstSeenBlock == nullptr)
+ FirstSeenBlock = B;
+ if (FirstSeenBlock != B) {
+ // We saw this origin in more than one block.
+ PersistentOrigins.set(OID.Value);
+ }
+ };
+
+ switch (F->getKind()) {
+ case Fact::Kind::Issue:
+ CheckOrigin(F->getAs<IssueFact>()->getOriginID());
+ break;
+ case Fact::Kind::OriginFlow: {
+ const auto *OF = F->getAs<OriginFlowFact>();
+ CheckOrigin(OF->getDestOriginID());
+ CheckOrigin(OF->getSrcOriginID());
+ break;
+ }
+ case Fact::Kind::ReturnOfOrigin:
+ CheckOrigin(F->getAs<ReturnOfOriginFact>()->getReturnedOriginID());
+ break;
+ case Fact::Kind::Use:
+ CheckOrigin(F->getAs<UseFact>()->getUsedOrigin());
+ break;
+ case Fact::Kind::Expire:
+ case Fact::Kind::TestPoint:
+ break;
+ }
+ }
+ }
+ return PersistentOrigins;
+}
+
namespace {
+
/// Represents the dataflow lattice for loan propagation.
///
/// This lattice tracks which loans each origin may hold at a given program
/// point.The lattice has a finite height: An origin's loan set is bounded by
/// the total number of loans in the function.
-/// TODO(opt): To reduce the lattice size, propagate origins of declarations,
-/// not expressions, because expressions are not visible across blocks.
struct Lattice {
/// The map from an origin to the set of loans it contains.
- OriginLoanMap Origins = OriginLoanMap(nullptr);
-
- explicit Lattice(const OriginLoanMap &S) : Origins(S) {}
+ /// Origins that appear in multiple blocks. Participates in join operations.
+ OriginLoanMap PersistentOrigins = OriginLoanMap(nullptr);
+ /// Origins confined to a single block. Discarded at block boundaries.
+ OriginLoanMap BlockLocalOrigins = OriginLoanMap(nullptr);
+
+ explicit Lattice(const OriginLoanMap &Persistent,
+ const OriginLoanMap &BlockLocal)
+ : PersistentOrigins(Persistent), BlockLocalOrigins(BlockLocal) {}
Lattice() = default;
bool operator==(const Lattice &Other) const {
- return Origins == Other.Origins;
+ return PersistentOrigins == Other.PersistentOrigins &&
+ BlockLocalOrigins == Other.BlockLocalOrigins;
}
bool operator!=(const Lattice &Other) const { return !(*this == Other); }
void dump(llvm::raw_ostream &OS) const {
OS << "LoanPropagationLattice State:\n";
- if (Origins.isEmpty())
+ OS << " Persistent Origins:\n";
+ if (PersistentOrigins.isEmpty())
OS << " <empty>\n";
- for (const auto &Entry : Origins) {
+ for (const auto &Entry : PersistentOrigins) {
+ if (Entry.second.isEmpty())
+ OS << " Origin " << Entry.first << " contains no loans\n";
+ for (const LoanID &LID : Entry.second)
+ OS << " Origin " << Entry.first << " contains Loan " << LID << "\n";
+ }
+ OS << " Block-Local Origins:\n";
+ if (BlockLocalOrigins.isEmpty())
+ OS << " <empty>\n";
+ for (const auto &Entry : BlockLocalOrigins) {
if (Entry.second.isEmpty())
OS << " Origin " << Entry.first << " contains no loans\n";
for (const LoanID &LID : Entry.second)
@@ -50,7 +128,8 @@ class AnalysisImpl
OriginLoanMap::Factory &OriginLoanMapFactory,
LoanSet::Factory &LoanSetFactory)
: DataflowAnalysis(C, AC, F), OriginLoanMapFactory(OriginLoanMapFactory),
- LoanSetFactory(LoanSetFactory) {}
+ LoanSetFactory(LoanSetFactory),
+ PersistentOrigins(computePersistentOrigins(F, C)) {}
using Base::transfer;
@@ -59,10 +138,10 @@ class AnalysisImpl
Lattice getInitialState() { return Lattice{}; }
/// Merges two lattices by taking the union of loans for each origin.
- // TODO(opt): Keep the state small by removing origins which become dead.
+ /// Only persistent origins are joined; block-local origins are discarded.
Lattice join(Lattice A, Lattice B) {
OriginLoanMap JoinedOrigins = utils::join(
- A.Origins, B.Origins, OriginLoanMapFactory,
+ A.PersistentOrigins, B.PersistentOrigins, OriginLoanMapFactory,
[&](const LoanSet *S1, const LoanSet *S2) {
assert((S1 || S2) && "unexpectedly merging 2 empty sets");
if (!S1)
@@ -74,16 +153,15 @@ class AnalysisImpl
// Asymmetric join is a performance win. For origins present only on one
// branch, the loan set can be carried over as-is.
utils::JoinKind::Asymmetric);
- return Lattice(JoinedOrigins);
+ return Lattice(JoinedOrigins, OriginLoanMapFactory.getEmptyMap());
}
/// A new loan is issued to the origin. Old loans are erased.
Lattice transfer(Lattice In, const IssueFact &F) {
OriginID OID = F.getOriginID();
LoanID LID = F.getLoanID();
- return Lattice(OriginLoanMapFactory.add(
- In.Origins, OID,
- LoanSetFactory.add(LoanSetFactory.getEmptySet(), LID)));
+ LoanSet NewLoans = LoanSetFactory.add(LoanSetFactory.getEmptySet(), LID);
+ return setLoans(In, OID, NewLoans);
}
/// A flow from source to destination. If `KillDest` is true, this replaces
@@ -98,7 +176,7 @@ class AnalysisImpl
LoanSet SrcLoans = getLoans(In, SrcOID);
LoanSet MergedLoans = utils::join(DestLoans, SrcLoans, LoanSetFactory);
- return Lattice(OriginLoanMapFactory.add(In.Origins, DestOID, MergedLoans));
+ return setLoans(In, DestOID, MergedLoans);
}
LoanSet getLoans(OriginID OID, ProgramPoint P) const {
@@ -106,14 +184,33 @@ class AnalysisImpl
}
private:
+ /// Returns true if the origin is persistent (referenced in multiple blocks).
+ bool isPersistent(OriginID OID) const {
+ return PersistentOrigins.test(OID.Value);
+ }
+
+ Lattice setLoans(Lattice L, OriginID OID, LoanSet Loans) {
+ if (isPersistent(OID))
+ return Lattice(OriginLoanMapFactory.add(L.PersistentOrigins, OID, Loans),
+ L.BlockLocalOrigins);
+ return Lattice(L.PersistentOrigins,
+ OriginLoanMapFactory.add(L.BlockLocalOrigins, OID, Loans));
+ }
+
LoanSet getLoans(Lattice L, OriginID OID) const {
- if (auto *Loans = L.Origins.lookup(OID))
+ const OriginLoanMap *Map =
+ isPersistent(OID) ? &L.PersistentOrigins : &L.BlockLocalOrigins;
+ if (auto *Loans = Map->lookup(OID))
return *Loans;
return LoanSetFactory.getEmptySet();
}
OriginLoanMap::Factory &OriginLoanMapFactory;
LoanSet::Factory &LoanSetFactory;
+ /// Boolean vector indexed by origin ID. If true, the origin appears in
+ /// multiple basic blocks and must participate in join operations. If false,
+ /// the origin is block-local and can be discarded at block boundaries.
+ llvm::BitVector PersistentOrigins;
};
} // namespace
diff --git a/clang/unittests/Analysis/LifetimeSafetyTest.cpp b/clang/unittests/Analysis/LifetimeSafetyTest.cpp
index 0c051847f4d47..f27b56eb518aa 100644
--- a/clang/unittests/Analysis/LifetimeSafetyTest.cpp
+++ b/clang/unittests/Analysis/LifetimeSafetyTest.cpp
@@ -529,7 +529,8 @@ TEST_F(LifetimeAnalysisTest, PointersInACycle) {
MyObj* temp = p1;
p1 = p2;
p2 = p3;
- p3 = temp;
+ p3 = temp;B
+ POINT(in_loop);
}
POINT(after_loop);
}
@@ -543,7 +544,11 @@ TEST_F(LifetimeAnalysisTest, PointersInACycle) {
EXPECT_THAT(Origin("p1"), HasLoansTo({"v1", "v2", "v3"}, "after_loop"));
EXPECT_THAT(Origin("p2"), HasLoansTo({"v1", "v2", "v3"}, "after_loop"));
EXPECT_THAT(Origin("p3"), HasLoansTo({"v1", "v2", "v3"}, "after_loop"));
- EXPECT_THAT(Origin("temp"), HasLoansTo({"v1", "v2", "v3"}, "after_loop"));
+
+ EXPECT_THAT(Origin("temp"), HasLoansTo({"v1", "v2", "v3"}, "in_loop"));
+ // 'temp' is a block-local origin and it's loans are not tracked outside the
+ // block.
+ EXPECT_THAT(Origin("temp"), HasLoansTo({}, "after_loop"));
}
TEST_F(LifetimeAnalysisTest, PointersAndExpirationInACycle) {
More information about the cfe-commits
mailing list