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Merged
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Jun 30, 2025
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[cxx-interop] Avoid copies when accessing pointee #82480

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47 changes: 41 additions & 6 deletions lib/ClangImporter/ImportDecl.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -4097,7 +4097,8 @@ namespace {
func->setSelfIndex(selfIdx.value());
if (Impl.SwiftContext.LangOpts.hasFeature(
Feature::AddressableParameters))
func->getImplicitSelfDecl()->setAddressable();
func->getAttrs().add(new (Impl.SwiftContext)
AddressableSelfAttr(true));
} else {
func->setStatic();
func->setImportAsStaticMember();
Expand Down Expand Up @@ -4145,6 +4146,34 @@ namespace {
return false;
}

// Inject lifetime annotations selectively for some STL types so we can use
// unsafeAddress to avoid copies.
bool inferSelfDependence(const clang::FunctionDecl *decl,
AbstractFunctionDecl *result, size_t returnIdx) {
const auto *method = dyn_cast<clang::CXXMethodDecl>(decl);
if (!method)
return false;
const auto *enclosing = method->getParent();
if (enclosing->isInStdNamespace() &&
(enclosing->getName() == "unique_ptr" ||
enclosing->getName() == "shared_ptr") &&
method->isOverloadedOperator() &&
method->getOverloadedOperator() == clang::OO_Star) {
Comment on lines +4157 to +4161
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Ideally we would use API Notes to inject this, but that isn't possible right now since we don't support annotating C++ operators in API Notes, so this looks good.

SmallVector<LifetimeDependenceInfo, 1> lifetimeDependencies;
SmallBitVector dependenciesOfRet(returnIdx);
dependenciesOfRet[result->getSelfIndex()] = true;
lifetimeDependencies.push_back(LifetimeDependenceInfo(
nullptr, IndexSubset::get(Impl.SwiftContext, dependenciesOfRet),
returnIdx,
/*isImmortal*/ false));
Impl.SwiftContext.evaluator.cacheOutput(
LifetimeDependenceInfoRequest{result},
Impl.SwiftContext.AllocateCopy(lifetimeDependencies));
return true;
}
return false;
}

void addLifetimeDependencies(const clang::FunctionDecl *decl,
AbstractFunctionDecl *result) {
if (decl->getTemplatedKind() == clang::FunctionDecl::TK_FunctionTemplate)
Expand All @@ -4163,10 +4192,19 @@ namespace {
CxxEscapability::Unknown) != CxxEscapability::NonEscapable;
};

auto swiftParams = result->getParameters();
bool hasSelf =
result->hasImplicitSelfDecl() && !isa<ConstructorDecl>(result);
auto returnIdx = swiftParams->size() + hasSelf;

if (inferSelfDependence(decl, result, returnIdx))
return;

// FIXME: this uses '0' as the result index. That only works for
// standalone functions with no parameters.
// See markReturnsUnsafeNonescapable() for a general approach.
auto &ASTContext = result->getASTContext();

SmallVector<LifetimeDependenceInfo, 1> lifetimeDependencies;
LifetimeDependenceInfo immortalLifetime(nullptr, nullptr, 0,
/*isImmortal*/ true);
Expand All @@ -4189,10 +4227,7 @@ namespace {
}
};

auto swiftParams = result->getParameters();
bool hasSelf =
result->hasImplicitSelfDecl() && !isa<ConstructorDecl>(result);
const auto dependencyVecSize = swiftParams->size() + hasSelf;
const auto dependencyVecSize = returnIdx;
SmallBitVector inheritLifetimeParamIndicesForReturn(dependencyVecSize);
SmallBitVector scopedLifetimeParamIndicesForReturn(dependencyVecSize);
SmallBitVector paramHasAnnotation(dependencyVecSize);
Expand Down Expand Up @@ -4271,7 +4306,7 @@ namespace {
? IndexSubset::get(Impl.SwiftContext,
scopedLifetimeParamIndicesForReturn)
: nullptr,
swiftParams->size() + hasSelf,
returnIdx,
/*isImmortal*/ false));
else if (auto *ctordecl = dyn_cast<clang::CXXConstructorDecl>(decl)) {
// Assume default constructed view types have no dependencies.
Expand Down
23 changes: 21 additions & 2 deletions lib/ClangImporter/SwiftDeclSynthesizer.cpp
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Expand Up @@ -1679,6 +1679,7 @@ SubscriptDecl *SwiftDeclSynthesizer::makeSubscript(FuncDecl *getter,
FuncDecl *getterImpl = getter ? getter : setter;
FuncDecl *setterImpl = setter;

// FIXME: support unsafeAddress accessors.
// Get the return type wrapped in `Unsafe(Mutable)Pointer<T>`.
const auto rawElementTy = getterImpl->getResultInterfaceType();
// Unwrap `T`. Use rawElementTy for return by value.
Expand Down Expand Up @@ -1761,6 +1762,23 @@ SubscriptDecl *SwiftDeclSynthesizer::makeSubscript(FuncDecl *getter,
return subscript;
}

static bool doesReturnDependsOnSelf(FuncDecl *f) {
if (!f->getASTContext().LangOpts.hasFeature(Feature::AddressableParameters))
return false;
if (!f->hasImplicitSelfDecl())
return false;
if (auto deps = f->getLifetimeDependencies()) {
for (auto dependence : *deps) {
auto returnIdx = f->getParameters()->size() + !isa<ConstructorDecl>(f);
if (!dependence.hasInheritLifetimeParamIndices() &&
dependence.hasScopeLifetimeParamIndices() &&
dependence.getTargetIndex() == returnIdx)
return dependence.getScopeIndices()->contains(f->getSelfIndex());
}
}
return false;
}

// MARK: C++ dereference operator

VarDecl *
Expand All @@ -1772,6 +1790,7 @@ SwiftDeclSynthesizer::makeDereferencedPointeeProperty(FuncDecl *getter,
FuncDecl *getterImpl = getter ? getter : setter;
FuncDecl *setterImpl = setter;
auto dc = getterImpl->getDeclContext();
bool resultDependsOnSelf = doesReturnDependsOnSelf(getterImpl);

// Get the return type wrapped in `Unsafe(Mutable)Pointer<T>`.
const auto rawElementTy = getterImpl->getResultInterfaceType();
Expand All @@ -1782,9 +1801,9 @@ SwiftDeclSynthesizer::makeDereferencedPointeeProperty(FuncDecl *getter,
// Use 'address' or 'mutableAddress' accessors for non-copyable
// types that are returned indirectly.
bool isNoncopyable = dc->mapTypeIntoContext(elementTy)->isNoncopyable();
bool isImplicit = !isNoncopyable;
bool isImplicit = !(isNoncopyable || resultDependsOnSelf);
bool useAddress =
rawElementTy->getAnyPointerElementType() && isNoncopyable;
rawElementTy->getAnyPointerElementType() && (isNoncopyable || resultDependsOnSelf);

auto result = new (ctx)
VarDecl(/*isStatic*/ false, VarDecl::Introducer::Var,
Expand Down
2 changes: 1 addition & 1 deletion test/Interop/Cxx/class/method/methods-addressable-silgen.swift
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Expand Up @@ -17,7 +17,7 @@ public func addressableTest(x: borrowing @_addressable NonTrivialInWrapper, y: i
// CHECK: %{{[0-9]+}} = apply %{{[0-9]+}}([[UNWRAPPED]], %{{[0-9]+}}) : $@convention(cxx_method) (@in_guaranteed NonTrivialInWrapper, @in_guaranteed HasMethods) -> ()
var m2 = HasMethods()
// CHECK: [[ACCESS:%[0-9]+]] = begin_access [modify] [unknown] [[INPUT2]]
// CHECK: %{{[0-9]+}} = apply %32([[ACCESS]], %{{[0-9]+}}) : $@convention(cxx_method) (@inout NonTrivialInWrapper, @in_guaranteed HasMethods) -> ()
// CHECK: %{{[0-9]+}} = apply %{{[0-9]+}}([[ACCESS]], %{{[0-9]+}}) : $@convention(cxx_method) (@inout NonTrivialInWrapper, @in_guaranteed HasMethods) -> ()
// CHECK-NEXT: end_access [[ACCESS]]
m2.nonTrivialTakesRef(&y)
}
22 changes: 22 additions & 0 deletions test/Interop/Cxx/stdlib/Inputs/custom-smart-ptr.h
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Original file line number Diff line number Diff line change
@@ -0,0 +1,22 @@
#pragma once

static int copies2 = 0;

struct CountCopies2 {
CountCopies2() = default;
CountCopies2(const CountCopies2& other) { ++copies2; }
~CountCopies2() {}

int getCopies() const { return copies2; }
void method() {}
void constMethod() const {}
int field = 42;
};

struct MySmartPtr {
CountCopies2& operator*() const [[clang::lifetimebound]] { return *ptr; }

CountCopies2* ptr;
};

inline MySmartPtr getPtr() { return MySmartPtr{new CountCopies2()}; }
6 changes: 6 additions & 0 deletions test/Interop/Cxx/stdlib/Inputs/module.modulemap
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Expand Up @@ -81,3 +81,9 @@ module NoCXXStdlib {
requires cplusplus
export *
}

module CustomSmartPtr {
header "custom-smart-ptr.h"
requires cplusplus
export *
}
15 changes: 15 additions & 0 deletions test/Interop/Cxx/stdlib/Inputs/std-unique-ptr.h
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Expand Up @@ -66,4 +66,19 @@ std::shared_ptr<std::string> makeStringShared() {
return std::make_unique<std::string>("Shared string");
}

static int copies = 0;

struct CountCopies {
CountCopies() = default;
CountCopies(const CountCopies& other) { ++copies; }
~CountCopies() {}

int getCopies() const { return copies; }
void method() {}
void constMethod() const {}
int field = 42;
};

inline std::unique_ptr<CountCopies> getCopyCountedUniquePtr() { return std::make_unique<CountCopies>(); }

#endif // TEST_INTEROP_CXX_STDLIB_INPUTS_STD_UNIQUE_PTR_H
41 changes: 41 additions & 0 deletions test/Interop/Cxx/stdlib/avoid-redundant-copies.swift
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@@ -0,0 +1,41 @@
// RUN: %target-run-simple-swift(-I %S/Inputs -cxx-interoperability-mode=upcoming-swift -enable-experimental-feature AddressableParameters)
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I don't have a strong preference here, but I'm curious to hear your thoughts around pros and cons of a run time test vs checking SIL or IR output

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I think runtime checks are less fragile. SIL or the LLVM IR can break easily when some passes or the IR changes. Runtime tests might be a bit less performant but I think in this case they are more directly test what we want here because we can make copies observable and directly observe it. It is not always possible to observe the property we want to test at runtime, SIL or IR tests are great for that. Also, I think here we do not care about the shape of the IRs, just the fact that there are no copies. I guess one caveat is, the runtime test does not tell us why are there no copies? Is it because there were no copies to begin with or is it because some optimisations got rid of them?

Despite this caveat, I prefer a runtime test in this case as I find it easier to read and write and less fragile in this case.


// REQUIRES: executable_test
// REQUIRES: swift_feature_AddressableParameters

// https://github.com/apple/swift/issues/70226
// UNSUPPORTED: OS=windows-msvc

import StdlibUnittest
import StdUniquePtr
import CustomSmartPtr
import CxxStdlib

var AvoidCopiesSuite = TestSuite("AvoidRedundantCopies")

AvoidCopiesSuite.test("The pointee does not copy when passed as self") {
let up = getNonCopyableUniquePtr()
expectEqual(up.pointee.method(1), 42)
expectEqual(up.pointee.method(1), 42)
let cup = getCopyCountedUniquePtr();
expectEqual(cup.pointee.getCopies(), 0)
cup.pointee.method()
cup.pointee.constMethod()
let _ = cup.pointee.field
expectEqual(cup.pointee.getCopies(), 0)
let copy = cup.pointee
expectEqual(copy.getCopies(), 1)
}

AvoidCopiesSuite.test("The custom smart pointer pointee does not copy when passed as self") {
let myptr = getPtr();
expectEqual(myptr.pointee.getCopies(), 0)
myptr.pointee.method()
myptr.pointee.constMethod()
let _ = myptr.pointee.field
expectEqual(myptr.pointee.getCopies(), 0)
let copy = myptr.pointee
expectEqual(copy.getCopies(), 1)
}

runAllTests()