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[Clang] Extend lifetime of temporaries in mem-default-init for P2718R0 (
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#86960)

Depends on [CWG1815](#108039).
Fixes #85613.

In [[Clang] Implement P2718R0 "Lifetime extension in range-based for
loops"](#76361), we've not
implement the lifetime extensions for the temporaries which in
`CXXDefaultInitExpr`. As the confirmation in
#85613, we should extend
lifetime for that.

To avoid modifying current CodeGen rules, in a lifetime extension
context, the cleanup of `CXXDefaultInitExpr` was ignored.

---------

Signed-off-by: yronglin <[email protected]>
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yronglin authored Oct 10, 2024
1 parent cb5fbd2 commit 25d9688
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3 changes: 3 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -171,6 +171,9 @@ C++23 Feature Support
^^^^^^^^^^^^^^^^^^^^^
- Removed the restriction to literal types in constexpr functions in C++23 mode.

- Extend lifetime of temporaries in mem-default-init for P2718R0. Clang now fully
supported `P2718R0 Lifetime extension in range-based for loops <https://wg21.link/P2718R0>`_.

C++20 Feature Support
^^^^^^^^^^^^^^^^^^^^^

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2 changes: 2 additions & 0 deletions clang/lib/Sema/SemaExpr.cpp
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Expand Up @@ -5649,6 +5649,8 @@ ExprResult Sema::BuildCXXDefaultInitExpr(SourceLocation Loc, FieldDecl *Field) {
runWithSufficientStackSpace(Loc, [&] {
MarkDeclarationsReferencedInExpr(E, /*SkipLocalVariables=*/false);
});
if (isInLifetimeExtendingContext())
DiscardCleanupsInEvaluationContext();
// C++11 [class.base.init]p7:
// The initialization of each base and member constitutes a
// full-expression.
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2 changes: 2 additions & 0 deletions clang/lib/Sema/SemaInit.cpp
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Expand Up @@ -763,6 +763,8 @@ void InitListChecker::FillInEmptyInitForField(unsigned Init, FieldDecl *Field,
SemaRef.currentEvaluationContext().DelayedDefaultInitializationContext =
SemaRef.parentEvaluationContext()
.DelayedDefaultInitializationContext;
SemaRef.currentEvaluationContext().InLifetimeExtendingContext =
SemaRef.parentEvaluationContext().InLifetimeExtendingContext;
DIE = SemaRef.BuildCXXDefaultInitExpr(Loc, Field);
}
if (DIE.isInvalid()) {
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59 changes: 59 additions & 0 deletions clang/test/AST/ast-dump-for-range-lifetime.cpp
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Expand Up @@ -449,4 +449,63 @@ void test13() {
for (auto e : dg<A>().r().g().r().g().r().g())
bar(e);
}

extern "C" void exit(int);

struct A14 {
int arr[1];
~A14() noexcept(false) { throw 42; }
};

struct B14 {
int x;
const A14 &a = A14{{0}};
const int *begin() { return a.arr; }
const int *end() { return &a.arr[1]; }
};

void test14() {
// The ExprWithCleanups in CXXDefaultInitExpr will be ignored.

// CHECK: FunctionDecl {{.*}} test14 'void ()'
// CHECK: -CXXForRangeStmt {{.*}}
// CHECK-NEXT: |-<<<NULL>>>
// CHECK-NEXT: |-DeclStmt {{.*}}
// CHECK-NEXT: | `-VarDecl {{.*}} implicit used __range1 'const int (&)[1]' cinit
// CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'const int[1]' lvalue
// CHECK-NEXT: | `-MemberExpr {{.*}} 'const int[1]' lvalue .arr {{.*}}
// CHECK-NEXT: | `-MemberExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue .a {{.*}}
// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'B14':'P2718R0::B14' xvalue extended by Var {{.*}} '__range1' 'const int (&)[1]'
// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'B14':'P2718R0::B14' functional cast to B14 <NoOp>
// CHECK-NEXT: | `-InitListExpr {{.*}} 'B14':'P2718R0::B14'
// CHECK-NEXT: | |-IntegerLiteral {{.*}} 'int' 0
// CHECK-NEXT: | `-CXXDefaultInitExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue has rewritten init
// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue extended by Var {{.*}} '__range1' 'const int (&)[1]'
// CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'const A14':'const P2718R0::A14' <NoOp>
// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'A14':'P2718R0::A14' functional cast to A14 <NoOp>
// CHECK-NEXT: | `-CXXBindTemporaryExpr {{.*}} 'A14':'P2718R0::A14' (CXXTemporary {{.*}})
// CHECK-NEXT: | `-InitListExpr {{.*}} 'A14':'P2718R0::A14'
// CHECK-NEXT: | `-InitListExpr {{.*}} 'int[1]'
// CHECK-NEXT: | `-IntegerLiteral {{.*}} 'int' 0
for (auto &&x : B14{0}.a.arr) { exit(0); }

// CHECK: -CXXForRangeStmt {{.*}}
// CHECK-NEXT: |-<<<NULL>>>
// CHECK-NEXT: |-DeclStmt {{.*}}
// CHECK-NEXT: | `-VarDecl {{.*}} col:19 implicit used __range1 'B14 &&' cinit
// CHECK-NEXT: | `-ExprWithCleanups {{.*}} 'B14':'P2718R0::B14' xvalue
// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'B14':'P2718R0::B14' xvalue extended by Var {{.*}} '__range1' 'B14 &&'
// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'B14':'P2718R0::B14' functional cast to B14 <NoOp>
// CHECK-NEXT: | `-InitListExpr {{.*}} 'B14':'P2718R0::B14'
// CHECK-NEXT: | |-IntegerLiteral {{.*}} 'int' 0
// CHECK-NEXT: | `-CXXDefaultInitExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue has rewritten init
// CHECK-NEXT: | `-MaterializeTemporaryExpr {{.*}} 'const A14':'const P2718R0::A14' lvalue extended by Var {{.*}} '__range1' 'B14 &&'
// CHECK-NEXT: | `-ImplicitCastExpr {{.*}} 'const A14':'const P2718R0::A14' <NoOp>
// CHECK-NEXT: | `-CXXFunctionalCastExpr {{.*}} 'A14':'P2718R0::A14' functional cast to A14 <NoOp>
// CHECK-NEXT: | `-CXXBindTemporaryExpr {{.*}} 'A14':'P2718R0::A14' (CXXTemporary {{.*}})
// CHECK-NEXT: | `-InitListExpr {{.*}} 'A14':'P2718R0::A14'
// CHECK-NEXT: | `-InitListExpr {{.*}} 'int[1]'
// CHECK-NEXT: | `-IntegerLiteral {{.*}} 'int' 0
for (auto &&x : B14{0}) { exit(0); }
}
} // namespace P2718R0
122 changes: 121 additions & 1 deletion clang/test/CXX/special/class.temporary/p6.cpp
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Expand Up @@ -463,6 +463,80 @@ template void default_arg_dependent_context2<int>();
template void default_arg_dependent_context3<int>();
} // namespace default_arg

namespace default_init {
template <class T>
struct DepA {
T arr[1];
~DepA() {}
};

template <class T>
struct DepB {
int x;
const DepA<T> &a = DepA<T>{{0}};
~DepB() {}
const int *begin() { return a.arr; }
const int *end() { return &a.arr[1]; }
};

template <typename T>
void default_init1_dependent() {
// CHECK-CXX23: void @_ZN7P2718R012default_init23default_init1_dependentINS0_4DepBIiEEEEvv()
// CHECK-CXX23-LABEL: for.cond.cleanup:
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepBIiED1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepAIiED1Ev(
for (auto &&x : T{0}) {}
}

template <typename T>
void default_init2_dependent() {
// CHECK-CXX23: void @_ZN7P2718R012default_init23default_init2_dependentINS0_4DepBIiEEEEvv()
// CHECK-CXX23-LABEL: for.cond.cleanup:
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepBIiED1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init4DepAIiED1Ev(
for (auto &&x : T{0}.a.arr) {}
}

template void default_init1_dependent<DepB<int>>();
template void default_init2_dependent<DepB<int>>();
} // namespace default_init

// -- Examples from https://wg21.link/p2718r0
extern void block_scope_begin_function();
extern void block_scope_end_function();
namespace std_examples {
using T = std::list<int>;
const T& f1(const T& t) { return t; }
const T& f2(T t) { return t; }
T g();
void foo() {
// CHECK-CXX23: define {{.*}} void @_ZN7P2718R012std_examples3fooEv()
// CHECK-CXX23: call void @_ZN7P2718R026block_scope_begin_functionEv
block_scope_begin_function();
{
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012std_examples1gEv
// CHECK-CXX23-NEXT: call {{.*}} @_ZN7P2718R012std_examples2f1ERKSt4listIiE
// CHECK-CXX23: for.cond.cleanup:
// CHECK-CXX23-NEXT: call void @_ZNSt4listIiED1Ev
for (auto e : f1(g())) {} // OK, lifetime of return value of g() extended
}
// CHECK-CXX23: call void @_ZN7P2718R024block_scope_end_functionEv
block_scope_end_function();

// The lifetime of temporary returned by g() in this case will not be extended.
// CHECK-CXX23: call void @_ZN7P2718R026block_scope_begin_functionEv
block_scope_begin_function();
{
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012std_examples1gEv
// CHECK-CXX23-NEXT: call {{.*}} @_ZN7P2718R012std_examples2f2ESt4listIiE
// CHECK-CXX23-NEXT: call void @_ZNSt4listIiED1Ev
for (auto e : f2(g())) {} // undefined behavior
}
// CHECK-CXX23: call void @_ZN7P2718R024block_scope_end_functionEv
block_scope_end_function();
}
} // namespace std_examples

namespace basic {
using T = std::list<int>;
const T& f1(const T& t) { return t; }
Expand Down Expand Up @@ -579,5 +653,51 @@ void default_arg3() {
for (auto e : C(0, C(0, C(0, C())))) {}
}
} // namespace default_arg
} // namespace P2718R0

namespace default_init {
struct X {
int x;
~X() {}
};

struct Y {
int y;
const X &x = X{1};
~Y() {}
};

struct A {
int arr[1];
const Y &y = Y{1};
~A() {}
};

struct B {
int x;
const A &a = A{{0}};
~B() {}
const int *begin() { return a.arr; }
const int *end() { return &a.arr[1]; }
};

void default_init1() {
// CHECK-CXX23: void @_ZN7P2718R012default_init13default_init1Ev()
// CHECK-CXX23-LABEL: for.cond.cleanup:
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1BD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1AD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1YD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1XD1Ev(
for (auto &&x : B{0}) {}
}

void default_init2() {
// CHECK-CXX23: void @_ZN7P2718R012default_init13default_init2Ev()
// CHECK-CXX23-LABEL: for.cond.cleanup:
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1BD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1AD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1YD1Ev(
// CHECK-CXX23-NEXT: call void @_ZN7P2718R012default_init1XD1Ev(
for (auto &&x : B{0}.a.arr) {}
}
} // namespace default_init
} // namespace P2718R0
9 changes: 1 addition & 8 deletions clang/www/cxx_status.html
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Expand Up @@ -475,14 +475,7 @@ <h2 id="cxx23">C++23 implementation status</h2>
<tr>
<td>Lifetime extension in range-based for loops</td>
<td><a href="https://wg21.link/P2718R0">P2718R0</a></td>
<td class="partial" align="center">
<details>
<summary>Clang 19 (Partial)</summary>
The lifetime extension of temporaries bound to member references
by default member initializers in aggregate initialization was
not supported now.
</details>
</td>
<td class="full" align="center">Clang 20</td>
</tr>
<!--Issaquah 2023 papers-->
<tr>
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