[clang] fbfd2c9 - [coroutines] Introduce [[clang::coro_lifetimebound]] (#72851)
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Wed Nov 22 03:51:28 PST 2023
Author: Utkarsh Saxena
Date: 2023-11-22T12:51:24+01:00
New Revision: fbfd2c9972695bc74a646bfa5e3e2d00477dabe2
URL: https://github.com/llvm/llvm-project/commit/fbfd2c9972695bc74a646bfa5e3e2d00477dabe2
DIFF: https://github.com/llvm/llvm-project/commit/fbfd2c9972695bc74a646bfa5e3e2d00477dabe2.diff
LOG: [coroutines] Introduce [[clang::coro_lifetimebound]] (#72851)
Adds attribute `[[clang::coro_lifetimebound]]`.
All arguments to a function are considered to be **lifetime bound** if
the function
returns a type annotated with ``[[clang::coro_lifetimebound]]`` and
``[[clang::coro_return_type]]``.
Added:
clang/test/SemaCXX/coro-lifetimebound.cpp
Modified:
clang/docs/ReleaseNotes.rst
clang/include/clang/Basic/Attr.td
clang/include/clang/Basic/AttrDocs.td
clang/lib/Sema/SemaInit.cpp
clang/test/Misc/pragma-attribute-supported-attributes-list.test
Removed:
################################################################################
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index b65106b9106d4d7..7e2c990d03e7f27 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -334,6 +334,10 @@ Attribute Changes in Clang
[[clang::code_align(A)]] for(;;) { }
}
+- Clang now introduced ``[[clang::coro_lifetimebound]]`` attribute.
+ All parameters of a function are considered to be lifetime bound if the function
+ returns a type annotated with ``[[clang::coro_lifetimebound]]`` and ``[[clang::coro_return_type]]``.
+
Improvements to Clang's diagnostics
-----------------------------------
- Clang constexpr evaluator now prints template arguments when displaying
diff --git a/clang/include/clang/Basic/Attr.td b/clang/include/clang/Basic/Attr.td
index c2fbdfc66c540d6..03ed6accf700c4e 100644
--- a/clang/include/clang/Basic/Attr.td
+++ b/clang/include/clang/Basic/Attr.td
@@ -1110,6 +1110,14 @@ def CoroWrapper : InheritableAttr {
let SimpleHandler = 1;
}
+def CoroLifetimeBound : InheritableAttr {
+ let Spellings = [Clang<"coro_lifetimebound">];
+ let Subjects = SubjectList<[CXXRecord]>;
+ let LangOpts = [CPlusPlus];
+ let Documentation = [CoroLifetimeBoundDoc];
+ let SimpleHandler = 1;
+}
+
// OSObject-based attributes.
def OSConsumed : InheritableParamAttr {
let Spellings = [Clang<"os_consumed">];
diff --git a/clang/include/clang/Basic/AttrDocs.td b/clang/include/clang/Basic/AttrDocs.td
index aa9987fdfa84b20..c74ec2bc1218aa6 100644
--- a/clang/include/clang/Basic/AttrDocs.td
+++ b/clang/include/clang/Basic/AttrDocs.td
@@ -7483,7 +7483,6 @@ generation of the other destruction cases, optimizing the above `foo.destroy` to
}];
}
-
def CoroReturnTypeAndWrapperDoc : Documentation {
let Category = DocCatDecl;
let Content = [{
@@ -7581,3 +7580,60 @@ alignment boundary. Its value must be a power of 2, between 1 and 4096
}];
}
+
+def CoroLifetimeBoundDoc : Documentation {
+ let Category = DocCatDecl;
+ let Content = [{
+The ``[[clang::coro_lifetimebound]]`` is a class attribute which can be applied
+to a `coroutine return type (CRT) <https://clang.llvm.org/docs/AttributeReference.html#coro-return-type>` _ (i.e.
+it should also be annotated with ``[[clang::coro_return_type]]``).
+
+All parameters of a function are considered to be lifetime bound. See documentation
+of ``[[clang::lifetimebound]]`` for more `details <https://clang.llvm.org/docs/AttributeReference.html#lifetimebound> _`.
+if the function returns a coroutine return type (CRT) annotated with ``[[clang::coro_lifetimebound]]``.
+
+Reference parameters of a coroutine are susceptible to capturing references to temporaries or local variables.
+
+For example,
+
+.. code-block:: c++
+
+ task<int> coro(const int& a) { co_return a + 1; }
+ task<int> dangling_refs(int a) {
+ // `coro` captures reference to a temporary. `foo` would now contain a dangling reference to `a`.
+ auto foo = coro(1);
+ // `coro` captures reference to local variable `a` which is destroyed after the return.
+ return coro(a);
+ }
+
+`Lifetime bound <https://clang.llvm.org/docs/AttributeReference.html#lifetimebound> _` static analysis
+can be used to detect such instances when coroutines capture references which may die earlier than the
+coroutine frame itself. In the above example, if the CRT `task` is annotated with
+``[[clang::coro_lifetimebound]]``, then lifetime bound analysis would detect capturing reference to
+temporaries or return address of a local variable.
+
+Both coroutines and coroutine wrappers are part of this analysis.
+
+.. code-block:: c++
+
+ template <typename T> struct [[clang::coro_return_type, clang::coro_lifetimebound]] Task {
+ using promise_type = some_promise_type;
+ };
+
+ Task<int> coro(const int& a) { co_return a + 1; }
+ Task<int> [[clang::coro_wrapper]] coro_wrapper(const int& a, const int& b) {
+ return a > b ? coro(a) : coro(b);
+ }
+ Task<int> temporary_reference() {
+ auto foo = coro(1); // warning: capturing reference to a temporary which would die after the expression.
+
+ int a = 1;
+ auto bar = coro_wrapper(a, 0); // warning: `b` captures reference to a temporary.
+
+ co_return co_await coro(1); // fine.
+ }
+ [[clang::coro_wrapper]] Task<int> stack_reference(int a) {
+ return coro(a); // warning: returning address of stack variable `a`.
+ }
+}];
+}
diff --git a/clang/lib/Sema/SemaInit.cpp b/clang/lib/Sema/SemaInit.cpp
index 80b51b09bf5445f..c0c321f0f200d21 100644
--- a/clang/lib/Sema/SemaInit.cpp
+++ b/clang/lib/Sema/SemaInit.cpp
@@ -7580,10 +7580,15 @@ static void visitLifetimeBoundArguments(IndirectLocalPath &Path, Expr *Call,
if (ObjectArg && implicitObjectParamIsLifetimeBound(Callee))
VisitLifetimeBoundArg(Callee, ObjectArg);
+ bool CheckCoroCall = false;
+ if (const auto *RD = Callee->getReturnType()->getAsRecordDecl()) {
+ CheckCoroCall = RD->hasAttr<CoroLifetimeBoundAttr>() &&
+ RD->hasAttr<CoroReturnTypeAttr>();
+ }
for (unsigned I = 0,
N = std::min<unsigned>(Callee->getNumParams(), Args.size());
I != N; ++I) {
- if (Callee->getParamDecl(I)->hasAttr<LifetimeBoundAttr>())
+ if (CheckCoroCall || Callee->getParamDecl(I)->hasAttr<LifetimeBoundAttr>())
VisitLifetimeBoundArg(Callee->getParamDecl(I), Args[I]);
}
}
diff --git a/clang/test/Misc/pragma-attribute-supported-attributes-list.test b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
index a57bc011c059483..dd91f4f88ad685b 100644
--- a/clang/test/Misc/pragma-attribute-supported-attributes-list.test
+++ b/clang/test/Misc/pragma-attribute-supported-attributes-list.test
@@ -56,9 +56,10 @@
// CHECK-NEXT: ConsumableAutoCast (SubjectMatchRule_record)
// CHECK-NEXT: ConsumableSetOnRead (SubjectMatchRule_record)
// CHECK-NEXT: Convergent (SubjectMatchRule_function)
+// CHECK-NEXT: CoroLifetimeBound (SubjectMatchRule_record)
// CHECK-NEXT: CoroOnlyDestroyWhenComplete (SubjectMatchRule_record)
// CHECK-NEXT: CoroReturnType (SubjectMatchRule_record)
-// CHECK-NEXT: CoroWrapper
+// CHECK-NEXT: CoroWrapper (SubjectMatchRule_function)
// CHECK-NEXT: CountedBy (SubjectMatchRule_field)
// CHECK-NEXT: DLLExport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
// CHECK-NEXT: DLLImport (SubjectMatchRule_function, SubjectMatchRule_variable, SubjectMatchRule_record, SubjectMatchRule_objc_interface)
diff --git a/clang/test/SemaCXX/coro-lifetimebound.cpp b/clang/test/SemaCXX/coro-lifetimebound.cpp
new file mode 100644
index 000000000000000..d3e2d673ebb3c0d
--- /dev/null
+++ b/clang/test/SemaCXX/coro-lifetimebound.cpp
@@ -0,0 +1,123 @@
+// RUN: %clang_cc1 -triple x86_64-apple-darwin9 %s -std=c++20 -fsyntax-only -verify -Wall -Wextra -Wno-error=unreachable-code -Wno-unused
+
+#include "Inputs/std-coroutine.h"
+
+using std::suspend_always;
+using std::suspend_never;
+
+template <typename T> struct [[clang::coro_lifetimebound, clang::coro_return_type]] Co {
+ struct promise_type {
+ Co<T> get_return_object() {
+ return {};
+ }
+ suspend_always initial_suspend();
+ suspend_always final_suspend() noexcept;
+ void unhandled_exception();
+ void return_value(const T &t);
+
+ template <typename U>
+ auto await_transform(const Co<U> &) {
+ struct awaitable {
+ bool await_ready() noexcept { return false; }
+ void await_suspend(std::coroutine_handle<>) noexcept {}
+ U await_resume() noexcept { return {}; }
+ };
+ return awaitable{};
+ }
+ };
+};
+
+Co<int> foo_coro(const int& b) {
+ if (b > 0)
+ co_return 1;
+ co_return 2;
+}
+
+int getInt() { return 0; }
+
+Co<int> bar_coro(const int &b, int c) {
+ int x = co_await foo_coro(b);
+ int y = co_await foo_coro(1);
+ int z = co_await foo_coro(getInt());
+ auto unsafe1 = foo_coro(1); // expected-warning {{temporary whose address is used as value of local variable}}
+ auto unsafe2 = foo_coro(getInt()); // expected-warning {{temporary whose address is used as value of local variable}}
+ auto safe1 = foo_coro(b);
+ auto safe2 = foo_coro(c);
+ co_return co_await foo_coro(co_await foo_coro(1));
+}
+
+[[clang::coro_wrapper]] Co<int> plain_return_co(int b) {
+ return foo_coro(b); // expected-warning {{address of stack memory associated with parameter}}
+}
+
+[[clang::coro_wrapper]] Co<int> safe_forwarding(const int& b) {
+ return foo_coro(b);
+}
+
+[[clang::coro_wrapper]] Co<int> unsafe_wrapper(int b) {
+ return safe_forwarding(b); // expected-warning {{address of stack memory associated with parameter}}
+}
+
+[[clang::coro_wrapper]] Co<int> complex_plain_return(int b) {
+ return b > 0
+ ? foo_coro(1) // expected-warning {{returning address of local temporary object}}
+ : bar_coro(0, 1); // expected-warning {{returning address of local temporary object}}
+}
+
+#define CORO_WRAPPER \
+ _Pragma("clang diagnostic push") \
+ _Pragma("clang diagnostic ignored \"-Wc++23-extensions\"") \
+ [[clang::coro_wrapper]] \
+ _Pragma("clang diagnostic pop")
+
+void lambdas() {
+ auto unsafe_lambda = [] CORO_WRAPPER (int b) {
+ return foo_coro(b); // expected-warning {{address of stack memory associated with parameter}}
+ };
+ auto coro_lambda = [] (const int&) -> Co<int> {
+ co_return 0;
+ };
+ auto unsafe_coro_lambda = [&] (const int& b) -> Co<int> {
+ int x = co_await coro_lambda(b);
+ auto safe = coro_lambda(b);
+ auto unsafe1 = coro_lambda(1); // expected-warning {{temporary whose address is used as value of local variable}}
+ auto unsafe2 = coro_lambda(getInt()); // expected-warning {{temporary whose address is used as value of local variable}}
+ auto unsafe3 = coro_lambda(co_await coro_lambda(b)); // expected-warning {{temporary whose address is used as value of local variable}}
+ co_return co_await safe;
+ };
+ auto safe_lambda = [](int b) -> Co<int> {
+ int x = co_await foo_coro(1);
+ co_return x + co_await foo_coro(b);
+ };
+}
+// =============================================================================
+// Safe usage when parameters are value
+// =============================================================================
+namespace by_value {
+Co<int> value_coro(int b) { co_return co_await foo_coro(b); }
+[[clang::coro_wrapper]] Co<int> wrapper1(int b) { return value_coro(b); }
+[[clang::coro_wrapper]] Co<int> wrapper2(const int& b) { return value_coro(b); }
+}
+
+// =============================================================================
+// Lifetime bound but not a Coroutine Return Type: No analysis.
+// =============================================================================
+namespace not_a_crt {
+template <typename T> struct [[clang::coro_lifetimebound]] CoNoCRT {
+ struct promise_type {
+ CoNoCRT<T> get_return_object() {
+ return {};
+ }
+ suspend_always initial_suspend();
+ suspend_always final_suspend() noexcept;
+ void unhandled_exception();
+ void return_value(const T &t);
+ };
+};
+
+CoNoCRT<int> foo_coro(const int& a) { co_return a; }
+CoNoCRT<int> bar(int a) {
+ auto x = foo_coro(a);
+ co_return 1;
+}
+} // namespace not_a_crt
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