Function Effect Analysis — Clang 22.1.8 documentation

# Clang 22.1.8 documentation

## Function Effect Analysis

## Introduction

Clang Function Effect Analysis is a language extension which can warn about “unsafe” constructs. The feature is currently tailored for the Performance Constraint attributes `nonblocking` and `nonallocating`; functions with these attributes are verified as not containing any language constructs or calls to other functions which violate the constraint. (See Attributes in Clang.)

## The `nonblocking` and `nonallocating` attributes

### Attribute syntax

The `nonblocking` and `nonallocating` attributes apply to function types, allowing them to be attached to functions, blocks, function pointers, lambdas, and member functions.

    // Functions
    void nonblockingFunction() [[clang::nonblocking]];
    void nonallocatingFunction() [[clang::nonallocating]];

    // Function pointers
    void (*nonblockingFunctionPtr)() [[clang::nonblocking]];

    // Typedefs, type aliases.
    typedef void (*NBFunctionPtrTypedef)() [[clang::nonblocking]];
    using NBFunctionPtrTypeAlias_gnu = __attribute__((nonblocking)) void (*)();
    using NBFunctionPtrTypeAlias_std = void (*)() [[clang::nonblocking]];

    // C++ methods
    struct Struct {
      void NBMethod() [[clang::nonblocking]];
    };

    // C++ lambdas
    auto nbLambda = []() [[clang::nonblocking]] {};

    // Blocks
    void (^nbBlock)() = ^() [[clang::nonblocking]] {};

The attribute applies only to the function itself. In particular, it does not apply to any nested functions or declarations, such as blocks, lambdas, and local classes.

This document uses the C++/C23 syntax `[[clang::nonblocking]]`, since it parallels the placement of the `noexcept` specifier, and the attributes have other similarities to `noexcept`. The GNU `__attribute__((nonblocking))` syntax is also supported. Note that it requires a different placement on a C++ type alias.

Like `noexcept`, `nonblocking` and `nonallocating` have an optional argument, a compile-time constant boolean expression. By default, the argument is `true`, so `[[clang::nonblocking]]` is equivalent to `[[clang::nonblocking(true)]]`, and declares the function type as never blocking.

### Attribute semantics

Together with `noexcept`, the `nonallocating` and `nonblocking` attributes define an ordered series of performance constraints. From weakest to strongest:

- `noexcept` (as per the C++ standard): The function type will never throw an exception.

- `nonallocating`: The function type will never allocate memory on the heap or throw an exception.

- `nonblocking`: The function type will never block on a lock, allocate memory on the heap, or throw an exception.

`nonblocking` includes the `nonallocating` guarantee.

While `nonblocking` and `nonallocating` are conceptually a superset of `noexcept`, neither attribute implicitly specifies `noexcept`. Further, `noexcept` has a specified runtime behavior of aborting if an exception is thrown, while the `nonallocating` and `nonblocking` attributes are mainly for compile-time analysis and have no runtime behavior, except in code built with Clang’s RealtimeSanitizer. Nonetheless, Clang emits a warning if, in C++, a function is declared `nonblocking` or `nonallocating` without `noexcept`. This diagnostic is controlled by `-Wperf-constraint-implies-noexcept`.

`nonblocking(true)` and `nonallocating(true)` apply to function types, and by extension, to function-like declarations. When applied to a declaration with a body, the compiler verifies the function, as described in the section “Analysis and warnings”, below.

`blocking` and `allocating` are synonyms for `nonblocking(false)` and `nonallocating(false)`, respectively. They can be used on a function-like declaration to explicitly disable any potential inference of `nonblocking` or `nonallocating` during verification. (Inference is described later in this document). `nonblocking(false)` and `nonallocating(false)` are legal, but superfluous when applied to a function type that is not part of a declarator: `float (int) [[nonblocking(false)]]` and `float (int)` are identical types.

For functions with no explicit performance constraint, the worst is assumed: the function allocates memory and potentially blocks, unless it can be inferred otherwise. This is detailed in the discussion of verification.

The following example describes the meanings of all permutations of the two attributes and arguments:

    void nb1_na1() [[clang::nonblocking(true)]] [[clang::nonallocating(true)]];
    // Valid; nonallocating(true) is superfluous but doesn't contradict the guarantee.

    void nb1_na0() [[clang::nonblocking(true)]] [[clang::nonallocating(false)]];
    // error: 'allocating' and 'nonblocking' attributes are not compatible

    void nb0_na1() [[clang::nonblocking(false)]] [[clang::nonallocating(true)]];
    // Valid; the function does not allocate memory, but may lock for other reasons.

    void nb0_na0() [[clang::nonblocking(false)]] [[clang::nonallocating(false)]];
    // Valid.

### Type conversions

A performance constraint can be removed or weakened via an implicit conversion. An attempt to add or strengthen a performance constraint is unsafe and results in a warning. The rules for this are comparable to that for `noexcept` in C++17 and later.

    void unannotated();
    void nonblocking() [[clang::nonblocking]];
    void nonallocating() [[clang::nonallocating]];

    void example()
    {
      // It's fine to remove a performance constraint.
      void (*fp_plain)();
      fp_plain = unannotated;
      fp_plain = nonblocking;
      fp_plain = nonallocating;

      // Adding/spoofing nonblocking is unsafe.
      void (*fp_nonblocking)() [[clang::nonblocking]];
      fp_nonblocking = nullptr;
      fp_nonblocking = nonblocking;
      fp_nonblocking = unannotated;
      // ^ warning: attribute 'nonblocking' should not be added via type conversion
      fp_nonblocking = nonallocating;
      // ^ warning: attribute 'nonblocking' should not be added via type conversion

      // Adding/spoofing nonallocating is unsafe.
      void (*fp_nonallocating)() [[clang::nonallocating]];
      fp_nonallocating = nullptr;
      fp_nonallocating = nonallocating;
      fp_nonallocating = nonblocking; // no warning because nonblocking includes nonallocating
      fp_nonallocating = unannotated;
      // ^ warning: attribute 'nonallocating' should not be added via type conversion
    }

### Virtual methods

In C++, when a virtual method has a performance constraint, overriding methods in subclasses inherit the constraint.

    struct Base {
      virtual void unsafe();
      virtual void safe() noexcept [[clang::nonblocking]];
    };

    struct Derived : public Base {
      void unsafe() [[clang::nonblocking]] override;
      // It's okay for an overridden method to be more constrained

      void safe() noexcept override;
      // This method is implicitly declared `nonblocking`, inherited from Base.
    };

### Redeclarations, overloads, and name mangling

The `nonblocking` and `nonallocating` attributes, like `noexcept`, do not factor into argument-dependent lookup and overloaded functions/methods.

First, consider that `noexcept` is integral to a function’s type:

    void f1(int);
    void f1(int) noexcept;
    // error: exception specification in declaration does not match previous
    //   declaration

Unlike `noexcept`, a redeclaration of `f2` with an added or stronger performance constraint is legal and propagates the attribute to the previous declaration:

    int f2();
    int f2() [[clang::nonblocking]]; // redeclaration with stronger constraint is OK.

This greatly eases adoption by making it possible to annotate functions in external libraries without modifying library headers.

A redeclaration with a removed or weaker performance constraint produces a warning, paralleling the behavior of `noexcept`:

    int f2() { return 42; }
    // warning: attribute 'nonblocking' on function does not match previous declaration

In C++14, the following two declarations of f3 are identical (a single function). In C++17 they are separate overloads:

    void f3(void (*)());
    void f3(void (*)() noexcept);

Similarly, the following two declarations of f4 are separate overloads. This pattern may pose difficulties due to ambiguity:

    void f4(void (*)());
    void f4(void (*)() [[clang::nonblocking]]);

The attributes have no effect on the mangling of function and method names.

### Objective-C

The attributes are currently unsupported on Objective-C methods.

## Analysis and warnings

### Constraints

Functions declared `nonallocating` or `nonblocking`, when defined, are verified according to the following rules. Such functions:

1. May not allocate or deallocate memory on the heap. The analysis follows the calls to `operator new` and `operator delete` generated by the `new` and `delete` keywords, and treats them like any other function call. The global `operator new` and `operator delete` aren’t declared `nonblocking` or `nonallocating` and so they are considered unsafe. (This is correct because most memory allocators are not lock-free. Note that the placement form of `operator new` is implemented inline in libc++’s `<new>` header, and is verifiably `nonblocking`, since it merely casts the supplied pointer to the result type.)

2. May not throw or catch exceptions. To throw, the compiler must allocate the exception on the heap. (Also, many subclasses of `std::exception` allocate a string). Exceptions are deallocated when caught.

3. May not make any indirect function call, via a virtual method, function pointer, or pointer-to-member function, unless the target is explicitly declared with the same `nonblocking` or `nonallocating` attribute (or stronger).

4. May not make direct calls to any other function, with the following exceptions:

> 1. The callee is also explicitly declared with the same `nonblocking` or `nonallocating` attribute (or stronger).

> 2. The callee is defined in the same translation unit as the caller, does not have the `false` form of the required attribute, and can be verified to have the same attribute or stronger, according to these same rules.

> 3. The callee is a built-in function that is known not to block or allocate.

> 4. The callee is declared `noreturn` and, if compiling C++, the callee is also declared `noexcept`. This special case excludes functions such as `abort()` and `std::terminate()` from the analysis. (The reason for requiring `noexcept` in C++ is that a function declared `noreturn` could be a wrapper for `throw`.)

5. May not invoke or access an Objective-C method or property, since `objc_msgSend()` calls into the Objective-C runtime, which may allocate memory or otherwise block.

6. May not access thread-local variables. Typically, thread-local variables are allocated on the heap when first accessed.

Functions declared `nonblocking` have an additional constraint:

7. May not declare static local variables (e.g. Meyers singletons). The compiler generates a lock protecting the initialization of the variable.

Violations of any of these rules result in warnings, in the `-Wfunction-effects` category:

    void notInline();

    void example() [[clang::nonblocking]]
    {
      auto* x = new int;
      // warning: function with 'nonblocking' attribute must not allocate or deallocate
      //   memory

      if (x == nullptr) {
        static Logger* logger = createLogger();
        // warning: function with 'nonblocking' attribute must not have static local variables

        throw std::runtime_warning{ "null" };
        // warning: 'nonblocking" function 'example' must not throw exceptions
      }
      notInline();
      // warning: 'function with 'nonblocking' attribute must not call non-'nonblocking' function
      //   'notInline'
      // note (on notInline()): declaration cannot be inferred 'nonblocking' because it has no
      //   definition in this translation unit
    }

### Inferring `nonblocking` or `nonallocating`

In the absence of a `nonblocking` or `nonallocating` attribute (whether `true` or `false`), a function that is called from a performance-constrained function may be analyzed to infer whether it has a desired attribute. This analysis happens when the function is not a virtual method, and it has a visible definition within the current translation unit (i.e. its body can be traversed).

    void notInline();
    int implicitlySafe() { return 42; }
    void implicitlyUnsafe() { notInline(); }

    void example() [[clang::nonblocking]]
    {
      int x = implicitlySafe(); // OK
      implicitlyUnsafe();
      // warning: function with 'nonblocking' attribute must not call non-'nonblocking' function
      //   'implicitlyUnsafe'
      // note (on implicitlyUnsafe): function cannot be inferred 'nonblocking' because it calls
      //   non-'nonblocking' function 'notInline'
      // note (on notInline()): declaration cannot be inferred 'nonblocking' because it has no
      //   definition in this translation unit
    }

### Lambdas and blocks

As mentioned earlier, the performance constraint attributes apply only to a single function and not to any code nested inside it, including blocks, lambdas, and local classes. It is possible for a nonblocking function to schedule the execution of a blocking lambda on another thread. Similarly, a blocking function may create a `nonblocking` lambda for use in a realtime context.

Operations which create, destroy, copy, and move lambdas and blocks are analyzed in terms of the underlying function calls. For example, the creation of a lambda with captures generates a function call to an anonymous struct’s constructor, passing the captures as parameters.

### Implicit function calls in the AST

The `nonblocking` / `nonallocating` analysis occurs at the Sema phase of analysis in Clang. During Sema, there are some constructs which will eventually become function calls, but do not appear as function calls in the AST. For example, `auto* foo = new Foo;` becomes a declaration containing a `CXXNewExpr` which is understood as a function call to the global `operator new` (in this example), and a `CXXConstructExpr`, which, for analysis purposes, is a function call to `Foo`’s constructor. Most gaps in the analysis would be due to incomplete knowledge of AST constructs which become function calls.

### Disabling diagnostics

Function effect diagnostics are controlled by `-Wfunction-effects`.

A construct like this can be used to exempt code from the checks described here:

    #define NONBLOCKING_UNSAFE(...)                                    \
      _Pragma("clang diagnostic push")                                 \
      _Pragma("clang diagnostic ignored \"-Wunknown-warning-option\"") \
      _Pragma("clang diagnostic ignored \"-Wfunction-effects\"")       \
      __VA_ARGS__                                                      \
      _Pragma("clang diagnostic pop")

Disabling the diagnostic allows for:

- constructs which do block, but which in practice are used in ways to avoid unbounded blocking, e.g. a thread pool with semaphores to coordinate multiple realtime threads;

- using libraries which are safe but not yet annotated;

- incremental adoption in a large codebase.

## Adoption

There are a few common issues that arise when adopting the `nonblocking` and `nonallocating` attributes.

### C++ exceptions

Exceptions pose a challenge to the adoption of the performance constraints. Common library functions which throw exceptions include:

Method | Alternative
--- | ---
`std::vector<T>::at()` | `operator[](size_t)`, after verifying that the index is in range.
`std::optional<T>::value()` | `operator*`, after checking `has_value()` or `operator bool()`.
`std::expected<T, E>::value()` | Same as for `std::optional<T>::value()`.

### `std::function<R(Args...)>`

`std::function<R(Args...)>` is generally incompatible with `nonblocking` and `nonallocating` code, because a typical implementation may allocate heap memory in the constructor.

Alternatives:

- `std::function_ref` (available in C++26 or as `llvm::function_ref`). This is appropriate and optimal when a functor’s lifetime does not need to extend past the function that created it.

- `inplace_function` from WG14. This solves the allocation problem by giving the functor wrapper a fixed size known at compile time and using an inline buffer.

While these alternatives both address the heap allocation of `std::function`, they are still obstacles to `nonblocking/nonallocating` verification, for reasons detailed in the next section.

### Interactions with type-erasure techniques

`std::function<R(Args...)>` illustrates a common C++ type-erasure technique. Using template argument deduction, it decomposes a function type into its return and parameter types. Additional components of the function type, including `noexcept`, `nonblocking`, `nonallocating`, and any other attributes, are discarded.

Standard library support for these components of a function type is not immediately forthcoming.

Code can work around this limitation in either of two ways:

1. Avoid abstractions like `std::function` and instead work directly with the original lambda type.

2. Create a specialized alternative, e.g. `nonblocking_function_ref<R(Args...)>` where all function pointers used in the implementation and its interface are `nonblocking`.

As an example of the first approach, when using a lambda as a Callable template parameter, the attribute is preserved:

    std::sort(vec.begin(), vec.end(),
      [](const Elem& a, const Elem& b) [[clang::nonblocking]] { return a.mem < b.mem; });

Here, the type of the `Compare` template parameter is an anonymous class generated from the lambda, with an `operator()` method holding the `nonblocking` attribute.

A complication arises when a Callable template parameter, instead of being a lambda or class implementing `operator()`, is a function pointer:

    static bool compare_elems(const Elem& a, const Elem& b) [[clang::nonblocking]] {
      return a.mem < b.mem; };

    std::sort(vec.begin(), vec.end(), compare_elems);

Here, the type of `compare_elems` is decomposed to `bool(const Elem&, const Elem&)`, without `nonblocking`, when forming the template parameter. This can be solved using the second approach, creating a specialized alternative which explicitly requires the attribute. In this case, it’s possible to use a small wrapper to transform the function pointer into a functor:

    template <typename>
    class nonblocking_fp;

    template <typename R, typename... Args>
    class nonblocking_fp<R(Args...)> {
    public:
      using impl_t = R (*)(Args...) [[clang::nonblocking]];

    private:
      impl_t mImpl{ nullptr_t };
    public:
      nonblocking_fp() = default;
      nonblocking_fp(impl_t f) : mImpl{ f } {}

      R operator()(Args... args) const
      {
        return mImpl(std::forward<Args>(args)...);
      }
    };

    // deduction guide (like std::function's)
    template< class R, class... ArgTypes >
    nonblocking_fp( R(*)(ArgTypes...) ) -> nonblocking_fp<R(ArgTypes...)>;

    // --

    // Wrap the function pointer in a functor which preserves ``nonblocking``.
    std::sort(vec.begin(), vec.end(), nonblocking_fp{ compare_elems });

Now, the `nonblocking` attribute of `compare_elems` is verified when it is converted to a `nonblocking` function pointer, as the argument to `nonblocking_fp`’s constructor. The template parameter is the functor class `nonblocking_fp`.

### Static local variables

Static local variables are often used for lazily-constructed globals (Meyers singletons). Beyond the compiler’s use of a lock to ensure thread-safe initialization, it is dangerously easy to inadvertently trigger initialization, involving heap allocation, from a `nonblocking` or `nonallocating` context.

Generally, such singletons need to be replaced by globals, and care must be taken to ensure their initialization before they are used from `nonblocking` or `nonallocating` contexts.

### Annotating libraries

It can be surprising that the analysis does not depend on knowledge of any primitives; it simply assumes the worst, that all function calls are unsafe unless explicitly marked as safe or able to be inferred as safe. With `nonblocking`, this appears to suffice for all but the most primitive of spinlocks.

At least for an operating system’s C functions, it is possible to define an override header which redeclares safe common functions (e.g. `pthread_self()`) with the addition of `nonblocking`. This may help in adopting the feature incrementally.

It also helps that many of the functions in the standard C libraries (notably `<math.h>`) are treated as built-in functions by Clang, which the diagnosis understands to be safe.

Much of the C++ standard library consists of inline templated functions which work well with inference. A small number of primitives may need explicit `nonblocking/nonallocating` attributes.