[clang] Add clang/docs/FunctionEffectAnalysis.rst. (PR #109855)
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Tue Sep 24 13:04:31 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Doug Wyatt (dougsonos)
<details>
<summary>Changes</summary>
Follow-on from #<!-- -->99656, which introduces 2nd pass caller/callee analysis for function effects.
Wrote a new documentation page, derived directly from the RFC posted to LLVM Discourse earlier this year.
---
Patch is 21.94 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/109855.diff
2 Files Affected:
- (added) clang/docs/FunctionEffectAnalysis.rst (+503)
- (modified) clang/docs/index.rst (+1)
``````````diff
diff --git a/clang/docs/FunctionEffectAnalysis.rst b/clang/docs/FunctionEffectAnalysis.rst
new file mode 100644
index 00000000000000..8c1cf8a483c5f1
--- /dev/null
+++ b/clang/docs/FunctionEffectAnalysis.rst
@@ -0,0 +1,503 @@
+========================
+Function Effect Analysis
+========================
+
+Introduction
+============
+
+Clang Function Effect Analysis is a C++ 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 :doc:`AttributeReference`.)
+
+
+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.
+
+.. code-block:: c++
+
+ // 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(true)]]``
+is equivalent to ``[[clang::nonblocking]]``, and declares the function type as never locking.
+
+
+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, and never throw an
+ exception.
+- ``nonblocking``: The function type will never block on a lock, never allocate memory on the heap,
+ and never throw an exception.
+
+``nonblocking`` includes the ``nonallocating`` guarantee.
+
+``nonblocking`` and ``nonallocating`` include the ``noexcept`` guarantee, but the presence of either
+attribute does not implicitly specify ``noexcept``. (It would be inappropriate for a Clang
+attribute, ignored by non-Clang compilers, to imply a standard language feature.)
+
+``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. Functions without an explicit
+performance constraint are not verified.
+
+``nonblocking(false)`` and ``nonallocating(false)`` are synonyms for the attributes ``blocking`` and
+``allocating``. 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*. ``float (int) [[nonblocking(false)]]`` and ``float (int)`` are
+identical types.
+
+For all functions with no explicit performance constraint, the worst is assumed, that the function
+allocates memory and potentially blocks, unless it can be inferred otherwise, as described in the
+discussion of verification.
+
+The following list describes the meanings of all permutations of the two attributes and arguments:
+
+- ``nonblocking(true)`` + ``nonallocating(true)``: valid; ``nonallocating(true)`` is superfluous but
+ does not contradict the guarantee.
+- ``nonblocking(true)`` + ``nonallocating(false)``: error, contradictory.
+- ``nonblocking(false)`` + ``nonallocating(true)``: valid; the function does not allocate memory,
+ but may lock for other reasons.
+- ``nonblocking(false)`` + ``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.
+
+.. code-block:: c++
+
+ 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 base class's virtual method has a performance constraint, overriding methods in
+subclasses inherit the attribute.
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ 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, in order to
+parallel the behavior of ``noexcept``:
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ void f4(void (*)());
+ void f4(void (*)() [[clang::nonblocking]]);
+
+The attributes have no effect on the mangling of function and method names.
+
+``noexcept``
+------------
+
+``nonblocking`` and ``nonallocating`` are conceptually similar to a stronger form of C++'s
+``noexcept``, but with further diagnostics, as described later in this document. Therefore, in C++,
+a ``nonblocking`` or ``nonallocating`` function, method, block or lambda should also be declared
+``noexcept``.[^6] If ``noexcept`` is missing, a warning is issued. In Clang, this diagnostic is
+controlled by ``-Wperf-constraint-implies-noexcept``.
+
+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 ``std::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:
+
+ a. The callee is also explicitly declared with the same ``nonblocking`` or ``nonallocating``
+ attribute (or stronger).
+ b. 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 be have the same attribute or stronger,
+ according to these same rules.
+ c. The callee is a built-in function (other than builtins which are known to block or allocate).
+ d. The callee is declared ``noreturn`` and, if compiling C++, the callee is also declared
+ ``noexcept``. This exception excludes functions such as ``abort()`` and ``std::terminate()``
+ from the analysis.
+
+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.
+
+Functions declared ``nonblocking`` have an additional constraint:
+
+6. 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:
+
+.. code-block:: c++
+
+ 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 locals
+
+ 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, when found to be called from a performance-constrained function, can 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).
+
+.. code-block:: c++
+
+ 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
+lock-free 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:
+
+.. code-block:: c++
+
+ #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()``. |
++----------------------------------+-----------------------------------------------------------------------+
+
+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<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:
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ 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:
+
+.. code-block:: c++
+
+ 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 ...
[truncated]
``````````
</details>
https://github.com/llvm/llvm-project/pull/109855
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