================
@@ -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
----------------
cjappl wrote:
I think more appropriately "the compiler may generate a lock" -- I'm not sure
it is always guaranteed to generate a lock, but will in some circumstances.
Someone with more experience than me can confirm or deny
https://github.com/llvm/llvm-project/pull/109855
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