https://github.com/dougsonos created
https://github.com/llvm/llvm-project/pull/109855
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.
>From 085965b324efde41168c5d51db3a368578d3458f Mon Sep 17 00:00:00 2001
From: Doug Wyatt <dwy...@apple.com>
Date: Mon, 23 Sep 2024 14:44:32 -0700
Subject: [PATCH] Add clang/docs/FunctionEffectAnalysis.rst.
---
clang/docs/FunctionEffectAnalysis.rst | 503 ++++++++++++++++++++++++++
clang/docs/index.rst | 1 +
2 files changed, 504 insertions(+)
create mode 100644 clang/docs/FunctionEffectAnalysis.rst
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 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 for many of the functions in ``<math.h>``, Clang generates
calls to built-in
+functions, 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.
diff --git a/clang/docs/index.rst b/clang/docs/index.rst
index 4a497f4d9bcc3c..1f4ff28d199b2d 100644
--- a/clang/docs/index.rst
+++ b/clang/docs/index.rst
@@ -26,6 +26,7 @@ Using Clang as a Compiler
ClangStaticAnalyzer
ThreadSafetyAnalysis
DataFlowAnalysisIntro
+ FunctionEffectAnalysis
AddressSanitizer
ThreadSanitizer
MemorySanitizer
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