[Clang] Introduce OverflowBehaviorType for fine-grained overflow control

clang/docs/OverflowBehaviorTypes.rst

@@ -0,0 +1,269 @@
+=====================
+OverflowBehaviorTypes
+=====================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+Clang provides a type attribute that allows developers to have fine-grained control
+over the overflow behavior of integer types. The ``overflow_behavior``
+attribute can be used to specify how arithmetic operations on a given integer
+type should behave upon overflow. This is particularly useful for projects that
+need to balance performance and safety, allowing developers to enable or
+disable overflow checks for specific types.
+
+The attribute can be enabled using the compiler option
+``-foverflow-behavior-types``.
+
+The attribute syntax is as follows:
+
+.. code-block:: c++
+
+  __attribute__((overflow_behavior(behavior)))
+
+Where ``behavior`` can be one of the following:
+
+* ``wrap``: Specifies that arithmetic operations on the integer type should
+  wrap on overflow. This is equivalent to the behavior of ``-fwrapv``, but it
+  applies only to the attributed type and may be used with both signed and
+  unsigned types. When this is enabled, UBSan's integer overflow and integer
+  truncation checks (``signed-integer-overflow``,
+  ``unsigned-integer-overflow``, ``implicit-signed-integer-truncation``, and
+  ``implicit-unsigned-integer-truncation``) are suppressed for the attributed
+  type.
+
+* ``no_wrap``: Specifies that arithmetic operations on the integer type should
+  be checked for overflow. When using the ``signed-integer-overflow`` sanitizer
+  or when using ``-ftrapv`` alongside a signed type, this is the default
+  behavior. Using this, one may enforce overflow checks for a type even when
+  ``-fwrapv`` is enabled globally.
+
+This attribute can be applied to ``typedef`` declarations and to integer types directly.
+
+Examples
+========
+
+Here is an example of how to use the ``overflow_behavior`` attribute with a ``typedef``:
+
+.. code-block:: c++
+
+  typedef unsigned int __attribute__((overflow_behavior(no_wrap))) non_wrapping_uint;
+
+  non_wrapping_uint add_one(non_wrapping_uint a) {
+    return a + 1; // Overflow is checked for this operation.
+  }
+
+Here is an example of how to use the ``overflow_behavior`` attribute with a type directly:
+
+.. code-block:: c++
+
+  int mul_alot(int n) {
+    int __attribute__((overflow_behavior(wrap))) a = n;
+    return a * 1337; // Potential overflow is not checked and is well-defined
+  }
+
+"Well-defined" overflow is consistent with two's complement wrap-around
+semantics and won't be removed via eager compiler optimizations (like some
+undefined behavior might).
+
+Overflow behavior types are implicitly convertible to and from built-in
+integral types.
+
+Note that C++ overload set formation rules treat promotions to and from
+overflow behavior types the same as normal integral promotions and conversions.
+
+Interaction with Command-Line Flags and Sanitizer Special Case Lists
+====================================================================
+
+The ``overflow_behavior`` attribute interacts with sanitizers, ``-ftrapv``,
+``-fwrapv``, and Sanitizer Special Case Lists (SSCL) by wholly overriding these
+global flags. The following table summarizes the interactions:
+
+.. list-table:: Overflow Behavior Precedence
+   :widths: 15 15 15 15 20 15
+   :header-rows: 1
+
+   * - Behavior
+     - Default(No Flags)
+     - -ftrapv
+     - -fwrapv
+     - Sanitizers
+     - SSCL
+   * - ``overflow_behavior(wrap)``
+     - Wraps
+     - No trap
+     - Wraps
+     - No report
+     - Overrides SSCL
+   * - ``overflow_behavior(no_wrap)``
+     - Traps
+     - Traps
+     - Traps
+     - Reports
+     - Overrides SSCL
+
+It is important to note the distinction between signed and unsigned types. For
+unsigned integers, which wrap on overflow by default, ``overflow_behavior(no_wrap)``
+is particularly useful for enabling overflow checks. For signed integers, whose
+overflow behavior is undefined by default, ``overflow_behavior(wrap)`` provides
+a guaranteed wrapping behavior.
+
+The ``overflow_behavior`` attribute can be used to override the behavior of
+entries from a :doc:`SanitizerSpecialCaseList`. This is useful for allowlisting
+specific types into overflow instrumentation.
+
+Promotion Rules
+===============
+
+The promotion rules for overflow behavior types are designed to preserve the
+specified overflow behavior throughout an arithmetic expression. They differ
+from standard C/C++ integer promotions but in a predictable way, similar to
+how ``_Complex`` and ``_BitInt`` have their own promotion rules.
+
+* **OBT and Standard Integer Type**: In an operation involving an overflow
+  behavior type (OBT) and a standard integer type, the result will have the
+  type of the OBT, including its overflow behavior, sign, and bit-width. The
+  standard integer type is implicitly converted to match the OBT.
+
+  .. code-block:: c++
+
+    typedef char __attribute__((overflow_behavior(no_wrap))) no_wrap_char;
+    // The result of this expression is no_wrap_char.
+    no_wrap_char c;
+    unsigned long ul;
+    auto result = c + ul;
+
+* **Two OBTs of the Same Kind**: When an operation involves two OBTs of the
+  same kind (e.g., both ``wrap``), the result will have the larger of the two
+  bit-widths. If the bit-widths are the same, an unsigned type is favored over
+  a signed one.
+
+  .. code-block:: c++
+
+    typedef unsigned char __attribute__((overflow_behavior(wrap))) u8_wrap;
+    typedef unsigned short __attribute__((overflow_behavior(wrap))) u16_wrap;
+    // The result of this expression is u16_wrap.
+    u8_wrap a;
+    u16_wrap b;
+    auto result = a + b;
+
+* **Two OBTs of Different Kinds**: In an operation between a ``wrap`` and a
+  ``no_wrap`` type, a ``no_wrap`` is produced. It is recommended to avoid such
+  operations, as Clang may emit a warning for such cases in the future.
+  Regardless, the resulting type matches the bit-width, sign and behavior of
+  the ``no_wrap`` type.
+
+Diagnostics
+===========
+
+Clang provides diagnostics to help developers manage overflow behavior types.
+
+-Wimplicitly-discarded-overflow-behavior
+----------------------------------------
+
+This warning is issued when an overflow behavior type is implicitly converted
+to a standard integer type, which may lead to the loss of the specified
+overflow behavior.
+
+.. code-block:: c++
+
+  typedef int __attribute__((overflow_behavior(wrap))) wrapping_int;
+
+  void some_function(int);
+
+  void another_function(wrapping_int w) {
+    some_function(w); // warning: implicit conversion from 'wrapping_int' to
+                      // 'int' discards overflow behavior
+  }
+
+To fix this, you can explicitly cast the overflow behavior type to a standard
+integer type.
+
+.. code-block:: c++
+
+  typedef int __attribute__((overflow_behavior(wrap))) wrapping_int;
+
+  void some_function(int);
+
+  void another_function(wrapping_int w) {
+    some_function(static_cast<int>(w)); // OK
+  }
+
+This warning acts as a group that includes
+``-Wimplicitly-discarded-overflow-behavior-pedantic`` and
+``-Wimplicitly-discarded-overflow-behavior-assignment``.
+
+-Wimplicitly-discarded-overflow-behavior-pedantic
+-------------------------------------------------
+
+A less severe version of the warning, ``-Wimplicitly-discarded-overflow-behavior-pedantic``,
+is issued for implicit conversions from an unsigned wrapping type to a standard
+unsigned integer type. This is considered less problematic because both types
+have well-defined wrapping behavior, but the conversion still discards the
+explicit ``overflow_behavior`` attribute.
+
+.. code-block:: c++
+
+  typedef unsigned int __attribute__((overflow_behavior(wrap))) wrapping_uint;
+
+  void some_function(unsigned int);
+
+  void another_function(wrapping_uint w) {
+    some_function(w); // warning: implicit conversion from 'wrapping_uint' to
+                      // 'unsigned int' discards overflow behavior
+                      // [-Wimplicitly-discarded-overflow-behavior-pedantic]
+  }
+
+-Wimplicitly-discarded-overflow-behavior-assignment
+---------------------------------------------------
+
+This warning is issued when an overflow behavior type is implicitly converted
+to a standard integer type as part of an assignment, which may lead to the
+loss of the specified overflow behavior. This is a more specific version of
+the ``-Wimplicitly-discarded-overflow-behavior`` warning, and it is off by
+default.
+
+.. code-block:: c++
+
+  typedef int __attribute__((overflow_behavior(wrap))) wrapping_int;
+
+  void some_function() {
+    wrapping_int w = 1;
+    int i = w; // warning: implicit conversion from 'wrapping_int' to 'int'
+               // discards overflow behavior
+               // [-Wimplicitly-discarded-overflow-behavior-assignment]
+  }
+
+To fix this, you can explicitly cast the overflow behavior type to a standard
+integer type.
+
+.. code-block:: c++
+
+  typedef int __attribute__((overflow_behavior(wrap))) wrapping_int;
+
+  void some_function() {
+    wrapping_int w = 1;
+    int i = static_cast<int>(w); // OK
+    int j = (int)w; // C-style OK
+  }
+
+
+-Woverflow-behavior-attribute-ignored
+-------------------------------------
+
+This warning is issued when the ``overflow_behavior`` attribute is applied to
+a type that is not an integer type.
+
+.. code-block:: c++
+
+  typedef float __attribute__((overflow_behavior(wrap))) wrapping_float;
+  // warning: 'overflow_behavior' attribute only applies to integer types;
+  // attribute is ignored [-Woverflow-behavior-attribute-ignored]
+
+  typedef struct S { int i; } __attribute__((overflow_behavior(wrap))) S_t;
+  // warning: 'overflow_behavior' attribute only applies to integer types;
+  // attribute is ignored [-Woverflow-behavior-attribute-ignored]
+

clang/docs/ReleaseNotes.rst

@@ -381,6 +381,8 @@ New Compiler Flags
 
 - New options ``-g[no-]key-instructions`` added, disabled by default. Reduces jumpiness of debug stepping for optimized code in some debuggers (not LLDB at this time). Not recommended for use without optimizations. DWARF only. Note both the positive and negative flags imply ``-g``.
 
+- New option ``-foverflow-behavior-types`` added to enable parsing of the ``overflow_behavior`` type attribute.
+
 Deprecated Compiler Flags
 -------------------------
 
@@ -481,6 +483,10 @@ related warnings within the method body.
 - Clang will print the "reason" string argument passed on to
   ``[[clang::warn_unused_result("reason")]]`` as part of the warning diagnostic.
 
+- Introduced a new type attribute ``__attribute__((overflow_behavior))`` which
+  currently accepts either ``wrap`` or ``no_wrap`` as an argument, enabling
+  type-level control over overflow behavior.
+
 Improvements to Clang's diagnostics
 -----------------------------------
 

clang/docs/SanitizerSpecialCaseList.rst

@@ -134,6 +134,40 @@ precedence. Here are a few examples.
   fun:*bar
   fun:bad_bar=sanitize
 
+Interaction with Overflow Behavior Types
+----------------------------------------
+
+The ``overflow_behavior`` attribute provides a more granular, source-level
+control that takes precedence over the Sanitizer Special Case List. If a type
+is given an ``overflow_behavior`` attribute, it will override any matching
+``type:`` entry in a special case list.
+
+This allows developers to enforce a specific overflow behavior for a critical
+type, even if a broader rule in the special case list would otherwise disable
+instrumentation for it.
+
+.. code-block:: bash
+
+  $ cat ignorelist.txt
+  # Disable signed overflow checks for all types by default.
+  [signed-integer-overflow]
+  type:*
+
+  $ cat foo.c
+  // Force 'critical_type' to always have overflow checks,
+  // overriding the ignorelist.
+  typedef int __attribute__((overflow_behavior(no_wrap))) critical_type;
+
+  void foo(int x) {
+    critical_type a = x;
+    a++; // Overflow is checked here due to the 'no_wrap' attribute.
+
+    int b = x;
+    b++; // Overflow is NOT checked here due to the ignorelist.
+  }
+
+For more details on overflow behavior types, see :doc:`OverflowBehaviorTypes`.
+
 Format
 ======
 

clang/docs/UndefinedBehaviorSanitizer.rst

@@ -380,6 +380,28 @@ This attribute may not be
 supported by other compilers, so consider using it together with
 ``#if defined(__clang__)``.
 
+Disabling Overflow Instrumentation with ``__attribute__((overflow_behavior(wrap)))``
+------------------------------------------------------------------------------------
+
+For more fine-grained control over how integer overflow is handled, you can use
+the ``__attribute__((overflow_behavior(wrap)))`` attribute. This attribute can
+be applied to ``typedef`` declarations and integer types to specify that
+arithmetic operations on that type should wrap on overflow. This can be used to
+disable overflow sanitization for specific types, while leaving it enabled for
+all other types.
+
+For more information, see :doc:`OverflowBehaviorTypes`.
+
+Enforcing Overflow Instrumentation with ``__attribute__((overflow_behavior(no_wrap)))``
+---------------------------------------------------------------------------------------
+
+Conversely, you can use ``__attribute__((overflow_behavior(no_wrap)))`` to
+enforce overflow checks for a specific type, even when ``-fwrapv`` is enabled
+globally. This is useful for ensuring that critical calculations are always
+checked for overflow, regardless of the global compiler settings.
+
+For more information, see :doc:`OverflowBehaviorTypes`.
+
 Suppressing Errors in Recompiled Code (Ignorelist)
 --------------------------------------------------
 

clang/docs/index.rst

@@ -40,6 +40,7 @@ Using Clang as a Compiler
    SanitizerCoverage
    SanitizerStats
    SanitizerSpecialCaseList
+   OverflowBehaviorTypes
    BoundsSafety
    BoundsSafetyAdoptionGuide
    BoundsSafetyImplPlans