P1105R0
Leaving no room for a lower-level language: A C++ Subset

1. Introduction

It is my intent that this be the least bad form of dialect, the proper subset. All valid freestanding libraries should be valid hosted libraries with compatible semantics.

In [P0829], I propose adding library features to freestanding mode that should work everywhere. This paper covers the removal and modification of features that don’t work everywhere. There is already standards precedent in support.signal for avoiding portions of all the features that I am making optional.

There are years, if not decades of field experience using C++ subsets similar to what I am proposing ([OSR], [APPLE_KERNEL]). The workarounds and compiler switches are mostly available today. The main places where this paper innovates is in places where we can keep more features than current compiler based switches allow.

In theory, this paper would result in large scale code breaks for existing freestanding users. In practice, there are almost no existing freestanding users because the current definition is not serving the stated purpose of working "without the benefit of an operating system". Existing implementations already provide mechanisms for disabling many of the features that this paper proposes to make optional. Updating these implementations to conform to this proposal would leave existing users largely unaffected, except that they would now be using a truly compliant C++ implementation.

I believe that the embedded and kernel C++ community is better served by making features optional, rather than providing conforming, but low quality, highly unsatisfactory implementations. Missing functionality sends a clear signal to library writers, where low quality implementations provide an easier to miss message.

Note that freestanding implementations can (and should) make available all the features that are implementable on their target environment. For example, there are many embedded systems where floating point operations are desirable, but heap allocations are not. Each cluster of features will get its own feature test macro. This has the effect of making all implementations compliant that are "between" the bare minimum freestanding and the full hosted implementation.

2. Value of standardization

First, I will answer those questions with another question: Why bring any proposal into the standard? Presumably the authors of those proposals could get work done without the proposal. Proposal authors are resourceful people, and can probably implement their papers in a fork of an existing compiler or standard library. Yet they go through the hassle and expense of presenting papers to WG21 anyway.

By making freestanding useful, I will be providing a target for toolchain and library authors. Library authors that wish to make their libraries as portable as possible will have a standardized lowest common denominator to write against. Purchasers will be better able to make requests of their vendors for freestanding compliant products. Educators will be better able to teach about the requirements of kernel and embedded programming. Tool vendors can better prioritize work on conforming compiler modes, and possibly reject new, ad-hoc non-conforming modes. Users can get uniform behavior on what is currently an inconsistent set of vendor extensions.

3. Before-and-after tables

3.1. Well-formed

void caller() {
    try {foo();}
    catch(const std::exception &e) {
        log_exception(e.what());
        throw;
    }
}

struct B {virtual ~B() {} };
void foo() {B b;}

3.2. Potentially ill-formed

struct B {virtual void f() {}};
struct D : B {virtual void f() {}};
D *func(B *b) {
    return dynamic_cast<D*>(b);
}

#include <typeinfo>
struct B {virtual void f() {}};
const bool func(B &b) {
    return typeid(b) == typeid(int);
}

int foo() {
    thread_local int x = 0;
    ++x;
    return x;
}

double doubler(double x) {
    return x * 2.0;
}

void handler();
void foo() {
    atexit(handler);
}

struct Obj {Obj();};
void foo() {
    static Obj obj;
}

struct BigData {
    int d[16];
};

void foo(
    std::atomic &lhs,
    const BigData &rhs)
{lhs = rhs;}

3.3. noexcept comparisons

extern "C" void extern_c();
void plain();
void noexcept_false() noexcept(false);
void noexcept_true() noexcept;

4. Features going optional

The feature macros are somewhat backwards from how the macros are normally defined. The macros are defined when the paper is adopted and the feature is missing. We can’t define the macros in the past to say the features are present. Testing for the "non-feature" macros is a safer and more backwards compatible way of determining whether the following features are present.

4.1. Exceptions

This section applies to "dynamic" exceptions. In other words, the exceptions we have had since C++98. [P0709] could add "static" exceptions. I am keeping static exceptions in mind with this design, but I’m not providing any wording against that proposal.

4.1.1. Why make this optional?

Even when exceptions aren’t thrown, there is a large space cost. Table based exception costs grow roughly in proportion to the size and complexity of the program, and not in the number of throw sites, catch sites, or frames traversed in an exception throw. Since table based exception costs grows with program size, rather than how much it is used, it is not zero overhead. setjmp / longjmp exception size costs are similar in these regards.

See [P0709] for further discussion on the problems with exceptions.

4.1.2. What isn’t changing?

4.1.3. What am I changing (and why)?

Evaluating a throw expression with operands in an environment without exception support is undefined behavior. We allow the programmer to compile with a throw to allow exception neutral code to be shared between freestanding and hosted implementations. The throw should never be evaluated, since we shouldn’t be able to get into a catch block.

Evaluating a throw expression without operands in an environment without exception support is also undefined behavior. This is a minor change in behavior. A rethrow without an active exception currently calls std::terminate. I don’t want unreachable code to increase binary size, and requiring a call to std::terminate would do that.

We allow throw expressions so that programmers in environments with exceptions can catch the exception, and either translate the exception to another type of exception, rethrow the exception in a "Lippincott" function, or handle the exception some other way. In these cases, we have the expectation that the code will never run in the exceptionless environment.

Implementations are encouraged to produce warnings on any throw expression with operands, as well as allow suppressions for informing the compiler when those throws are actually there for exception translation purposes.

When a function without a noexcept specification is passed to a noexcept expression, noexcept will return true if exception support is not present. This will speed up operations like move_if_noexcept and containers with strong exception guarantees. This also differs somewhat from existing practices. Visual Studio, clang, and gcc do not currently adjust the value of noexcept when exceptions are off. The Intel compiler makes the noexcept operator unconditionally return true when exceptions are turned off. In addition, Visual Studio has a compiler mode, /EHsc, that takes extern "C" functions and makes them noexcept. Note that this approach leaves the door open for static exceptions to use noexcept.

Both the literal form of noexcept(false) and the conditional form noexcept(noexcept(foo())) that evaluates to 'noexcept(false)' should be treated with suspicion in exceptionless environments. Code that claims that it could throw when throwing isn’t allowed seems wrong. This isn’t normative, but it would be a useful place for compilers to warn. The behavior is being kept as is to leave room for future exception handling mechanisms, like [P0709].

4.1.4. Alternative designs

1. No change to noexcept expression

noexcept would continue to assume that a function without a noexcept specifier could throw. We have the most experience with this option (Clang, gcc, and Visual Studio implement it), but it also leaves the most unexploited performance, as it pessimizes move_if_noexcept, vector, and other facilities that query noexcept.

2. noexcept expressions always return true

Intel icc currently implements this option. This opens us up to breakage if some other proposal gives meaning to noexcept on platforms without dynamic exceptions. Having noexcept directly contradict a noexcept(false) specification seems wrong as well.

3. Make noexcept ill-formed

Visual Studio currently warns when it sees a noexcept specifier while exceptions are disabled. This option would make it very difficult to share code between freestanding and hosted. Libraries attempting to target both would most likely resort to a macro that conditionally expands to noexcept.

4. throw UB vs. ill-formed vs. std::terminate

We could make some or all throw expressions ill-formed. The benefit is that compilers could more reliably produce diagnostics. The cost is that it would be more difficult to share exception neutral code between hosted and freestanding.

We could make throw call std::terminate rather than have it be UB. This would give reliable behavior that is similar to what no-except variants of today’s standard libraries do. UB likely optimizes better. Compiler would be able to remove any code that leads to the UB. If throw calls std::terminate, that would count as an ODR-use, and would prevent the linker from discarding the definition of std::terminate.

5. try and catch allowed vs. ill-formed

If we made try and catch ill-formed, we would severely impact the portability of libraries across the exception and non-exception worlds. However, this is basically the status quo today, so we have experience with this pain.

If we adopt everything else in this paper, while banning try and catch, we would be able to claim that freestanding C++ is signal safe C++.

6. Only allow catch(...) and throw;

Logging exceptions and translating exceptions are less common use cases than simple catch and rethrow use cases. Allowing catch(type) takes us down the path of pulling in std::exception, as well as making it difficult to diagnose inappropriate throw obj; statements.

4.2. Parts of <exception> header

4.2.1. What isn’t changing?

std::terminate will still be available. Various language features, most recently contracts, rely on std::terminate. Freestanding will keep std::terminate rather than respecify how all those features signal unrecoverable errors.

4.2.2. What am I changing?

• terminate_handler, get_terminate_handler and set_terminate_handler

• uncaught_exceptions

• exception_ptr, current_exception, rethrow_exception, and make_exception_ptr

• bad_exception and nested_exception

• throw_with_nested and rethrow_if_nested

4.2.3. Why?

uncaught_exceptions relies on thread-local storage (see §4.5 Thread local storage). Hard coding a return value of zero would work for existing implementations, but it would close off potential future designs (see §5.1 [P0709] Zero-overhead deterministic exceptions).

The exception_ptr and throw_with_nested facilities require heap allocations and/or thread-local storage.

4.2.4. Alternative designs

1. Omit std::exception and its children.

This alternative would make it so that clients could only catch(...) and catch their own client defined types. This removes the ability of those clients to log or translate exceptions. However, it would likely require less work on the implementation side, seeing as the current exception classes don’t work in kernel and embedded environments.

2. Omit the entire <exception> header.

In addition to the issues in the above alternative, we would also need to ensure that all the other library features and core language features didn’t call std::terminate in freestanding mode.

4.3. RTTI

4.3.1. What am I changing?

4.3.2. Why?

The slot in the vtable itself is also a place where space is wasted.

If typeid and dynamic_cast can’t be called, implementations can safely remove the type_info objects, saving space. Some ABIs will even permit reclaiming the vtable slot.

4.4. Default heap storage

4.4.1. What isn’t changing?

4.4.2. What am I changing?

The presence of a virtual destructor shall not require ::operator delete to be provided unless an instance of the object is created with new. Constructors and destructors will not ODR-use non-placement allocation and deallocation functions. Instead new and delete expressions will ODR-use the non-placement allocation and dealloction functions. (basic.def.odr)

4.4.3. Why?

Many kernel systems have multiple pools of memory, none of which is suitable as a default. In the Microsoft Windows kernel, developers have the choice of paged pool, which is plentiful and dangerous; and non-paged pool, which is safe and scarce. The National Instruments codebase has had experience using each of those options as a default, and both have proven problematic. The Microsoft Visual Studio compiler switch /kernel already implements the lack of default allocation functions. [kernel_switch]

In current implementations of virtual destructors, the class’s vtable points at a stub function that calls the "real" destructor, then calls ::operator delete. This places a burden on freestanding users of hosted code, even when the freestanding users aren’t using new and delete. It seems reasonable to allow a freestanding class to have a virtual destructor, so long as the class is never newed or deleteed. Hosted uses of the class can new and delete all they want.

4.5. Thread local storage

4.5.1. What am I changing?

4.5.2. Why?

For embedded platforms, there may not be an operating system. Implementing thread local storage on those platforms would be extra runtime overhead.

For kernel platforms, and drivers in particular, the operating system may be owned by a third party. The third party may not provide arbitrary thread local storage for plugins. Neither Linux, Microsoft Windows, Apple OSX, FreeBSD, nor OpenRTOS support arbitrary thread local storage in the kernel.

4.6. Floating point

4.6.1. What am I changing?

<cfloat> is not required to be present in environments without floating point support. numeric_limits<floating point type> is not required to be present in environments without floating point support.

4.6.2. Why?

In kernel environments, floating point operations are avoided. The system call interface from user mode to kernel mode normally does a partial context switch, where it saves off the old values of registers, so that they can be restored when returning to user mode. In order to make user / kernel transitions fast, operating systems usually don’t automatically save or restore the floating point state. This means that carelessly using floating point in the kernel ends up corrupting the user mode program’s floating point state.

4.7. Program start-up and termination

• __cpp_freestanding_no_static_initialization.

• __cpp_freestanding_no_dynamic_initialization.

• __cpp_freestanding_no_termination.

4.7.1. What isn’t changing

std::abort and std::terminate will remain in the library. _Exit will be in the library assuming [P0829] is accepted.

4.7.2. Rationalization for the status quo

All code which runs before the user’s code could be considered unwanted overhead in some applications. All code that runs after the user’s code could also be considered unwanted overhead. Also, the "early" code that does initialization needs to be written in some language, and if we require zero initialization to happen before anything else, then that excludes C++ from being used to write early startup code.

In practice, I expect zero initialization and static initialization to be the most used freestanding extension.

std::abort and _Exit do not call global destructors, global registration functions, or flush file I/O. std::terminate does not call destructors or flush file I/O, but it does call a global registration function. §4.2 Parts of <exception> header makes the getters and setters for the global registration function optional, so a freestanding std::terminate doesn’t necessarily have a registration function either. That leaves these as three functions that will end the program in an implementation defined way.

4.7.3. What am I changing?

4.7.4. Why?

4.8. Language mandated blocking synchronization

• __cpp_freestanding_no_locked_atomics.

• __cpp_freestanding_no_dynamic_static_init. This implies that __cpp_threadsafe_static_init is undefined.

4.8.1. What am I changing?

In practice, this won’t require changes from toolchain vendors. On unknown environments, the C++ runtime functions necessary to implement locked atomics and dynamic initialization of function statics generally aren’t provided. This results in linker errors, satisfying the ill-formed requirement. This change will make such a toolchain conforming.

This change would break code migrating from C++98 to C++Next, as it will remove function static initialization that previously worked. That same code would likely break in the C++98 to C++11 migration, as the function static initialization would require facilities not present in the environment. Implementations would likely continue to provide compiler flags to aid the migration.

4.8.2. Why?

On a system without an OS, your main blocking choices are disabling interrupts and spin locks. Spin locks are needed to synchronize among multiple hardware threads, and disabling interrupts is required when synchronizing a processor with itself. Neither blocking technique is universally applicable, even when limited to the realm of OS-less systems.

In the Windows kernel, there are multiple types of locks. No one lock type is appropriate in all situations.

The CRECT RTOS [CRECT] doesn’t have independent locks like many other OSes do. All locks are explicitly associated with a particular resource. Jobs must list all resources they use so that scheduling priorities can be calculated at compile-time. This effectively means that a CRECT application has N distinct lock types, used only by that application. None of these locks are known to the maintainers of CRECT, and none of them are known to the C++ runtime. Current compiler ABIs do not provide the C++ runtime with information about the type or address of the function static being initialized.

Some OSes and applications are trying to meet hard real time guarantees. Spin locks and disabled interrupts can add potentially unbounded jitter and latency to time critical operations, even when the operation isn’t performed on a time critical code path.

Some OSes aren’t scheduled in a time-sliced manner. Spin locks on these systems are a bad idea. You could get in the middle of static initialization, get an interrupt that causes you to change threads, then get stuck on the initialization of the same static. Forward progress will be halted until another interrupt happens at some indeterminate point in the future.

All of these concerns are also concerns with regards to signals. support.signal already calls out that locked atomics result in UB when invoked from a signal. Dynamic initialization of a static variable is also UB when invoked from a signal. If we are willing to make special rules for signals, shouldn’t we be willing to make special rules for embedded and kernel... especially if the rules are largely the same?

5. Related works in progress, and future work

5.1. [P0709] Zero-overhead deterministic exceptions

5.2. [P0784] Standard containers and constexpr

One possible avenue for the std::allocator special case is for the implementation to provide declarations of all the methods, but provide no implementations. The declarations may prove sufficient for the constexpr use case, while triggering linker errors in the runtime case.

Or maybe, this could be tackled with conditionally constexpr! functions...

5.3. [P1073] constexpr! functions

5.4. Explicit control of program startup and termination

6. Common QoI issues

6.1. Pure virtual functions

6.2. Symbol name length

The C++ standard does not govern name mangling, and this paper makes no concrete recommendations. Implementations should strive to allow users to make useful tradeoffs between symbol name length, legibility, and ABI compatibility.

7. Acknowledgments