src/cxa_exception.cpp

//===------------------------- cxa_exception.cpp --------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is dual licensed under the MIT and the University of Illinois Open
// Source Licenses. See LICENSE.TXT for details.
//
//  
//  This file implements the "Exception Handling APIs"
//  http://www.codesourcery.com/public/cxx-abi/abi-eh.html
//  
//===----------------------------------------------------------------------===//

#include "cxxabi.h"

#include <exception>        // for std::terminate
#include <cstdlib>          // for malloc, free
#include <string>           // for memset
#include <pthread.h>

#include "cxa_exception.hpp"

namespace __cxxabiv1 {
static const uint64_t kOurExceptionClass          = 0x434C4E47432B2B00; // CLNGC++\0
static const uint64_t kOurDependentExceptionClass = 0x434C4E47432B2B01; // CLNGC++\1
                                                    
//  Utility routines
static __cxa_exception *exception_from_thrown_object(void *p) throw() {
    return ((__cxa_exception *) p) - 1;
}
    
static void * thrown_object_from_exception(void *p) throw() {
    return (void *) (((__cxa_exception *) p) + 1 );
}

static size_t object_size_from_exception_size(size_t size) throw() {
    return size + sizeof (__cxa_exception);
}

//  Get the exception object from the unwind pointer.
//  Relies on the structure layout, where the unwind pointer is right in
//  front of the user's exception object
static __cxa_exception *
exception_from_exception_object(void *ptr) throw() {
    _Unwind_Exception *p = reinterpret_cast<_Unwind_Exception *>(ptr);
    return exception_from_thrown_object(p + 1 );
}

static void setExceptionClass(_Unwind_Exception *unwind) throw() {
    unwind->exception_class = kOurExceptionClass;
}

static void setDependentExceptionClass(_Unwind_Exception *unwind) throw() {
    unwind->exception_class = kOurDependentExceptionClass;
}

//  Is it one of ours?
static bool isOurExceptionClass(_Unwind_Exception *unwind) throw() {
    return(unwind->exception_class == kOurExceptionClass)||
               (unwind->exception_class == kOurDependentExceptionClass);
}

static bool isDependentException(_Unwind_Exception *unwind) throw() {
    return (unwind->exception_class & 0xFF) == 0x01;
}

//	TODO: This needs to be atomic
static int incrementHandlerCount(__cxa_exception *exception) throw() {
	return ++exception->handlerCount;
}

//	TODO: This needs to be atomic
static int decrementHandlerCount(__cxa_exception *exception) throw() {
	return --exception->handlerCount;
}

#include "fallback_malloc.cpp"

//  Allocate some memory from _somewhere_
static void *do_malloc(size_t size) throw() {
    void *ptr = std::malloc(size);
    if (NULL == ptr) // if malloc fails, fall back to emergency stash
        ptr = fallback_malloc(size);
    return ptr;
}

//  Didn't know you could "return <expression>" from a void function, did you?
//  Well, you can, if the type of the expression is "void" also.
static void do_free(void *ptr) throw() {
    return is_fallback_ptr(ptr) ? fallback_free(ptr) : std::free(ptr);
}

/*  Howard says:
    If reason isn't _URC_FOREIGN_EXCEPTION_CAUGHT, then the terminateHandler
    stored in exc is called.  Otherwise the exceptionDestructor stored in 
    exc is called, and then the memory for the exception is deallocated.
*/
static void exception_cleanup_func(_Unwind_Reason_Code reason, struct _Unwind_Exception* exc) {
    __cxa_exception *exception = exception_from_exception_object(exc);
    if (_URC_FOREIGN_EXCEPTION_CAUGHT != reason)
        exception->terminateHandler ();
        
    void * thrown_object = thrown_object_from_exception(exception);
    if (NULL != exception->exceptionDestructor)
        exception->exceptionDestructor(thrown_object);
    __cxa_free_exception(thrown_object);
}

static LIBCXXABI_NORETURN void failed_throw(__cxa_exception *exception) throw() {
//  Section 2.5.3 says:
//      * For purposes of this ABI, several things are considered exception handlers:
//      ** A terminate() call due to a throw.
//  and
//      * Upon entry, Following initialization of the catch parameter, 
//          a handler must call:
//      * void *__cxa_begin_catch(void *exceptionObject );
    (void) __cxa_begin_catch(&exception->unwindHeader);
    std::terminate();
}

extern "C" {

//  Allocate a __cxa_exception object, and zero-fill it.
//  Reserve "thrown_size" bytes on the end for the user's exception
//  object. Zero-fill the object. If memory can't be allocated, call
//  std::terminate. Return a pointer to the memory to be used for the
//  user's exception object.
void * __cxa_allocate_exception (size_t thrown_size) throw() {
    size_t actual_size = object_size_from_exception_size(thrown_size);
    void *ptr = do_malloc(actual_size);
    if (NULL == ptr)
        std::terminate();
    std::memset(ptr, 0, actual_size);
    return thrown_object_from_exception(ptr);
}


//  Free a __cxa_exception object allocated with __cxa_allocate_exception.
void __cxa_free_exception (void * thrown_exception) throw() {
    do_free(exception_from_thrown_object(thrown_exception));
}


//  This function shall allocate a __cxa_dependent_exception and
//  return a pointer to it. (Really to the object, not past its' end).
//  Otherwise, it will work like __cxa_allocate_exception.
void * __cxa_allocate_dependent_exception () throw() {
    size_t actual_size = sizeof(__cxa_dependent_exception);
    void *ptr = do_malloc(actual_size);
    if (NULL == ptr)
        std::terminate();
    std::memset(ptr, 0, actual_size);
//  bookkeeping here ?
    return ptr;
}


//  This function shall free a dependent_exception.
//  It does not affect the reference count of the primary exception.
void __cxa_free_dependent_exception (void * dependent_exception) throw() {
//  I'm pretty sure there's no bookkeeping here
    do_free(dependent_exception);
}


// 2.4.3 Throwing the Exception Object
/*
After constructing the exception object with the throw argument value,
the generated code calls the __cxa_throw runtime library routine. This
routine never returns.

The __cxa_throw routine will do the following:

* Obtain the __cxa_exception header from the thrown exception object address,
which can be computed as follows: 
 __cxa_exception *header = ((__cxa_exception *) thrown_exception - 1); 
* Save the current unexpected_handler and terminate_handler in the __cxa_exception header.
* Save the tinfo and dest arguments in the __cxa_exception header. 
* Set the exception_class field in the unwind header. This is a 64-bit value
representing the ASCII string "XXXXC++\0", where "XXXX" is a
vendor-dependent string. That is, for implementations conforming to this
ABI, the low-order 4 bytes of this 64-bit value will be "C++\0".
* Increment the uncaught_exception flag. 
* Call _Unwind_RaiseException in the system unwind library, Its argument is the
pointer to the thrown exception, which __cxa_throw itself received as an argument.
__Unwind_RaiseException begins the process of stack unwinding, described
in Section 2.5. In special cases, such as an inability to find a
handler, _Unwind_RaiseException may return. In that case, __cxa_throw
will call terminate, assuming that there was no handler for the
exception.
*/
LIBCXXABI_NORETURN void 
__cxa_throw(void * thrown_exception, std::type_info * tinfo, void (*dest)(void *)) {
    __cxa_eh_globals *globals = __cxa_get_globals();
    __cxa_exception *exception = exception_from_thrown_object(thrown_exception);
    
    exception->unexpectedHandler = std::get_unexpected();
    exception->terminateHandler  = std::get_terminate();
    exception->exceptionType = tinfo;
    exception->exceptionDestructor = dest;
    setExceptionClass(&exception->unwindHeader);
    exception->referenceCount = 1;  // This is a newly allocated exception, no need for thread safety.
    globals->uncaughtExceptions += 1;   // Not atomically, since globals are thread-local

    exception->unwindHeader.exception_cleanup = exception_cleanup_func;
    _Unwind_RaiseException(&exception->unwindHeader);
    
//  If we get here, some kind of unwinding error has occurred.
    failed_throw(exception);
}


// 2.5.3 Exception Handlers
extern void * __cxa_get_exception_ptr(void * exceptionObject) throw() {
    return exception_from_exception_object(exceptionObject);
}
    

/*
This routine:
* Increment's the exception's handler count.
* Places the exception on the stack of currently-caught exceptions if it is not 
  already there, linking the exception to the previous top of the stack.
* Decrements the uncaught_exception count.
* Returns the adjusted pointer to the exception object.
*/
void * __cxa_begin_catch(void * exceptionObject) throw() {
    __cxa_eh_globals *globals = __cxa_get_globals();
    __cxa_exception *exception = exception_from_exception_object(exceptionObject);

//  TODO add stuff for dependent exceptions.

//  TODO - should this be atomic?
//  Increment the handler count, removing the flag about being rethrown
//  assert(exception->handlerCount != 0);
    exception->handlerCount = exception->handlerCount < 0 ?
        -exception->handlerCount + 1 : exception->handlerCount + 1;

//  place the exception on the top of the stack if it's not there.
    if (exception != globals->caughtExceptions) {
        exception->nextException = globals->caughtExceptions;
        globals->caughtExceptions = exception;
    }
        
    globals->uncaughtExceptions -= 1;   // Not atomically, since globals are thread-local
    return thrown_object_from_exception(exception);
}


/*
Upon exit for any reason, a handler must call:
    void __cxa_end_catch ();

This routine:
* Locates the most recently caught exception and decrements its handler count.
* Removes the exception from the caught exception stack, if the handler count goes to zero.
* Destroys the exception if the handler count goes to zero, and the exception was not re-thrown by throw.
*/
void __cxa_end_catch() {
    __cxa_eh_globals *globals = __cxa_get_globals();
    __cxa_exception *current_exception = globals->caughtExceptions;
    
    if (NULL != current_exception) {
        if (current_exception->handlerCount < 0) {
        //  The exception has been rethrown
            if (0 == incrementHandlerCount(current_exception)) {
                globals->caughtExceptions = current_exception->nextException;
            //	Howard says: If the exception has been rethrown, don't destroy.
        	}
        }
        else {
            if (0 == decrementHandlerCount(current_exception)) {
            //  Remove from the chain of uncaught exceptions
                globals->caughtExceptions = current_exception->nextException;
                if (!isDependentException(&current_exception->unwindHeader))
                    _Unwind_DeleteException(&current_exception->unwindHeader);
                else {
                //  TODO: deal with a dependent exception
                }
            }
        }       
    }
}


std::type_info * __cxa_current_exception_type() {
//  get the current exception
    __cxa_eh_globals *globals = __cxa_get_globals();
    __cxa_exception *current_exception = globals->caughtExceptions;
    if (NULL == current_exception)
        return NULL;        //  No current exception
//  TODO add stuff for dependent exceptions.
    return current_exception->exceptionType;
}

// 2.5.4 Rethrowing Exceptions
/*  This routine 
* marks the exception object on top of the caughtExceptions stack 
  (in an implementation-defined way) as being rethrown. 
* If the caughtExceptions stack is empty, it calls terminate() 
  (see [C++FDIS] [except.throw], 15.1.8). 
* It then returns to the handler that called it, which must call 
  __cxa_end_catch(), perform any necessary cleanup, and finally 
  call _Unwind_Resume() to continue unwinding.
*/
extern LIBCXXABI_NORETURN void __cxa_rethrow() {
    __cxa_eh_globals *globals = __cxa_get_globals();
    __cxa_exception *exception = exception_from_exception_object(globals->caughtExceptions );

    if (NULL == exception)   // there's no current exception!
        std::terminate ();

//  Mark the exception as being rethrown
    exception->handlerCount = -exception->handlerCount ;	// TODO: Atomic
    
#if __arm__
    (void) _Unwind_SjLj_Resume_or_Rethrow(&exception->unwindHeader);
#else
    (void) _Unwind_Resume_or_Rethrow     (&exception->unwindHeader);
#endif

//  If we get here, some kind of unwinding error has occurred.
    failed_throw(exception);
}

}  // extern "C"

}  // abi