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The as-if rule

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Lenguaje C++
Los temas generales
Original:
General topics
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Control de flujo
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Flow control
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Sentencias condicionales ejecución
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Conditional execution statements
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Instrucciones de iteración
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Iteration statements
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Ir declaraciones
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Jump statements
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Funciones
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Functions
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declaración de la función
lambda declaración de la función
función de plantilla
especificador inline
especificaciones de excepción (obsoleto)
noexcept especificador (C++11)
Excepciones
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Exceptions
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Los espacios de nombres
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Namespaces
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Tipos
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Types
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decltype specifier (C++11)
Los especificadores
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Specifiers
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cv especificadores
almacenamiento duración especificadores
constexpr especificador (C++11)
auto especificador (C++11)
alignas especificador (C++11)
Inicialización
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Initialization
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Los literales
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Literals
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Expresiones
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Expressions
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representaciones alternativas
Utilidades
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Utilities
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Tipos
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Types
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typedef declaration
alias de tipo de declaración (C++11)
atributos (C++11)
Lanza
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Casts
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conversiones implícitas
const_cast conversion
static_cast conversion
dynamic_cast conversion
reinterpret_cast conversion
C-cast estilo y funcional
De asignación de memoria
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Memory allocation
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Clases
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Classes
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Clase propiedades específicas de la función
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Class-specific function properties
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Funciones especiales miembro
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Special member functions
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Plantillas
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Templates
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clase de plantilla
función de plantilla
plantilla de especialización
paquetes de parámetros (C++11)
Varios
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Miscellaneous
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Montaje en línea
 
Permite a cualquier y todas las transformaciones de código que no cambian el comportamiento observable del programa
Original:
Allows any and all code transformations that do not change the observable behavior of the program
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Contenido

[editar] Explicación

El compilador de C + + se permite realizar ningún cambio en el programa, siempre y cuando las siguientes es cierta:
Original:
The C++ compiler is permitted to perform any changes to the program as long as the following remains true:
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  • Accesos (lee y escribe) a los objetos volátiles se producen en el mismo orden escrita .
    Original:
    Accesses (reads and writes) to the volatile objects occur in the same order as written.
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  • Al terminar el programa, los datos escritos en todos los archivos es exactamente como si el programa se ejecuta como está escrito .
    Original:
    At program termination, the data written to all files is exactly as if the program was executed as written.
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  • Todas las operaciones de entrada y salida se producen en el mismo orden y con el mismo contenido que si el programa se ejecuta como está escrito .
    Original:
    All input and output operations occur in the same order and with the same content as if the program was executed as written.
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[editar] Notas

Debido a que el compilador es (generalmente) no puede analizar el código de una biblioteca externa para determinar si tiene o no realizar el acceso I / O o volátil, de terceros biblioteca pide, asimismo, no se ven afectados por la optimización. Sin embargo, las llamadas estándar de la biblioteca puede ser sustituido por otras llamadas, eliminado, o añadido al programa durante la optimización .
Original:
Because the compiler is (usually) unable to analyze the code of an external library to determine whether it does or does not perform I/O or volatile access, third-party library calls also aren't affected by optimization. However, standard library calls may be replaced by other calls, eliminated, or added to the program during optimization.
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Los programas con un comportamiento indefinido, por ejemplo, debido al acceso a una matriz fuera de los límites, la modificación de un objeto const, evaluación orden violaciónes, etc, están libres de la regla como-si: a menudo cambiar el comportamiento observable cuando vuelve a compilar con diferentes ajustes de optimización. Por ejemplo, si una prueba de desbordamiento entero con signo se basa en el resultado de desbordamiento que, por ejemplo, if(n+1 < n) abort();, it is removed entirely by some compilers porque desbordamiento con signo es un comportamiento indefinido y el optimizador es libre de asumir que nunca sucede y la prueba es redundante .
Original:
Programs with undefined behavior, e.g. due to access to an array out of bounds, modification of a const object, evaluación orden violations, etc, are free from the as-if rule: they often change observable behavior when recompiled with different optimization settings. For example, if a test for signed integer overflow relies on the result of that overflow, e.g. if(n+1 < n) abort();, it is removed entirely by some compilers because desbordamiento con signo es un comportamiento indefinido and the optimizer is free to assume it never happens and the test is redundant.
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Copia elisión es el único bien definido excepción a la regla como-si .
Original:
Copia elisión is the only well-defined exception from the as-if rule.
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[editar] Ejemplo

int& preinc(int& n) { return ++n; }
int add(int n, int m) { return n+m; }
 
// volatile input to prevent constant folding
volatile int input = 7;
 
// volatile output to make the result a visible side-effect
volatile int result;
 
int main()
{
    int n = input;
// using built-in operators would invoke undefined behavior
//    int m = ++n + ++n;
// but using functions makes sure the code executes as-if 
// the functions were not overlapped
    int m = add(preinc(n), preinc(n));
    result = m;
}

Output:

# full code of the main() function as produced by the GCC compiler
# x86 (Intel) platform:
        movl    input(%rip), %eax   # eax = input
        leal    3(%rax,%rax), %eax  # eax = 3 + eax + eax
        movl    %eax, result(%rip)  # result = eax
        xorl    %eax, %eax          # eax = 0 (the return value of main())
        ret
 
# PowerPC (IBM) platform:
        lwz 9,LC..1(2)
        li 3,0          # r3 = 0 (the return value of main())
        lwz 11,0(9)     # r11 = input;
        slwi 11,11,1    # r11 = r11 << 1;
        addi 0,11,3     # r0 = r11 + 3;
        stw 0,4(9)      # result = r0;
        blr
 
# Sparc (Sun) platform:
        sethi   %hi(result), %g2
        sethi   %hi(input), %g1
        mov     0, %o0                 # o0 = 0 (the return value of main)
        ld      [%g1+%lo(input)], %g1  # g1 = input
        add     %g1, %g1, %g1          # g1 = g1 + g1
        add     %g1, 3, %g1            # g1 = 3 + g1
        st      %g1, [%g2+%lo(result)] # result = g1
        jmp     %o7+8
        nop
 
# in all cases, the side effects of preinc() were eliminated, and the
# entire main() function was reduced to the equivalent of result = 2*input + 3;

[editar] Ver también