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__assume

Microsoft Specific

The __assume compiler intrinsic passes a hint to the optimizer. The optimizer assumes that the condition represented by expression is true at the point where the keyword appears and remains true until expression is altered (for example, by assignment to a variable). Selective use of hints passed to the optimizer by __assume can improve optimization.

Use __assume in an ASSERT only when the assert is not recoverable. __assume should not be used in an assert for which you have subsequent error recovery code; the compiler may optimize away the error handling code.

__assume(expression)

The most common use of __assume is with the default case of a switch statement, as shown below.

Example

// compiler_intrinsics__assume.cpp
#ifdef DEGUG
# define ASSERT(e)    ( ((e) || assert(__FILE__, __LINE__) )
#else
# define ASSERT(e)    ( __assume(e) )
#endif

void func1(int i)
{
}

int main(int p)
{
   switch(p){
      case 1:
         func1(1);
         break;
      case 2:
         func1(-1);
         break;
      default:
         __assume(0);
            // This tells the optimizer that the default
            // cannot be reached. As so it does not have to generate
            // the extra code to check that 'p' has a value 
            // not represented by a case arm. This makes the switch 
            // run faster.
   }
}

Remarks

The use of __assume(0) tells the optimizer that the default case cannot be reached. As a result, the compiler does not generate code to test whether p has a value not represented in a case statement. Note that __assume(0) must be the first statement in the body of the default case for this to work.

Because the compiler generates code based on __assume, that code may not run correctly if the expression inside the __assume statement is false at run time. If you are not sure that the expression will always be true at run time, you can use the assert function to protect the code:

Example

# define ASSERT(e)    ( ((e) || assert(__FILE__, __LINE__)), __assume(e) )

Unfortunately, this use of assert prevents the compiler from performing the default-case optimization shown above. Therefore, you may want to use a separate macro instead:

#ifdef DEBUG
# define NODEFAULT   ASSERT(0)
#else
# define NODEFAULT   __assume(0)
#endif

   default:
      NODEFAULT;

END Microsoft Specific

See Also

Compiler Intrinsics | C++ Keywords

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