expression

Any expression that is assumed to evaluate to true.

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.

__assume is not a true intrinsic. It does not need to be declared as a function and cannot be used in a #pragma intrinsic directive. No code is generated, although it affects the code generated by the optimizer.

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 might optimize away the error handling code.

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

// compiler_intrinsics__assume.cpp
#ifdef DEBUG
# 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.
   }
}

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 might 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:

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

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

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

   default:
      NODEFAULT;

When the following code example is compiled with the /Og compiler option, the identifier x is assumed to have a value of 1 wherever it appears after the __assume statement. The preincrement operation in the __assume statement does not actually occur and does not affect the value of x. The assumed value is printed and returned.

When the example is compiled with the /Od compiler option, the default, no assumptions are made about the value of x and its actual value is printed and returned.

#include <stdio.h>
int main()
{
    int x=100;
    __assume( (++x) == 1 );
    printf("%d", x);
    return x;
}

This example yields the following results:

Reference