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C compiler

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Doc ID 018512 Rev 2

12.3.8

The __builtin_constant_p() operator

When the GCC extensions setting is on, the compiler recognizes the

__builtin_constant_p()

operator. This compile-time operator takes a single argument

and returns 1 if the argument is a constant expression or 0 if it is not.

12.3.9

Forward declarations of static arrays

When the GCC extensions setting is on, the compiler will not issue an error when you

declare a static array without specifying the number of elements in the array if you later

declare the array completely.Example 43 shows an example.

Example 43 Forward declaration of an empty array

static int a[]; /* Allowed only when GCC extensions are on. */

/* ... */

static int a[10]; /* Complete declaration. */

12.3.10

Omitted operands in conditional expressions

When the GCC extensions setting is on, you may skip the second expression in a

conditional expression. The default value for this expression is the first expression.Example

44 shows an example.

Example 44 Using the shorter form of the conditional expression

void f(int i, int j)

{

int a = i ? i : j;

int b = i ?: j; /* Equivalent to int b = i ? i : j; */

/* Variables a and b are both assigned the same value. */

}

12.3.11

The __builtin_expect() operator

When the GCC extensions setting is on, the compiler recognizes the

__builtin_expect()

operator. Use this compile-time operator in an if or while

statement to specify to the compiler how to generate instructions for branch prediction.

This compile-time operator takes two arguments:

●

The first argument must be an integral expression

●

The second argument must be a literal value

The second argument is the most likely result of the first argument.Example 45 shows an

example.

Example 45 Example for __builtin_expect() operator

void search(int *array, int size, int key)

{

int i;

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