Altera Nios II C2H Compiler Instrukcja Użytkownika Pobierz pdf (Strona 74)

3–34 9.1 Altera Corporation

Nios II C2H Compiler User Guide November 2009

Scheduling

compile-time due to the possibility of aliasing. This dependency causes

the read operation from ptr_b to be scheduled at State 2, rather than at

State 0.

__restrict__ Pointer Type Qualifier to Break Dependencies

If you know that a pointer never overlaps with another, you can inform

the compiler by declaring the pointer to be a restricted pointer. The

restrict type qualifier is introduced in the ISO C 99 specification. The

compiler ignores any or all aliasing implications of a pointer qualified by

__restrict__. The C99 specification states that if a pointer “p” is

declared with restrict, and another pointer “q” accesses any location also

accessed by “p,” the behavior is undefined. In other words, a restricted

pointer promises to never alias another pointer.

Example 3–28 demonstrates several pointers declared using the

__restrict__ type qualifier.

1 The qualifier comes after the *; __restrict__ qualifies the

pointer type, not the type that the pointer points to.

Example 3–28. Pointer Declarations with __restrict__

int * __restrict__ my_restricted_pointer_to_integer;

const int * __restrict__ my_restricted_pointer_to_constant_integer;

int * const __restrict__ my_constant_restricted_pointer_to_integer;

Figure 3–15 shows the dependency graph for Example 3–29, which uses

the __restrict__ type qualifier to inform the C2H Compiler that

ptr_a and ptr_b do not alias:

Example 3–29. Using __restrict__

void foo(int * __restrict__ ptr_a,

int * __restrict__ ptr_b)

{

int a, b;

a = *ptr_a;

*ptr_a = a + 7;

b = *ptr_b;

*ptr_b = b + 8;

}

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Altera Nios II C2H Compiler Instrukcja Użytkownika Strona 74