Altera Nios II C2H Compiler Manual de usuario descargar pdf (Pagina 67)

Altera Corporation 9.1 3–27

November 2009 Nios II C2H Compiler User Guide

C-to-Hardware Mapping Reference

Although an array is considered to be a pointer type, dereferencing an

array variable does not always mean the same thing as dereferencing a

pointer. For example, dereferencing or indexing once into a

multidimensional array returns a pointer to the first element in another

array. For an N-dimensional array, a dereference or index into any of the

first (N-1) dimensions does not read a value from the array memory; it

computes an offset from the array's base address to determine the address

of a subsection of the array.

Example 3–18 demonstrates that indexing to the first level of a two-

dimensional array does not result in a memory access.

Example 3–18. Indexing a Multidimensional Array without Causing a Memory Access

char a[LENGTH][WIDTH]; // Here's a two-dimensional array

// The following assignments are all equivalent.

char *subscripting = a[3];

char *dereferencing = *(a + 3);

char *offset = (char *) (a + 3);

char *ptr_arithmetic = (char *) ((void *)a + 3*WIDTH);

Indexing into any of the first (N-1) dimensions of an N-dimensional array

requires a multiplication operation, as demonstrated by the evaluation of

ptr_arithmetic in Example 3–18. If the size of the resultant array is an

integer power of two, then the multiplication operation is reduced to a

constant-shift operation, which does not require a hardware multiplier.

(Refer to section “Unregistered Operations and Assignments” on

page 3–3.)

A series of subscript operations that index into all N dimensions of an N-

dimensional array is equivalent to an indirection operation, which creates

an Avalon-MM master port. Example 3–19 illustrates several cases that

generate an Avalon-MM master port to dereference an array variable.

Example 3–19. Indexing an Array and Causing a Memory Access

char a[LENGTH][WIDTH]; /* a is a two-dimensional array */

// The following assignments are equivalent.

char *subscripting_a = a[3][2];

char *dereferencing_a = *(*(a + 3) + 2);

char b[LENGTH]; /* b is a one-dimensional array */

// The following assignments are equivalent.

char *subscripting_b = b[1];

char *dereferencing_b = *(b + 1);

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Altera Nios II C2H Compiler Manual de usuario Pagina 67