Altera Nios II C2H Compiler Manual do Utilizador descarregar pdf (Página 73)

Altera Corporation 9.1 3–33

November 2009 Nios II C2H Compiler User Guide

C-to-Hardware Mapping Reference

operations that come before it. Because arrays and structures are

equivalent to pointer operations, the same considerations apply when

indexing into an array or structures.

This section describes the implications of aliasing on the C2H Compiler

and outlines methods to prevent unnecessary dependencies.

Figure 3–14 shows the dependency graph for Example 3–27.

Example 3–27. Pointer Aliasing

void foo(int *ptr_a, int *ptr_b)

{

int a, b;

a = *ptr_a;

*ptr_a = a + 7;

b = *ptr_b;

*ptr_b = b + 8;

}

Figure 3–14. Pointer-Related Data Dependency

In this example, the C2H Compiler cannot determine whether or not

ptr_a and ptr_b ever point to the same address. Therefore, it schedules

conservatively, under the assumption that they do. The dependency

graph shows that the read operation from ptr_b depends on the write

operation to ptr_a. This is not a dependency on the variable ptr_a, but

rather a dependency on a location in memory that is unknown at

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Altera Nios II C2H Compiler Manual do Utilizador Página 73