Altera Nios II C2H Compiler Uživatelský manuál stáhnout pdf (Strana 59)

Altera Corporation 9.1 3–19

November 2009 Nios II C2H Compiler User Guide

C-to-Hardware Mapping Reference

Example 3–12 demonstrates another case of non equivalent pointers.

Example 3–12 is similar to Example 3–10, but a value is written to address

some_other_pointer between the reads from address

(packed_data + i).

Example 3–12. Nonequivalent Pointers Due to Potential Aliasing

void nonequivalent_pointers(char *packed_data,

int *some_other_pointer,

int len)

{

int i = 0;

while (i < len)

{

char ms_nibble = *(packed_data + i) >> 4;

char ls_nibble;

*some_other_pointer = i;

ls_nibble = *(packed_data + i) & 0x0f;

...

}

In this code, the C2H Compiler cannot determine if

some_other_pointer and packed_data overlap addresses (known

as aliasing), which would affect the result of the second evaluation of

*(packed_data + i). Therefore, the C2H Compiler creates a separate

master port for each dereference, creating a total of three master ports. For

details on how to inform the C2H Compiler that two pointers do not alias,

see section “Pointer Aliasing” on page 3–32.

Volatile Type Qualifier

The C2H Compiler does not consolidate or optimize pointers for

dereferenced types that use the volatile type qualifier. The volatile

type qualifier forces the variable to be evaluated strictly according to the

rules of the language. volatile is normally used to access non-memory

peripherals, such as timers and communication devices.

1 The volatile type qualifier overrides the __restrict__

pointer qualifier. For further information, see “Pointer Aliasing”

on page 3–32.

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Altera Nios II C2H Compiler Uživatelský manuál Strana 59