Altera Nios II C2H Compiler Manuel d'utilisateur télécharger pdf (Page 43)

Altera Corporation 9.1 3–3

November 2009 Nios II C2H Compiler User Guide

C-to-Hardware Mapping Reference

There are two types of exceptions to this rule:

■ Assignments that require zero logic elements in hardware

■ Assignments that use multiple registers to pipeline complex

arithmetic operations

The following sections discuss these exceptions.

Unregistered Operations and Assignments

Certain logical and bit-wise operations involving constants are trivial and

require no logic. In hardware, they are performed simply by

manipulating wires. Table 3–2 lists the applicable operators and

conditions. If an assignment consists solely of such operations, then its

result is not registered.

The following assignment is an example of a zero logic-element

operation.

int masked_data = data_in & 0x000fffff;

The C2H Compiler generates no register for the variable masked_data,

because its value is represented simply by concatenating 12 bits of zeroes

with the lower 20 bits of data_in.

Additional examples of unregistered assignments:

shift_by_constant = data_in << 3;

or_with_constant = data_in | 0xf0f0f0f0;

invert_shift_and_consts = (~data_in & 0xff) << 8;

Table 3–2. Operators That Can Result in Unregistered Assignments

Operator Description Required Condition

Right bit-wise shift Right-hand side is constant

Left bit-wise shift Right-hand side is constant

bit-wise AND Either operand is constant

bit-wise inclusive OR Either operand is constant

bit-wise exclusive OR Either operand is constant

bit-wise inversion Right-hand side is unregistered

)

Type cast Right-hand side is unregistered

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Altera Nios II C2H Compiler Manuel d'utilisateur Page 43