Altera Nios II C2H Compiler - Manual (Page 107)

Altera Corporation 9.1 4–13
November 2009 Nios II C2H Compiler User Guide
Understanding the C2H View
Figure 4–5. CRC Dependency Graph
Since the critical path does not involve the pointer data the only
operations are on local scalar types and a read operation from the array
table . The calculation of lut_addr only depends on the scalar critical
loop variable crc , while the calculation of crc depends on a memory
reference to critical loop array variable table .
Each time the C2H accelerator finishes calculating the value of crc for
loop iteration n, it can start calculating crc for iteration n+1. On the same
clock cycle, it can also start calculating the value of lut_addr for
iteration n+2. This means that the accelerator always gets a one-loop head
start on calculating
lut_addr . Thus, although the accelerator requires
lut_addr to calculate crc , lut_addr does not limit the loop speed,
because it is always ready as soon as crc is.
The report shows that the critical path is either 0--->6 , or 6--->11 .
Since the C2H Compiler pipelines the logic contained in loops, multiple
states are active concurrently.
Figure 4–6 represents the pipelined timing
of Example 4–2 on page 4–10 . Notice that since the assignment of crc is
the critical path, the accelerator begins each execution of that statement as
crc=(crc>>8)^ table [lut_addr];
crc=(crc>>8)^table[ lut_addr ];
lut_addr=( crc &0xFF)^*data++; lut_addr=(crc&0xFF)^ *data++ ;