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AR# 62859

Vivado HLS - How to achieve PIPELINE II=1 of floating point accumulation?


How can I achieve PIPELINE II=1 of floating point accumulation?

I am using the code below:
#define FT float
FT ybd(FT din[1024]){
FT sum=0;
LOOP:for(int i=0;i<1024;i++){
return sum;

However HLS reports the following warning:

@W [SCHED-68] Unable to enforce a carried constraint (II = 9)
   between 'fadd' operation ('sum', ybd/ybd.cpp:7) and 'fadd' operation ('sum', ybd/ybd.cpp:7).


Because the latency of fadd in this case is 11 clock cycles, you will need to fill up and pipeline inside fadd so it does one addition every clock cycle.

XAPP599 covers this in detail.

Below is the modified code.

The II for the function is 1034:

#define FT float
#define FADD_LAT 11
FT ybd(FT din[1024]){
FT sum_p[FADD_LAT];
#pragma HLS ARRAY_PARTITION variable=sum_p complete dim=1
FT sum;

loop_init: for(int i=0;i<FADD_LAT;i++) {
#pragma HLS UNROLL
sum_p[i] = 0;
sum = 0;

LOOP:for(int i=0;i<1024;i+=FADD_LAT){
#pragma HLS PIPELINE II=11 rewind
for (int j=0; j<FADD_LAT; j++)

loop_sum_f: for (int k=0; k<FADD_LAT; k++)
#pragma HLS UNROLL
sum += sum_p[k];
return sum;

AR# 62859
Date 11/26/2014
Status Active
Type General Article
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