I’ve spent these last couple of days to perform head-to-head comparisons of Xilinx Vivado HLS against HercuLeS on HLS-generated digital circuits (from input C code).

I believe that HercuLeS lived up to the challenge; it is competitive to Vivado HLS. The reader should take account that:

1. Both tools have been used (almost) out-of-the-box. Vivado HLS was configured with no bufg inclusion, and in “out_of_context” mode. These mean that no clock buffers and I/O pins were routed.
2. HercuLeS does not (yet) customize the generated HDL in order to fit better specific architectural features (DSP blocks, embedded SRL units).
3. Vivado HLS had some TOTAL FAILURES on some relatively simple codes such as a simple perfect number detector (positive integers equal to the sum of their divisors), a 1D wavelet code, and easter date calculation. It seems that Vivado HLS experiences some hard time with integer modulo/remainder. Codes are provided to anyone interested.

The following table provides a summary of the results:

NOTES:

• Measurements where obtained for the KC705 development board device: xc7k325t-ffg900-2
• TET is Total Execution Time in ns.
• VHLS is a shortened form for Vivado HLS.
• Vivado HLS 2013.1 was used.
• Bold denotes smaller area and lower execution time.
• Italic denotes an inconclusive comparison.
• For the cases of edgedet and sieve, VHLS identifies a BRAM; HercuLeS does not. In these cases, HercuLeS saves a BRAM while VHLS saves on LUTs and FFs (Registers).

Overall, there are about 30% wins for HercuLeS and ~70% wins for Vivado HLS. Not too bad for a tool like HercuLeS; producing generic, portable, vendor-independent code. I estimate that HercuLeS development effort is around 1-5% to Vivado HLS.

I believe that HercuLeS will do much better in the out-of-the-box experience (which is of high importance in order to draw more software-minded engineers in the game) in the near future.

Both HercuLeS and Vivado HLS have optimization features (e.g. loop unrolling). HercuLeS applies optimizations by using a source-to-source C code optimizer. Vivado HLS mostly resorts to end-user directives. These coding aspects will be taken into account in a followup comparison; they also yield a much more extensive solution space.