[GitHub] [incubator-tvm-vta] remotego commented on pull request #8: [Hardware][Xilinx] explicitly specify acc dep distance to avoid hidden pitfall

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remotego commented on pull request #8:
URL: https://github.com/apache/incubator-tvm-vta/pull/8#issuecomment-621593259


   > Thanks @zhanghaohit, for catching this error. I agree with the fact that the dependence should not be hardcoded. However in order to not to add too many parameters in the config file, can we derive the value from the VTA target (i.e. FPGA type) in `pkg_config`?
   > 
   > Also from what I understand is that the dependence distance is generally a property of the memory part that the FPGA is using. For instance that distance will be different if one uses BRAM vs. Ultra-RAM? And on a different FPGA family this number will change. Correct?
   
   Thanks @tmoreau89 for the advice. We also thought of putting the value into the pkg_config. However, later we found out the dependence distance is not tightly related to the device family/type.
   
   In my opinion, the compiler will decide the II based on multiple factors. But eventually, it is based on the number of cycles on the datapath from "Read Mem" -> "Perform Ops" -> "Write Back", as we need to avoid RAW data hazard.
   
   In the "Read Mem" and "Write Back" stage, it is definitely related to the properties of the memory (eg. device family, uram vs bram, etc). But it is also related to the H/W circuit of accessing the data. For example, multiplexers as there are multiple accesses. The compile will judge based on the complexity of the overall circuit, and it could add registers (increase II) if the desired frequency target could not be satisfied.
   
   In "Perform Ops" stage, the cycles needed is mainly related to the op itself. For example, a 32-bit multiplication may require more cycles than a 8-bit multiplication.


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