Block rams not inferred with Vivado 2015.2 with user defined types

[adamwalker]

Vivado 2015.2 fails to infer block ram when using the blockRamPow2 function when the type of element being stored is a user defined type. See:

https://gist.github.com/adamwalker/0b39f86bdb4925e918cf
Specifically, in the first case synthesis (in Vivado) runs forever which I presume is because it is synthesizing the entire array.

Perhaps it is not the intention to allow arbitrary types to be stored in block ram, but it is allowed by the type system and I couldn't find any documentation stating that you shouldn't do that.

Thanks
Adam

[christiaanb]

It is supposed to work for arbitrary types, and I'm quite sure it works with Altera's Quartus tool. Still, I want to make sure it will work with Xilinx' tools also. I know how to fix this: I have to update the code-generator to only store bitvectors (std_logic_vector in VHDL) in blockRAMs.

Anyhow, as a work-around, I see two options:

• Use seperate blockRam's for each field of your tuple
• Have CLaSH generate Verilog instead of VHDL (using the :verilog command in interactive mode, or the --verilog flag in batch mode): in Verilog we only have bitvector basically, so it should work.

[adamwalker]

I wouldn't be suprised if it works with Xilinx ISE. From what I understand, Vivado is a complete rewrite of ISE that is still lacking features and has bugs that do not exist in ISE.

[ggreif]

@adamwalker If you find out that is a vivado bug, please make sure that it gets reported! Thanks!

[adamwalker]

Haha. I'm guessing you haven't tried to report a bug to Xilinx before. It will be ignored :)

[ggreif]

I am just running synthesis with Vivado 2015.3. It does seem to loop. Simulation was fast, though.

[christiaanb]

I just tried with Altera Quartus Prime 15.1.0, I can go as high as a 2^14 number of elements in the blockRam (e.g. a write and read addres of Unsigned 14). Higher than that, and Quartus reports it ran out of memory. Indeed, for Unsigned 14 it was already using 6 GB (on my 8 GB machine).

The VHDL that I'm generating is almost an exact copy of both the Xilinx and Altera recommended VHDL for blockRam inference. I guess for blockRams with really large address spaces, you are forced to use something like Xilinx' Coregen.

I don't think there's an easy fix for the out-of-memory problem. Aside from generating Xilinx/Altera specific blockRam VHDL, instead of the current vendor-independent VHDL.