| # Comparing ASAP7 and sky130hd with respect to ALU operations | |
| Subcategory: Usage question | |
| ## Conversation | |
| ### oharboe | |
| The [mock-alu](https://github.com/The-OpenROAD-Project/OpenROAD-flow-scripts/tree/master/flow/designs/src/mock-alu) allows studying the speed and area of various typical ALU operations. | |
| The mock-alu has two 64 bit inputs and one 64 bit output. The inputs and outputs are registered within the mock-alu. This is to have clear boundary conditions when studying the guts of the mock-alu. | |
| The mock-alu implements a number of operations and variants of these operations to study the area and minimum clock period for these operations. | |
| The operations fall into a couple of categories: | |
| - *add, subtract and compare*: ADD, SUB, SETCC_EQ(equal), SETCC_NE (not equal), SETCC_LT (less than), SETT_LE (less than or equal), SETCC_ULT (unsigned less than), SETCC_ULE(unsigned less than or equal) | |
| - *barrel shifter*: SHR(logical shift right), SRA(arithmetic shift right), SHL(shift left) | |
| - *bitwise logic*: OR(bitwise or), AND(bitwise and), XOR(bitwise xor) | |
| - *multiplication*: 64 bit multiply. There are various algorithms used, default Han Carlson. The implementation is PDK specific and comes from https://github.com/antonblanchard/vlsiffra/ | |
| - *multiplexor*: MUX1..8. This is not really an ALU operation. All that is happening here is that bits from the input as selected using a mux and put into the output. This allows studying the performance of the mux that sits before the output of an ALU, which is selecting between the various supported operations. | |
| Next, the mock-alu allows implementing any combination of these operations. This allows implementing an ALU that only supports the shift operations, which can be labelled "SHR,SHL,SRA". This shift operation only mock-alu has a single shared barrelshifter. Similarly, a bitwise logic only mock-alu, can be labelled "OR,AND,XOR". | |
| At this point, we can plot various mock-alu implementations for ASAP7 and sky130hd: | |
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| Here 8,16,32 and 64 wide ADD operations are plotted: | |
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| Various multiplication algorithms for 64 bit multiplication, 4 pipeline stages: | |
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| Plotting Han Carlson multiply algorithm with 8, 16, 32 and 64 bit bit width: | |
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| Thoughts? | |
| ### maliberty | |
| Its interesting that the various multiply algorithms have the same delay (excluding ripple). | |
| I don't see a MULT for sky130 | |
| ### oharboe | |
| Here are the `vlsi-adder` algorith, excluding ripple that was ca. 2000+ ps. It would have made the graph harder to read: | |
|  | |
| ### maliberty | |
| Do all your runs end in negative slack at the end of the cts & the complete flow? Once we reach zero slack we stop optimizing. | |
| ### maliberty | |
| One thing worth noting is that asap7 doesn't have a full or half adder cells in the library (sky130 does). | |
| ### mithro | |
| BTW Have you seen Teo's spreadsheet @ https://docs.google.com/spreadsheets/d/1pTGzZo5XYU7iuUryxorfzJwNuE9rM3le5t44wmLohy4/edit#gid=126548956 ? | |
| ### mithro | |
| BTW I would love to get a similar spreadsheet to Teo's for GF180MCU and ASAP7 too. | |
| Sadly, Teo got distracted by the mathematical theory and then was stolen by NVIDIA before he could get to that. | |
| ### tspyrou | |
| @oharboe did you try using set_clock_uncertainty to force the slack to be negative and make the tool work harder. | |
| As @maliberty mentioned optimization will stop once timing passes. | |
| ### mithro | |
| @oharboe - Any chance you could do a write up of what you discovered? | |
| ### oharboe | |
| > There is a lot bunch of back and forth and I'm unclear what the final results are (and exactly how you produced them). | |
| > | |
| > Various questions include; | |
| > | |
| > * How do I reproduce your results and graphs? | |
| Run this script: https://github.com/The-OpenROAD-Project/OpenROAD-flow-scripts/blob/master/flow/designs/src/mock-alu/plot-area-min-clock-period.py | |
| Some tinkering required. | |
| > * What settings did you end up using and why did you end up using those settings? (Particularly around making the tool work harder?) | |
| I didn't study how to make a best possible ALU, I was only interested the relationship between the ALU operations. | |
| > * How do the various implementations compare? Do you understand why the compare this way? | |
| The various implementation of additions and multiplications? | |
| It is a mystery why there is essentially no difference between multiplication implementation clock periods... | |
| > * How do SKY130 and ASAP7 compare in the end here? | |
| The lessons learned on the relative size and speed of ALU operations appear to be much the same with SKY130 and ASAP7. Which is surprising. Ca. 15 years separate them... | |
| > * Do the relative "positions" between the implementations hold across SKY130 and ASAP7? | |
| Pretty much. | |
| > * What was the most interesting / unexpected thing you discovered? | |
| That relative size and speed of simple ALU operations are essentially unchanged across process nodes. | |
| Also, it would appear that if an ALU operation is 200ps on x86 7nm, yielding 5GHz, then one could choose to divide clock period of ASAP7+OpenROAD by 4 for simple ALU operations when one models and decide to take the lessons learned and apply them to architectural exploration. Further choose to treat the speed of ASAP7+OpenROAD as not terribly important in terms of making architctural choices as the choices will be the same if everything is optimized. | |
| By this I mean that to drive your architctural exploration, as a [first order approximation](https://en.wikipedia.org/wiki/Order_of_approximation#First-order), write the RTL in an idiomatic way, run them through ASAP7+OpenROAD and if your design is 4x the desired clock period, your design isn't completely off. | |
| Nobody who have information on commercial tools and PDKs can challenge me here. :-) Not because I'm right, but because PDKs and commercial tools are under strict NDAs... This also explains why there are a lot of unsaid things in this thread... | |
| > Writing it up as a nice coherent blog post would be pretty awesome but totally understand if you do nt have the time to do so. | |
| Agreed. At least I summarize a bit here. I'm happy to hear that there are some that are interested in this. | |
| Perhaps you would like to write a blog-post using the script above? | |