| # Comparing ASAP7 and sky130hd with respect to ALU operations |
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| Subcategory: Usage question |
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| ## Conversation |
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| ### 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.
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| 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.
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| The mock-alu implements a number of operations and variants of these operations to study the area and minimum clock period for these operations.
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| The operations fall into a couple of categories:
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| - *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)
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| - *barrel shifter*: SHR(logical shift right), SRA(arithmetic shift right), SHL(shift left)
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| - *bitwise logic*: OR(bitwise or), AND(bitwise and), XOR(bitwise xor)
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| - *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/
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| - *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.
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| 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".
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| 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? |
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| ### maliberty |
| Its interesting that the various multiply algorithms have the same delay (excluding ripple).
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| I don't see a MULT for sky130 |
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| ### oharboe |
| Here are the `vlsi-adder` algorith, excluding ripple that was ca. 2000+ ps. It would have made the graph harder to read:
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| ### 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. |
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| ### maliberty |
| One thing worth noting is that asap7 doesn't have a full or half adder cells in the library (sky130 does). |
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| ### mithro |
| BTW Have you seen Teo's spreadsheet @ https://docs.google.com/spreadsheets/d/1pTGzZo5XYU7iuUryxorfzJwNuE9rM3le5t44wmLohy4/edit#gid=126548956 ? |
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| ### mithro |
| BTW I would love to get a similar spreadsheet to Teo's for GF180MCU and ASAP7 too.
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| Sadly, Teo got distracted by the mathematical theory and then was stolen by NVIDIA before he could get to that. |
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| ### tspyrou |
| @oharboe did you try using set_clock_uncertainty to force the slack to be negative and make the tool work harder.
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| As @maliberty mentioned optimization will stop once timing passes. |
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| ### mithro |
| @oharboe - Any chance you could do a write up of what you discovered? |
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| ### 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).
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| > Various questions include;
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| > * How do I reproduce your results and graphs?
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| 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
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| Some tinkering required.
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| > * What settings did you end up using and why did you end up using those settings? (Particularly around making the tool work harder?)
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| I didn't study how to make a best possible ALU, I was only interested the relationship between the ALU operations.
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| > * How do the various implementations compare? Do you understand why the compare this way?
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| The various implementation of additions and multiplications?
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| It is a mystery why there is essentially no difference between multiplication implementation clock periods...
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| > * How do SKY130 and ASAP7 compare in the end here?
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| 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...
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| > * Do the relative "positions" between the implementations hold across SKY130 and ASAP7?
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| Pretty much.
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| > * What was the most interesting / unexpected thing you discovered?
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| That relative size and speed of simple ALU operations are essentially unchanged across process nodes.
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| 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.
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| 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.
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| 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...
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| > 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.
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| Agreed. At least I summarize a bit here. I'm happy to hear that there are some that are interested in this.
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| Perhaps you would like to write a blog-post using the script above?
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