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'datapath' Dialect Rationale
This document outlines the rationale of the datapath dialect, a dialect used for the efficient construction of arithmetic circuits.
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Introduction to the datapath dialect
Datapath design refers to the process of lowering arithmetic circuits to gate-level designs. When lowering arithmetic operations such as comb.add and comb.mul to gate-level implementations there are many component-level implementations to choose from. At the core of efficient gate-level implementations are a collection of circuit constructs that are composed to perform an arithmetic
operation on bitvectors. For example, in computer arithmetic a comb.mul would be comprised of three steps:
- Partial Product generation
- Compressor tree reduction to carry-save format
- Carry-propagate adder
Decomposing arithmetic operators into their efficient sub-circuits creates opportunities to optimise and fuse arithmetic operations together.
Reto Zimmerman's work provides a good overview of some of the optimisations
that the datapath dialect can expose. The core idea is to avoid inserting expensive carry-propagate adders and to leverage carry-save representations.
The datapath operations themselves should be thought of more as generators than concretelty specified mathematical operations.
Intended Usage
The datapath dialect is designed to be used as part of a synthesis flow, lowering a subset of comb operations to datapath operations. Optimisation passes on the datapath dialect can merge operations to form larger datapath blocks, where carry-propagation is deferred. The datapath operations can then be lowered into a subset of comb, consisting of only gate-level operations.
Example
In a simple example, we can fold a*b+c using the datapath dialect to remove a carry-propagate adder:
%0 = comb.mul %a, %b : i4
%1 = comb.add %0, %c : i4
Which is equivalent to:
%0:4 = datapath.partial_product %a, %b : (i4, i4) -> (i4, i4, i4, i4)
%1:2 = datapath.compress %0#0, %0#1, %0#2, %0#3, %c : i4 [5 -> 2]
%2 = comb.add %1#0, %1#1 : i4