Higher-level: Behavioural or Declarative ?

There are two primary, high-level entry styles we can consider (in practice use a blend of them ?):

• Behavioural Expression: Using imperative software-like code, where threads have stacks and pass between modules, and so on...

• Declarative/Logical Expression: Constraining assertions about the allowable behaviour are given, but any ordering constraints are implicit (e.g. SQL queries).

There are also purely generative approaches, like the Lava and Chisel hardware construction languages (HCLs), that just 'print out' a circuit diagram. These cannot handle data-dependent IF statements. »Lava Compiler (Singh)

Using a parallel set of guarded atomic actions is pure RTL: which is declarative (since no threads).

(The core of Bluespec RTL is declarative and much nicer to use than pure RTL. It has a scheduler (cf DBMS query planner), a behavioural-sub language for when imperative is best, and an advanced generative elaborator that is functional language.)

All higher-level styles are amenabale to automatic datapath and schedule generation, including re-encoding and re-pipelining to meet timing closure and power budgets.

Classical HLS converts a behavioural thread to a static schedule (fixed at compile time).