CAPI2: Variables

[ArkoshEternal]

CAPI2 Variables

Introducing flag-like variables feature discussed originally in #675

High Level Feature Description:

Add the ability to assign CAPI2 entries to a variable, denoted by a string which starts with $
Globally resolve variables either from defaults set in a target, or from CLI arguments

Change Details:

• Add variables section to target which allows for the definition of int, string, or bool variables which can be defined as key-value pairs
• Add support for new meta-character, $, which can be used in a manner similar to bash variables wherever a flag ternary operation can currently be used
• Add new cli for fusesoc run, --var key=value , which can supplement target held variables
• Add testing to ensure functionality of the variables infrastructure
• Add variables to build_system documentation

Design Decisions

• Add variables as coequal to flags instead of as redesigning flags as a data class to prevent any major re-writes of the flags system, although unifying is appealing at a later time
• Intentionally did not fuse them with parameters because of parameters isolation to use in the backend rather than the front end

Pain Points Solved

• ** Allows for upstream control of downstream generators**
• Allows for less complex flag ternary trees downstream
• Simplifies duplicated tool options (e.g. same command passed to vlogan/vhdlan and vcs in vcs simulations)
• Gives end users more flexibility in implementing their vision

Considered Extensions

• Allow for inline variable defaults (e.g. my_key: $my_value:=<default_value>)

The implementation was intentionally simple, happy to iterate on it more if there's specific pain points that can be solved in this change.