Although these options can be thought of as optimizations, they are
essentially orthogonal to the core of -O, which is managing signal
buffering and scope. Going from -O4 to -O2 means going from limited
to complete design visibility, yet in both cases proc and flatten
are desirable.
Before this commit, Verilog expressions like `x && 1` would result in
references to `logic_and_us` in generated CXXRTL code, which would
not compile. After this commit, since cells like that actually behave
the same regardless of signedness attributes, the signedness is
ignored, which also reduces the template instantiation pressure.
Constant wires can represent a significant chunk of the design in
generic designs or after optimization. Emitting them in VCD files
significantly improves usability because gtkwave removes all traces
that are not present in the VCD file after reload, and iterative
development suffers if switching a varying signal to a constant
disrupts the workflow.
Alias wires can represent a significant chunk of the design in highly
hierarchical designs; in Minerva SRAM, there are 273 member wires and
527 alias wires. Showing them in every hierarchy level significantly
improves usability.
whitequark