This commit introduces the negopt pass with pre/post optimization modes
for handling negation patterns in arithmetic circuits.
Pre-optimization patterns (expose for tree balancing):
- manual2sub: (a + ~b) + 1 → a - b
- sub2neg: a - b → a + (-b)
- negexpand: -(a + b) → (-a) + (-b) [with output width fix]
- negneg: -(-a) → a
- negmux: -(s ? a : b) → s ? (-a) : (-b)
Post-optimization patterns (cleanup/rebuild):
- negrebuild: (-a) + (-b) → -(a + b)
- muxneg: s ? (-a) : (-b) → -(s ? a : b)
- neg2sub: a + (-b) → a - b
All patterns use nusers() for fanout checking (standard Yosys style).
Comprehensive test coverage with positive/negative cases and formal
verification via equiv_opt.
Akash Levy
Abhinav Tondapu