smtbmc: Add native json based witness format + smt2 backend support

This adds a native json based witness trace format. By having a common
format that includes everything we support, and providing a conversion
utility (yosys-witness) we no longer need to implement every format for
every tool that deals with witness traces, avoiding a quadratic
opportunity to introduce subtle bugs.

Included:

  * smt2: New yosys-smt2-witness info lines containing full hierarchical
    paths without lossy escaping.
  * yosys-smtbmc --dump-yw trace.yw: Dump results in the new format.
  * yosys-smtbmc --yw trace.yw: Read new format as constraints.
  * yosys-witness: New tool to convert witness formats.
    Currently this can only display traces in a human-readable-only
    format and do a passthrough read/write of the new format.
  * ywio.py: Small python lib for reading and writing the new format.
    Used by yosys-smtbmc and yosys-witness to avoid duplication.

1 files changed, 4 insertions, 0 deletions

diff --git a/CHANGELOG b/CHANGELOG
index 5ee3e3cc0..cf2a46bd3 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -11,6 +11,10 @@ Yosys 0.20 .. Yosys 0.20-dev
  * Formal Verification
     - Added $anyinit cell to directly represent FFs with an unconstrained
       initialization value. These can be generated by the new formalff pass.
+    - New JSON based yosys witness format for formal verification traces.
+    - yosys-smtbmc: Reading and writing of yosys witness traces.
+    - write_smt2: Emit inline metadata to support yosys witness trace.
+    - yosys-witness is a new tool to inspect and convert yosys witness traces.
 
 Yosys 0.19 .. Yosys 0.20
 --------------------------