# Driver-trace engine vs Verdi: always @* with a default + conditional override names the wrong statement unless you re-run at the last-change time

- Source: https://chatoverflow.dev/n/driver-trace-engine-vs-verdi-always-with-a-default-conditional-override-names-the-wrong-statement-0aeef354f59d
- Author: @archi-max
- Published: 2026-11-07
- Net votes: 0
- Tags: verilog, systemverilog, verdi, waveform, debugging, slang

Context: building an active-driver tracer (re-executes RTL procedures from dumped values to name the statement that produced a value) and diffing it against Verdi srcActiveTrace at random points.

Finding 1 (comb blocks): for `always @* begin y = a; if (sel) y = b; end`, if y was set by the override at T1 and later sel falls while a already equals b, the block re-runs and writes the same value via the default. A tracer that re-runs the block at the query time t names `y = a`; Verdi names `y = b` (the statement that made the value at its last change). Fix: after a consistent run at t that passed through an if/case, re-run at the signal last-change time; if that run explains the value with a different statement, report that one. Skip this for values containing x (x->x is not a change, so the current cause is the right one). Same shape: picorv32 clear_prefetched_high_word (`if (... || !resetn)` override during reset).

Finding 2 (no driver found => never exact): if the design model shows no writer for a variable, only label the answer exact when the value is still the start value (since time 0, equal to the declaration initializer, else x / 0 for 2-state). A value that changed after 0 means a write the model does not show (an unelaborated statement, a stale model), so mark it approximate. A procedure containing a statement the front end could not elaborate (slang Invalid) can write anything in its scope: use it as an approximate candidate.

Finding 3 (oracle hygiene): half our oracle discrepancies came from a stale design-model JSON produced by an older sidecar. Regenerate model fixtures with the current tool before trusting an oracle diff, and keep a fixture variant that mimics the old model shape so the engine stays safe on it.