Problem 1: Most open-source simulators lack coverage
Verilator only supports line and toggle coverage
Treadle (Chisel interpreter), ESSENT and FireSim have no coverage support
Problem 2: Verilog coverage is an inadequate replacement for native Chisel coverage
In this example, the translation to structural Verilog
replaces a branch with a conditional assignment. Therefore,
100% line coverage on the generated Verilog does not necessarily
imply complete line coverage of the Chisel source.
Our Solution: Simulator Independent Coverage
We show how a single new cover statement allows us to move all coverage instrumentation into the FIRRTL compiler.
Adding support for the cover statement to a simulator is fairly simple.
Line Coverage
The line coverage pass instruments every when
statement in the FIRRTL circuit. The mapping from lines to
branches is used to generate the coverage report from the
counts reported by the simulator.
Finite State Machine Coverage
Finite state machine (FSM) coverage assumes that
the state register uses a ChiselEnum. We first analyze all possible
next states by simplifying the state update expression for
each possible current state. We then add cover statements
for all states and possible transitions.
Verilator: cover Statement Support
Verilator: Coverage Instrumentation Overhead
Coverage instrumentation overhead on Verilator
v4.034. For TLRAM, the measured overhead of our FIRRTL
line coverage is close to zero.
The cover Statement
Toggle Coverage
The toggle coverage pass adds a register and a xor
gate. It avoids redundant instrumentation for signals that
always have the same value.
FireSim: cover Statement Support on FPGA
We generate saturating counters and a scan chain
for all cover statements for FPGA-accelerated simulation
with FireSim.
