VCDDiff - VCD Waveform Comparison Tool

VCDDiff is a high-performance tool for comparing VCD (Value Change Dump) waveforms, designed specifically for RTL development and verification workflows. It helps identify differences between a reference implementation and a test implementation by comparing signal values across clock cycles.

Features

• Fast, cycle-accurate comparison of VCD waveforms
• Scope-based filtering to focus on specific modules
• Efficient signal caching for improved performance
• Progress bar visualization for long-running comparisons
• Detailed reporting of first divergence points for each signal
• Support for multi-bit signals and various signal types (binary, hex)

Use Cases

VCDDiff is particularly useful when:

1. Developing RTL Compilers:

   • Compare compiler output against a reference implementation
   • Verify correctness of compiler optimizations
   • Debug transformation passes

2. RTL Simulation:

   • Validate simulator implementations against reference simulators
   • Verify timing consistency across different simulation tools
   • Debug race conditions and timing issues

3. Hardware Verification:

   • Compare pre-synthesis vs post-synthesis behavior
   • Verify equivalence after RTL modifications
   • Debug signal mismatches in different implementation stages

Installation

# Install dependencies
cargo install flamegraph  # Required for performance profiling

# Build the project
cargo build --release

# Run the tool
cargo run --release -- \
    --vcd1 path/to/reference.vcd \
    --vcd2 path/to/test.vcd \
    --clock "top.clk" \
    --reset "top.reset"

# Print hierarchy
cargo run --release -- \
    --vcd1 path/to/reference.vcd \
    --vcd2 path/to/test.vcd \
    --clock "top.clk" \
    --reset "top.reset" \
    --print-hier

Example output

Found differences in 3 signals:
Signal 'top.module1.signal_a' first diverged at cycle 42: 0x1 vs 0x0
Signal 'top.module1.signal_b' first diverged at cycle 57: 0x3F vs 0x2F
Signal 'top.module2.flag' first diverged at cycle 103: 1 vs 0

Flamegraph profiling

just flamegraph