{fast, correct}
SystemVerilog, choose two.
The fast, correct, open SystemVerilog engine. Run your existing designs
and testbenches unmodified, with the language features real
verification depends on — and get them back dramatically sooner.
No compromises.
$ clotho run cpu.sv tb.sv
Get startedSUPPORTS
- Classes & interfaces
- SVA
- CRV
- Full UVMCOMING SOON
84.7%*
(and rising) of the entire 2023 LRM, meaning your existing UVM testbenches run unmodified.
* measured on sv-tests
HOW CLOTHO WORKS
Your RTL, compiled on the fly.
The fastest RTL simulators compile your design to efficient C++ and then machine code. Clotho goes further: it JITs your design on the fly, keeping the performance of compiled code at a fraction of the startup time.
Clotho uses LLVM to skip the C++ altogether, enabling better optimization and reducing compile time dramatically. Combined with build caching, re-runs and quick fixes start up in milliseconds.
Clotho uses full event-driven semantics: stratified event scheduling, accurate time modeling, four-state (0, 1, X, Z) logic. No compromises.
THE TOOLBOX
Big toolbox, one command.
No long TCL scripts, no generated C++ to manage, no wrappers. Point Clotho at your code and it elaborates, compiles, and runs. Edit and re-run in milliseconds.
-
clotho run
Elaborate, compile, and simulate — one command, automatic caching.
-
clotho lint
Catch latch inference, width mismatches, and dead code before simulating.
-
clotho fmt
Highly customizable formatting and SystemVerilog to Verilog conversion.
-
clotho synthSOON
Elaborate to a gate-level netlist using the same front end.
-
clotho lspSOON
All the same tools in your favorite editor, plus auto-complete.
module hello;
initial begin
$display("Hello, world!");
#10 $display("t = %0t", $time);
$finish;
end
endmodule$ clotho run hello.sv
Hello, world!
t = 10
$ clotho run hello.sv --wave hello.fst
Hello, world!
t = 10
wrote hello.fstHOW IS IT SO FAST?
Designed from the ground up, performance first.
Clotho’s speed comes from careful design, inspired by Verilator and the fastest software compilers and JITs — V8, Julia, and Clang.
01 · THE FOUNDATION
Built on LLVM
Clotho generates code through LLVM — the same production compiler backend behind Clang, Swift, and Julia. Clotho inherits the most sophisticated code generator and optimizer for free.
Running real machine code is fundamentally faster than walking a design step by step in an interpreter — the difference shows up on every run, and it compounds over a day of iteration.
Clotho models four state logic very efficiently, and opportunistically drops down to two-state logic where it doesn't matter — without sacrificing behavioral correctness.
Inspired by Verilator, Clotho reorders and combines a simulation's work to cut scheduling overhead while producing identical results.
Clotho caches the machine code it generates, so re-running an unchanged design — or one with only small edits — skips the build and starts up in milliseconds.
{FAST, CORRECT} — CHOOSE TWO
Speed never comes at the cost of correctness.
Clotho was designed carefully to follow IEEE 1800-2023, and its behavior is rigorously tested against established simulators. It doesn't cover the entire language yet, but coverage is growing quickly and it's able to run many real cores.
- Correct four state (0, 1, x, z) logic modeling
- Fully event-driven, stratified queue scheduler
- Behavior verified against sv-tests suite
- Strong LRM (IEEE 1800-2023) coverage
- UVM readyCOMING SOON
Try it on your own design.
Clotho is open source — no license fees, no per-seat limits. Clone it, point it at your RTL, and turn the hours you lose waiting on simulation into more iterations per day.
We benchmark Clotho comprehensively across real designs — the full results are here. We're confident it will save you time, and encourage you to try it on your own suite.