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Formal, Logic Simulation, hardware emulation/acceleration. Benefits and Limitations

Stephen Bailey, Director of Emerging Technologies, Mentor Graphics

Verification and Validation are key terms used and have the following differentiation:  Verification (specifically, hardware verification) ensures the design matches R&D’s functional specification for a module, block, subsystem or system. Validation ensures the design meets the market requirements, that it will function correctly within its intended usage.

Software-based simulation remains the workhorse for functional design verification. Its advantages in this space include:

-          Cost:  SW simulators run on standard compute servers.

-          Speed of compile & turn-around-time (TAT):  When verifying the functionality of modules and blocks early in the design project, software simulation has the fastest turn-around-time for recompiling and re-running a simulation.

-          Debug productivity:  SW simulation is very flexible and powerful in debug. If a bug requires interactive debugging (perhaps due to a potential UVM testbench issue with dynamic – stack and heap memory based – objects), users can debug it efficiently & effectively in simulation. Users have very fine level controllability of the simulation – the ability to stop/pause at any time, the ability to dynamically change values of registers, signals, and UVM dynamic objects.

-          Verification environment capabilities: Because it is software simulation, a verification environment can easily be created that peeks and pokes into any corner of the DUT. Stimulus, including traffic generation / irritators can be tightly orchestrated to inject stimulus at cycle accuracy.

-          Simulation’s broad and powerful verification and debug capabilities are why it remains the preferred engine for module and block verification (the functional specification & implementation at the “component” level).

If software-based simulation is so wonderful, then why would anyone use anything else?  Simulation’s biggest negative is performance, especially when combined with capacity (very large, as well as complex designs). Performance, getting verification done faster, is why all the other engines are used. Historically, the hardware acceleration engines (emulation and FPGA-based prototyping) were employed latish in the project cycle when validation of the full chip in its expected environment was the objective. However, both formal and hardware acceleration are now being used for verification as well. Let’s continue with the verification objective by first exploring the advantages and disadvantages of formal engines.

-          Formal’s number one advantage is its comprehensive nature. When provided a set of properties, a formal engine can exhaustively (for all of time) or for a, typically, broad but bounded number of clock cycles, verify that the design will not violate the property(ies). The prototypical example is verifying the functionality of a 32-bit wide multiplier. In simulation, it would take far too many years to exhaustively validate every possible legal multiplicand and multiplier inputs against the expected and actual product for it to be feasible. Formal can do it in minutes to hours.

-          At one point, a negative for formal was that it took a PhD to define the properties and run the tool. Over the past decade, formal has come a long way in usability. Today, formal-based verification applications package properties for specific verification objectives with the application. The user simply specifies the design to verify and, if needed, provides additional data that they should already have available; the tool does the rest. There are two great examples of this approach to automating verification with formal technology:

  • CDC (Clock Domain Crossing) Verification:  CDC verification uses the formal engine to identify clock domain crossings and to assess whether the (right) synchronization logic is present. It can also create metastability models for use with simulation to ensure no metastability across the clock domain boundary is propagated through the design. (This is a level of detail that RTL design and simulation abstract away. The metastability models add that level of detail back to the simulation at the RTL instead of waiting for and then running extremely long full-timing, gate-level simulations.)
  • Coverage Closure:  During the course of verification, formal, simulation and hardware accelerated verification will generate functional and code coverage data. Most organizations require full (or nearly 100%) coverage completion before signing-off the RTL. But, today’s designs contain highly reusable blocks that are also very configurable. Depending on the configuration, functionality may or may not be included in the design. If it isn’t included, then coverage related to that functionality will never be closed. Formal engines analyze the design, in its actual configuration(s) that apply, and does a reachability analysis for any code or (synthesizable) functional coverage point that has not yet been covered. If it can be reached, the formal tool will provide an example waveform to guide development of a test to achieve coverage. If it cannot be reached, the manager has a very high-level of certainty to approving a waiver for that coverage point.

-          With comprehensibility being its #1 advantage, why doesn’t everyone use and depend fully on formal verification:

  • The most basic shortcoming of formal is that you cannot simulate or emulate the design’s dynamic behavior. At its core, formal simply compares one specification (the RTL design) against another (a set of properties written by the user or incorporated into an automated application or VIP). Both are static specifications. Human beings need to witness dynamic behavior to ensure the functionality meets marketing or functional requirements. There remains no substitute for “visualizing” the dynamic behavior to avoid the GIGO (Garbage-In, Garbage-Out) problem. That is, the quality of your formal verification is directly proportional to the quality (and completeness) of your set of properties. For this reason, formal verification will always be a secondary verification engine, albeit one whose value rises year after year.
  • The second constraint on broader use of formal verification is capacity or, in the vernacular of formal verification:  State Space Explosion. Although research on formal algorithms is very active in academia and industry, formal’s capacity is directly related to the state space it must explore. Higher design complexity equals more state space. This constraint limits formal usage to module, block, and (smaller or well pruned/constrained) subsystems, and potentially chip levels (including as a tool to help isolate very difficult to debug issues).

The use of hardware acceleration has a long, checkered history. Back in the “dark ages” of digital design and verification, gate-level emulation of designs had become a big market in the still young EDA industry. Zycad and Ikos dominated the market in the late 1980’s to mid/late-1990’s. What happened?  Verilog and VHDL plus automated logic synthesis happened. The industry moved from the gate to the register-transfer level of golden design specification; from schematic based design of gates to language-based functional specification. The jump in productivity from the move to RTL was so great that it killed the gate-level emulation market. RTL simulation was fast enough. Zycad died (at least as an emulation vendor) and Ikos was acquired after making the jump to RTL, but had to wait for design size and complexity to compel the use of hardware acceleration once again.

Now, 20 years later, it is clear to everyone in the industry that hardware acceleration is back. All 3 major vendors have hardware acceleration solutions. Furthermore, there is no new technology able to provide a similar jump in productivity as did the switch from gate-level to RTL. In fact, the drive for more speed has resulted in emulation and FPGA prototyping sub-markets within the broader market segment of hardware acceleration. Let’s look at the advantages and disadvantages of hardware acceleration (both varieties).

-          Speed:  Speed is THE compelling reason for the growth in hardware acceleration. In simulation today, the average performance (of the DUT) is perhaps 1 kHz. Emulation expectations are for +/- 1 MHz and for FPGA prototypes 10 MHz (or at least 10x that of emulation). The ability to get thousands of more verification cycles done in a given amount of time is extremely compelling. What began as the need for more speed (and effective capacity) to do full chip, pre-silicon validation driven by Moore’s Law and the increase in size and complexity enabled by RTL design & design reuse, continues to push into earlier phases of the verification and validation flow – AKA “shift-left.”  Let’s review a few of the key drivers for speed:

  • Design size and complexity:  We are well into the era of billion gate plus design sizes. Although design reuse addressed the challenge of design productivity, every new/different combination of reused blocks, with or without new blocks, creates a multitude (exponential number) of possible interactions that must be verified and validated.
  • Software:  This is also the era of the SoC. Even HW compute intensive chip applications, such as networking, have a software component to them. Software engineers are accustomed to developing on GHz speed workstations. One MHz or even 10’s of MHz speeds are slow for them, but simulation speeds are completely intolerable and infeasible to enable early SW development or pre-silicon system validation.
  • Functional Capabilities of Blocks & Subsystems:  It can be the size of input data / simuli required to verify a block’s or subsystem’s functionality, the complexity of the functionality itself, or a combination of both that drives the need for huge numbers of verification cycles. Compute power is so great today, that smartphones are able to record 4k video and replay it. Consider the compute power required to enable Advanced Driver Assistance Systems (ADAS) – the car of the future. ADAS requires vision and other data acquisition and processing horsepower, software systems capable of learning from mistakes (artificial intelligence), and high fault tolerance and safety. Multiple blocks in an ADAS system will require verification horsepower that would stress the hardware accelerated performance available even today.

-          As a result of these trends which appear to have no end, hardware acceleration is shifting left and being used earlier and earlier in the verification and validation flows. The market pressure to address its historic disadvantages is tremendous.

  • Compilation time:  Compilation in hardware acceleration requires logic synthesis and implementation / mapping to the hardware that is accelerating the simulation of the design. Synthesis, placement, routing, and mapping are all compilation steps that are not required for software simulation. Various techniques are being employed to reduce the time to compile for emulation and FPGA prototype. Here, emulation has a distinct advantage over FPGA prototypes in compilation and TAT.
  • Debug productivity:  Although simulation remains available for debugging purposes, you’d be right in thinking that falling back on a (significantly) slower engine as your debug solution doesn’t sound like the theoretically best debug productivity. Users want a simulation-like debug productivity experience with their hardware acceleration engines. Again, emulation has advantages over prototyping in debug productivity. When you combine the compilation and debug advantages of emulation over prototyping, it is easy to understand why emulation is typically used earlier in the flow, when bugs in the hardware are more likely to be found and design changes are relatively frequent. FPGA prototyping is typically used as a platform to enable early SW development and, at least some system-level pre-silicon validation.
  • Verification capabilities:  While hardware acceleration engines were used primarily or solely for pre-silicon validation, they could be viewed as laboratory instruments. But as their use continues to shift to earlier in the verification and validation flow, the need for them to become 1st class verification engines grows. That is why hardware acceleration engines are now supporting:
    • UPF for power-managed designs
    • Code and, more appropriately, functional coverage
    • Virtual (non-ICE) usage modes which allow verification environments to be connected to the DUT being emulated or prototyped. While a verification environment might be equated with a UVM testbench, it is actually a far more general term, especially in the context of hardware accelerated verification. The verification environment may consist of soft models of things that exist in the environment the system will be used in (validation context). For example, a soft model of a display system or Ethernet traffic generator or a mass storage device. Soft models provide advantages including controllability, reproducibility (for debug) and easier enterprise management and exploitation of the hardware acceleration technology. It may also include a subsystem of the chip design itself. Today, it has become relatively common to connect a fast model written in software (usually C/C++) to an emulator or FPGA prototype. This is referred to as hybrid emulation or hybrid prototyping. The most common subsystem of a chip to place in a software model is the processor subsystem of an SoC. These models usually exist to enable early software development and can run at speeds equivalent to ~100 MHz. When the processor subsystem is well verified and validated, typically a reused IP subsystem, then hybrid mode can significantly increase the verification cycles of other blocks and subsystems, especially driving tests using embedded software and verifying functionality within a full chip context. Hybrid mode can rightfully be viewed as a sub-category of the virtual usage mode of hardware acceleration.
    • As with simulation and formal before it, hardware acceleration solutions are evolving targeted verification “applications” to facilitate productivity when verifying specific objectives or target markets. For example, a DFT application accelerates and facilitates the validation of test vectors and test logic which are usually added and tested at the gate-level.

In conclusion, it may seem that simulation is being used less today. But, it is all relative. The total number of verification cycles is growing exponentially. More simulation cycles are being performed today even though hardware acceleration and formal cycles are taking relatively larger pieces of the overall verification pie. Formal is growing in appeal as a complementary engine. Because of its comprehensive verification nature, it can significantly bend the cost curve for high-valued (difficult/challenging) verification tasks and objectives. The size and complexity of designs today require the application of all verification engines to the challenges of verifying and validating (pre-silicon) the hardware design and enabling early SW development. The use of hardware acceleration continues to shift-left and be used earlier in the verification and validation flow causing emulation and FPGA prototyping to evolve into full-fledged verification engines (not just ICE validation engines).

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