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Treat AI-generated RTL and HLS code as an untrusted implementation: compare it with a written behavioral and interface contract, check it in the project’s real build configuration, and run self-checking tests at the right abstraction. For HLS, C simulation can check the source before synthesis; checking the RTL generated by HLS requires a later C/RTL co-simulation after C synthesis. No single passing check proves behavior that the check did not exercise.
What should you establish before reviewing the code?
Write the intended behavior down independently of the generated source and its tests. The contract is the reference for review; comments and test cases produced alongside the implementation are not independent evidence of what it is supposed to do.
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Define behavior and boundaries
- Function: Specify what the block or function must compute, including rounding, saturation, overflow, or other arithmetic behavior where relevant.
- Legal inputs: Record valid ranges, parameter values, and how the design should treat invalid inputs if the interface defines that behavior.
- Outputs and timing: State expected outputs, latency, throughput, and any ordering requirements.
- Clock and reset: Specify clock domains, reset polarity and timing assumptions, and any startup behavior.
- Interface protocol: Describe handshakes, transaction boundaries, backpressure, and whether transactions may overlap.
This contract lets a reviewer distinguish an implementation defect from an underspecified requirement. If a requirement is missing, resolve it before treating a generated answer or its apparent behavior as authoritative.
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A plausible-looking snippet is not a validated design. First confirm that the code is being interpreted in the intended project context, then inspect its semantics against the contract.
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Check the actual configuration
- Confirm the HDL or HLS language dialect and supported constructs.
- Verify the top module or HLS top function, source files, include paths, macros, parameters, and tool settings.
- Run the project’s parsing and elaboration checks, or the equivalent front-end checks for the HLS flow, using that configuration.
- Read warnings and investigate unresolved or implicit behavior; do not equate a successful parse with correct functionality.
Inspect likely semantic failure points
For RTL, compare widths and signedness through every expression, check casts and truncation, and trace reset behavior, sequential assignments, state holds, and branch completeness. Review interface handshakes and parameter corner cases against the contract. For HLS, confirm that the C/C++ behavior, arithmetic widths, and supported constructs match the intended hardware interface. The exact synthesis rules and checklist depend on the project and tool.
These are targeted review questions, not a substitute for the tool’s supported-language rules. A source can be syntactically valid yet implement different arithmetic, reset, or transaction behavior than the contract requires.
How should you validate RTL before synthesis?
Use the project-supported language mode and build configuration for front-end checks, then run self-checking simulation and assertions where appropriate. Lint, simulation, and formal analysis answer different questions; none should be treated as a general certificate of correctness.
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Build tests around expected behavior
Use a testbench whose expected results are defined independently of the generated implementation. Exercise multiple transactions and varied values, and make the checker report failure reliably—for example, by returning a nonzero status when an expected result does not match. Waveform inspection can help diagnose behavior, but a visual review alone is not a reliable pass/fail check.
AMD’s Vitis HLS User Guide UG1399, “Writing a Test Bench,” 2026.1, says the testbench should call the top-level function for multiple transactions with different data values, and warns that simulation results are only as good as the testbench. The advice is specific to AMD’s documented flow, but the underlying review principle is practical across verification work: a broken checker can produce a false pass.
Exercise the cases most likely to expose defects
- Minimum and maximum legal values, zero, and signed-value boundaries.
- Overflow, truncation, or saturation boundaries when the contract specifies them.
- Reset, start, stop, and restart sequences.
- Backpressure, stalls, and overlapping transactions where the protocol permits them.
- Invalid inputs, but only where the contract defines the required response.
Randomized tests can broaden input coverage. Preserve the seed and use a checker or invariant so the result is reproducible and machine-checkable rather than dependent on visual waveform inspection.
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Use assertions and formal checks for stated properties
Assertions can capture protocol, state, and safety invariants; formal property checking may help with finite-state control or arithmetic properties that can be expressed precisely. IEEE 1800-2023, published 2024-02-28 and listed as active by the IEEE Standards Association, describes SystemVerilog facilities for RTL and gate-level modeling, testbenches, assertions, coverage, and constrained-random verification. The standard provides language facilities; it does not prescribe this particular review sequence.
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Assertion syntax in a source file is not proof that a property is complete, enabled, or proved. Confirm which properties ran, under what assumptions, and what the tool reported.
What does pre-synthesis validation mean for HLS?
For HLS, the source-level check before synthesis is C simulation. AMD recommends validating the HLS function with a testbench in C simulation before synthesis. It checks the C/C++ function against the testbench’s expectations; it does not yet check the RTL that HLS will generate.
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Make C simulation self-checking
- Use the intended HLS top function and the project’s actual source and parameter configuration.
- Call the function for multiple transactions and varied inputs, including contract-defined boundary cases.
- Compare outputs with expected values computed independently of the generated implementation.
- Make mismatches produce an unambiguous failure status, and retain the test inputs and results.
Review the testbench as carefully as the function. If expected values reproduce the same mistaken assumptions as the implementation, or failures do not affect the reported status, a green run may be misleading.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you check the RTL generated from HLS?
Use C/RTL co-simulation after C synthesis when the goal is to compare the generated RTL’s behavior with the C reference. In AMD’s Vitis HLS flow, co-simulation uses inputs captured during C simulation, runs them on the synthesized RTL in RTL simulation, and checks the RTL outputs in the testbench. It is therefore a post-C-synthesis check, not a way to inspect generated RTL before synthesis.
Pay attention to dataflow designs
For HLS DATAFLOW designs, review channel behavior and FIFO depth. AMD documents that insufficient FIFO depth can stall simulation. A stall should be investigated against the design’s channel interactions and configuration rather than dismissed as an ordinary functional mismatch.
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Co-simulation only checks the transactions and behaviors exercised by its testbench. It does not establish that untested inputs, interface conditions, or system interactions are correct.
Which validation method answers which question?
| Method | What it checks | What it does not establish by itself |
|---|---|---|
| RTL parsing, elaboration, and lint | Whether the source is accepted and checked in the selected language mode and build configuration; lint can flag issues covered by its rules. | That the implementation meets the behavioral contract for all relevant inputs and sequences. |
| RTL self-checking simulation | Behavior exercised by testbench transactions and checked against expected results. | Behavior outside the exercised cases or properties the checker does not verify. |
| Assertions and formal property checking | Properties that are stated and, for formal work, analyzed under the selected assumptions and tool setup. | Unstated properties or correctness beyond the assumptions, scope, and results of the analysis. |
| HLS C simulation | Source-level HLS function behavior exercised by the C testbench before synthesis; AMD recommends this before synthesis. | The behavior of the RTL generated later by HLS. |
| HLS C/RTL co-simulation | Generated RTL behavior on vectors from C simulation, compared with the C reference after C synthesis in AMD’s flow. | Unexercised vectors or integration behavior outside the block-level test. |
| Hardware emulation or system-level checks | Integration behavior in the relevant system context; AMD distinguishes hardware emulation as an integration check. | Properties not exercised or observed by the integration scenario. |
This distinction matters when reporting a result. “The HLS C simulation passed” supports a claim about tested source behavior; it is not interchangeable with “the generated RTL passed co-simulation” or “the design passed system integration.”
When are block-level checks not enough?
AMD characterizes C simulation and C/RTL co-simulation as block-level verification, while hardware emulation is an integration check. If the block interacts with other hardware or software, add the project’s system-level checks for those interactions. Passing a block test does not establish that the integrated system behaves correctly.
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What evidence should you keep for a reproducible review?
Record enough detail for another engineer to understand what was checked and repeat it:
- Source revision, including the generated-code or prompt revision where relevant.
- Tool and version, language mode, top selection, parameters, defines, and other configuration.
- Test vectors or random seed, expected-result checker, assertions, and formal assumptions where used.
- Commands or flow steps, logs, and the specific pass/fail status.
- Warnings that were waived and the reason for each waiver.
This evidence bundle is workflow advice, not a complete record mandated by the cited standard. Its purpose is to make the scope and reproducibility of a pass clear.
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