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Xiushan Feng* ASIC Verification Nvidia Corporation Assertion-Based Design Partition 1 TM Jayanta Bhadra, Ross Patterson {JayantaBhadra, {JayantaBhadra, Freescale Semiconductor *The work is done when the author worked at Freescale

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Docket – MT11774TS Problem Statement – Assertion-Based Design Verification Challenges ► Assertion-Based Verification is broadly used in circuit verification Increase the visibility of internal signals Use assertions as checkers. Pass of assertions == pass of verification Verify the correctness of design – improve the quality ► For large designs, simulation speed is slow and may blow up formal verification due to exponential state space. Slow down verification process – more resource, longer design cycles. For certain cases, it is not feasible to verify certain assertions. ► Most time, partial design is good enough to verify assertions. For example, for SOC PAD connectivity check, assertions are written to verify that signals can propagate from source to target under certain conditions. We don’t need logic that drives sources, loads values from targets. Unneeded logic usually is VERY big, removing them can achieve huge gains ► However, manually identifying and removing unneeded logic is very hard. Need to understand assertions and the design indent Failing to include logic can cause false verification results. ► An automatic solution is possible using Assertion-Based Partition 2

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Docket – MT11774TS Verification (simulation or formal verification) The Current Verification Environment 3 RTL Constraints Pass/Fail & Circuit Debug Compiled database is huge. Verification is slowed down More resources needed (licenses, engineering hours, computing machines) This technique attempts to reduce verification cycles and improve verification quality RTL Compile Assertions

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Docket – MT11774TS RTL Compile The Proposed Verification Environment 4 LEGEND: Yellow: NEW White: Prior Art RTL Assertions Pass/Fail & Circuit Debug 1.Signal propagation Signal reachability 2. Get boundary signals in ABV Constraints 3. Condition-base design partition with formal prove Assertions1 Assertions2 Verification (simulation or formal verification) RTL(partitioned)

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Docket – MT11774TS Definition of Signal Propagation and Reachability 5 a a' ABCD Signal Propagation: a is directly connected by wires to a’. i.e., a and a’ are interchangeable inside the assertions, assign B = A;assign D = C; Signal Reachability: If signal S1 is inside fan-in logic cone of signal S2, then S2 is reachable by S1. D is reachable by A, B, but not C. A B C D E E D C A B partially ordered graph assertion: assert property (condition |->a == ….) update assertion: assert property (condition |->a’ == …) foobar RTL: FOO foo (.A(a),.B(X)…); BAR bar (.C(X),.D(a’)…); X

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Docket – MT11774TS Signal Propagation and Reachability Computation 6 For each assertion, compute the boundary points For each pair of signals x, y of the assertion, compute whether y can be (forward/backward) reached by x. Reachable ? Y assertions state stage partially ordered graph Build directed edge from z to y Compute state stage graph with all signals of assertions replace x by z in assertions. X tied to z && z is on the path from x to y Y N RTL

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Docket – MT11774TS Example 7 inputs ast0 inputs ast0 state stage partially ordered graph State points (Latch/Flop) output Signal propagation and reachability

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Docket – MT11774TS Get Boundary Signals 8 partially ordered graph Keep lower bound and upper bounds Boundary signals State points (Latch/Flop) inputs ast0 output inputs ast0 output

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Docket – MT11774TS Conditional RTL Partition with Formal Prove 9 inputs ast0 ast1 output lower bounds inputs (IN) upper bounds outputs (OUT) Boundary signals Assertions1 Conditional compute cone of influence for all OUT Constraints RTL (partitioned) RTL Assertions2 (Optimized)

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Docket – MT11774TS Conditional RTL Partition with Formal Prove – Cont. 10 For each OUT, backward partition RTL. When hit a condition, build symbolic expression for condition and prove the condition Assertions1 compute cone of influence for both cases Constraints RTL (partitioned) RTL Assertions2 (Optimized) prove? N Partition RTL only for proven case. Update assertion based proven results Formal prove engine

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Docket – MT11774TS Build Symbolic Expression for Conditions and Prove 11 R Constraints cond R: Register IN: Input Recursively build expression for each fan-in f_in is R/IN? Y Terminate from the recursive procedure get fan-ins (f_ins) of cond expr(cond) = f (f_ins) N symbolic expression Constraints Formal prove engine True/False Light-weight No expression blow up

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Docket – MT11774TS Example 12 a b' ABCD assign B = A; b cond For example: Input X, Y; b’ = (cond)? b : C … If we can prove cond is always true, then we don’t need to trace path for signal C. we know b’ = b, and use it to update assertions and propagate signal. If we can prove cond is always false, then, we just need to trace signal C and propagate signals if feasible. R

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Docket – MT11774TS Assertion Grouping and Parallel ► Concept of assertion groups Assertions have hidden “locations” Logic regions Assertion types Assertion-based partition can be done on assertion groups ► Assertion groups can be verified in parallel with design partitions Groups are verified independently. Verification results inside one region can be re-used by others. E.g., verified proves can be assumptions for others, etc. ► Assume-and-guarantee is done at the boundary of groups. One assertion can be re-written into multiple ones that belong to different groups. Abstraction techniques are done differently for groups. E.g., Assertions of analog behavior models are verified inside analog groups. Verification of digital groups has analog blackboxed and analog drivers properly constrained, which will be proven by analog assertions. 13

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Docket – MT11774TS Assertion Grouping Example 14 R1 R2 inputs R3 output ► Several groups of assertions that verify a few logic regions Logic needed to prove a group may not be necessary for another Inputs of a region can be outputs of another cutpoints can force internal wires as inputs ► One assertion can be broken down into multiple assertions. For example, enable and reg R1; foo, bar, A, B R1 ast: enable && reg[19] && bar |-> A == B can be replaced by ast1: enable && reg[19] |-> foo ast2: foo && bar -> A ==B

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Docket – MT11774TS Results ► We applied the ideas on Freescale PAD IO verification. ► Without the ideas, original formal verification ran weeks and required VERY large machines (>100GB) A smaller and simpler design took >1week on large machines but didn’t get good coverage. This highlighted the need for this work ► With ABP, we are able to prove 25+K assertions within 2.5 hours. ► Critical SOC-level bugs were caught ! 15 With this disclosure Without this disclosure Memory Usage<4G (normal machine is enough)> 100G Licenses used<30100 Simulation RuntimeReduced Significantly – 2.5 hours (> 100X reduction) Could be several weeks DebugDebugging is possible and easier (therefore quicker) GUI Debugging is not feasible

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