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Branch Scenario

Concept WIKI v1 · 5/31/2026

A Branch Scenario is a constrained instruction-generation concept used in processor verification to make branch behavior both meaningful and bounded. It increases the chance of exercising forward-branch decision logic and prevents backward branches from degenerating into excessively long or endless loops.

Overview

A Branch Scenario is a constrained verification scenario for processor instruction generation that specifically targets branch behavior. In unconstrained random generation, branch conditions are often ineffective: random 32-bit register values are very unlikely to satisfy equality tests, so forward branches tend to fall through, while backward branches can remain in loops for a very long time. Branch scenarios address this by constraining nearby instructions and operands so branch outcomes become useful for verification.

Why branch scenarios are needed

Branches are difficult for automatic instruction generation because pure random operand values rarely create interesting control-flow behavior. In the example discussed in the source, a forward BEQ is unlikely to be taken because two random registers rarely match, and a backward BNE can keep looping because two random registers may take a very long time to converge.

Forward-branch handling

For forward branches, the goal is to increase the probability that the branch condition is evaluated in both taken and not-taken cases. The recommended constraint is to place an ADDI instruction immediately before the forward branch, using the same operands and a small immediate value. The source gives the example R1 = R2 + {-2:2}, which raises the probability of R1 == R2 to 20%.

Backward-branch handling

For backward branches, the branch can be treated as a loop scenario. The recommended constraint is to place an ADDI instruction immediately before the backward branch with the same operands and a small negative value. Inside the loop, that operand acts as a loop index and is incremented by 1 just before the branch operation. To keep the loop well formed, the two branch-operand registers should not be modified elsewhere in the loop; equivalently, other destination registers in the loop should not reuse those branch operands.

This structure avoids absurdly long loops while still creating a range of outcomes, from simple fall-through cases to loops with several iterations.

Boundary conditions

Branch scenarios also need explicit boundary-condition handling. The source notes that pathological cases must be prevented, such as a backward BGT R1, R2, LABEL_X that is always taken when R2 holds the smallest possible number.

Role in stimulus generation

The branch-scenario approach appears in a broader constrained-random, object-based stimulus-generation flow. The source describes expressing relationships between instruction objects as constraints in a common instruction scenario base class, with scenario generation implemented in SystemVerilog and driven by a scenario generator. In that context, branch scenarios are one constrained mechanism for producing useful processor programs instead of ineffective pure-random instruction streams.

LINKED ENTITIES

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Object-Oriented Stimulus Generation USES

CITATIONS

7 sources
7 citations
[1] Branches are challenging for automatic instruction generation because random register values rarely satisfy branch conditions, causing forward branches to miss branch-condition logic and backward branches to loop for a very long time. Applying constrained-random verification to microprocessors
[2] Constrained scenarios can make forward-branch behavior useful by initializing the comparing operands just before the branch; the example R1 = R2 + {-2:2} gives a 20 percent probability of equality. Applying constrained-random verification to microprocessors
[3] For forward branches, the recommended constraint is that the instruction immediately preceding the branch be an ADDI using the same operands and a small immediate value. Applying constrained-random verification to microprocessors
[4] For backward branches, the recommended constraint is an ADDI with the same operands and a small negative value before the branch, with the loop-index operand incremented by 1 inside the loop just before the branch. Applying constrained-random verification to microprocessors
[5] To keep a backward-branch loop valid, the two branch-operand registers should not be modified elsewhere inside the loop; other destination registers should not reuse those operands. Applying constrained-random verification to microprocessors
[6] Branch scenarios must also account for boundary conditions, such as preventing a backward BGT from being always taken when one operand is the smallest possible number. Applying constrained-random verification to microprocessors
[7] Branch scenarios are part of a broader constrained-random, object-based stimulus-generation approach in which relationships between instruction objects are expressed as constraints in a common instruction scenario base class and generated via SystemVerilog scenario generation. Applying constrained-random verification to microprocessors