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Branch Scenario Verification

Concept WIKI v1 · 5/28/2026

Branch Scenario Verification is a constrained-random microprocessor verification approach for generating useful forward and backward branch instruction scenarios. It raises the chance that forward branches exercise taken/not-taken logic and constrains backward branches as bounded loop scenarios to avoid excessively long or endless execution.

Overview

Branch Scenario Verification addresses a common weakness of automatic microprocessor instruction generation: unconstrained random register values rarely create useful branch behavior. For example, random 32-bit register contents make equality comparisons such as BEQ R1, R2 unlikely to be true, so forward branches tend to fall through and may miss branch-condition evaluation logic. Likewise, a backward branch such as BNE R3, R4 can keep execution in a loop for a very long time when the compared registers almost never become equal. [C1]

The technique uses constrained scenarios to restrict relationships among nearby instructions and operands. These constraints make branch outcomes more controllable while preserving randomness in the generated instruction stream. [C2]

Forward branches

For forward branches, the goal is to make both taken and not-taken outcomes reasonably likely. A cited approach is to initialize the compared operands immediately before the branch. For example, an ADDI can assign R1 = R2 + {-2:2} before a BEQ R1, R2, making equality occur when the immediate is zero; in the cited example, this raises the probability of equality to 20%. [C3]

In constraint terms, the instruction immediately preceding a forward branch is constrained to be an ADDI using the same operands as the branch and a small immediate value. This links the branch comparison to a controlled arithmetic setup while still allowing randomized immediate selection within a small range. [C4]

Backward branches

Backward branches are handled as loop scenarios. Rather than allowing arbitrary register values that may cause extremely long loops, constraints make the loop index converge toward the terminating condition. In the cited example, a negative immediate sets up the compared registers so that an increment inside the loop brings the values closer by one each iteration. When the values become equal, the backward BNE falls through and the loop exits. This creates cases ranging from immediate fall-through to loops with several iterations, while avoiding absurdly long loops. [C5]

A practical constraint pattern is:

  • place an ADDI immediately before the backward branch, using the same operands and a small negative immediate value;
  • increment the loop-index operand by one inside the loop, just before the branch;
  • prevent other instructions inside the loop from modifying the two branch-operand registers. [C6]

The testbench should also consider boundary conditions. For example, a backward branch such as BGT R1, R2, LABEL_X can become always taken if R2 has the smallest possible value. [C7]

Role in constrained-random stimulus generation

Branch Scenario Verification is part of a broader constrained-random verification strategy for microprocessors. Pure random instruction streams are often not useful because hardware-level rules, such as aligned load/store accesses unless an exception is intended, must be respected. A common instruction-scenario base class can encode relationships among instruction objects as constraints, and selected constraints can be disabled when exception-producing cases such as misaligned memory operations are desired. [C8]

Scenario generators can combine constrained-random, directed-random, and directed scenarios. Directed scenarios may load pre-assembled program traces, while directed-random scenarios can preload special data values before randomized arithmetic streams. This allows branch-focused constraints to coexist with broader processor stimulus strategies. [C9]

LINKED ENTITIES

3 links
Forward Branch Probability Enhancement USES
Backward Branch Loop Control USES
Constraint-Based Randomization USES

CITATIONS

9 sources
9 citations
[1] Random register values make equality-based forward branches unlikely to be taken and can cause backward branch loops to run for a very long time. Applying constrained-random verification to microprocessors
[2] Constrained scenarios can restrict sequences of operations so that forward branch probability becomes reasonable and endless backward-branch loops are avoided. Applying constrained-random verification to microprocessors
[3] Initializing branch comparison operands with an ADDI such as R1 = R2 + {-2:2} can make equality occur with 20% probability in the cited example. Applying constrained-random verification to microprocessors
[4] A forward branch can be constrained so that the immediately preceding operation is an ADDI with the same operands and a small immediate value. Applying constrained-random verification to microprocessors
[5] Backward branches can be treated as loop scenarios in which register values are brought closer each iteration until the branch falls through. Applying constrained-random verification to microprocessors
[6] Backward-branch constraints include using a preceding ADDI with the same operands and a small negative value, incrementing the loop index inside the loop just before the branch, and preventing other loop instructions from modifying the branch operands. Applying constrained-random verification to microprocessors
[7] Boundary conditions must be considered because a backward branch such as BGT R1, R2, LABEL_X can be always taken if R2 has the smallest possible value. Applying constrained-random verification to microprocessors
[8] Pure random instruction sequences are often not useful for processor verification, and instruction-scenario base classes can encode constraints such as memory-alignment rules that may be disabled to generate exception cases. Applying constrained-random verification to microprocessors
[9] Processor stimulus generation can use constrained-random scenarios, directed-random scenarios, and directed scenarios loaded from pre-assembled program traces. Applying constrained-random verification to microprocessors