vstart

vstart is a value used in vector-processing retry handling to indicate where an unfinished vector instruction should resume. In the cited VPU/OVI verification context, a retry can occur when the vector processing unit cannot handle all loaded cache lines sent by the scalar core. The instruction then completes while specifying a vstart value that represents the first element it could not write into the vector registers. The instruction is treated as unfinished and must be re-executed starting from the element identified by the stored vstart value. [retry-semantics]

Handling such retries also requires killing instructions after the retrying instruction and re-issuing the retrying instruction from the vstart element. The same evidence notes that determining the correct vstart can be complex, because the implementation may need to choose among the lowest received index, mask, or data chunk. [retry-control-flow]

In the referenced VPU verification work, retry behavior and different vstart values were important enough to be covered by directed tests added to the regression suite. Retries were described as one of the primary sources of VPU errors, and the environment randomized retry likelihood using UVM configuration objects. [verification-coverage]