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DescriptionQ688

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Q688FWQualcommASIC interview problem

Compute wrapped performance-counter deltas

TechniquesSiliconReliabilityDebugComputedeltas

DifficultyMedium

TopicPost-Silicon & Reliability

LanguageC++

Requirements4 checkpoints

Problem

Validation takes two snapshots of four free-running 32-bit hardware counters. Write a C++ function that returns modulo-2^32 deltas and the busiest counter.

Starting declarationC++

bool counter_delta(const uint32_t before[4],
const uint32_t after[4], uint32_t delta[4],
unsigned *max_i);

Example input and output

Use this case to check your interpretation

Input

before={100,0xFFFFFFFE,8,2} and after={125,1,17,11}.

Output

delta={25,3,9,9} and max_i=0.

Explanation

Unsigned subtraction handles the wrapped second counter, and the largest delta is 25 at the lowest matching index 0.

Requirements (4)

Require every pointer; return false without writing outputs if any is null.
For each index compute delta[i]=after[i]-before[i] using uint32_t modulo arithmetic.
Set max_i to the index of the largest delta; on a tie choose the lower index.
Compute all temporary results before writing outputs so aliased before/after and delta arrays still work.