wave_read_first

wave_read_first :: (T x) -> T {...}

pipeline DivergentBroadcast {
  pass "Main" {
    compute "Main" [64, 1, 1] {
    // Some lanes might be inactive due to divergence
    if (condition) {
        // Within divergent code, get value from first active lane
        float myValue = computeValue(input.local_index);
        float firstValue = wave_read_first(myValue);

        // All active lanes have first active lane's value
        processWithReference(myValue, firstValue);
    }
    }
  }
}

// Broadcast within divergent control flow
if (needsProcessing) {
    float computed = expensiveComputation();
    float reference = wave_read_first(computed);
    // Use reference for comparisons
}

// First active lane becomes leader
float leaderValue = wave_read_first(myValue);
bool isLeader = (myValue == leaderValue);  // Note: may have ties

// Read from first lane that has valid data
if (hasValidData) {
    float data = loadData(dataIndex);
    float sharedData = wave_read_first(data);
    // All active lanes get the first valid data
}

// Get one representative value from active lanes
float3 position = getPosition(input.local_index);
float3 representativePos = wave_read_first(position);

// Get first item to define batch parameters
int batchId = wave_read_first(itemBatchId);
// Process all items with same batch parameters
if (itemBatchId == batchId) {
    processItem();
}

// Requires GL_KHR_shader_subgroup_ballot
subgroupBroadcastFirst(x)

WaveReadLaneFirst(x)