swift3 - 使用 getter 时 UnsafeMutableBufferPointer 很慢

Question

我有一个带有自定义 AURenderCallback 的 AVAudioEngine。当我运行引擎时，cpu 会飙升，这似乎与 UnsafeMutableBufferPointer.getter 和 .setter 操作有关：

func performRender(ioActionFlags: UnsafeMutablePointer<AudioUnitRenderActionFlags>, inTimeStamp: UnsafePointer<AudioTimeStamp>, inBusNumber: UInt32, inNumberFrames: UInt32, ioData: UnsafeMutablePointer<AudioBufferList>?) -> OSStatus {

    let blI = UnsafeMutableAudioBufferListPointer(ioData)

    let numSamples = Int((blI?[0].mDataByteSize)! / UInt32(MemoryLayout<Float32>.stride))

    for input in 0 ..< blI!.count{

        let bI = blI?[input]

        guard let inputData = bI?.mData else {

            //assert(false)

            return kAudioUnitErr_Uninitialized
        }

        let samplesBI = UnsafeMutablePointer<Float32>(inputData.assumingMemoryBound(to: Float32.self))

        let samplesI = UnsafeMutableBufferPointer<Float32>(start: samplesBI, count: numSamples)

        for sampleIndex in 0 ..< samplesI.count {

            samplesI[sampleIndex] *= 0.5
        }

    }

    return noErr
}

什么会导致获取和设置指针数据的这种低效行为？因为它正在处理音频样本，所以效率低下会导致音频信号出现断断续续的情况。

Answer 1

您可以跳过 UnsafeMutableBufferPointer() 调用，并直接索引样本指针：

let bufferListPtr = UnsafeMutableAudioBufferListPointer(ioData)
...
let mBuffer : AudioBuffer = bufferListPtr[0]
let count = Int(mBuffer.mDataByteSize) / yourSampleSizeInBytes
let dataPointer = UnsafeMutableRawPointer(mBuffer.mData)
if let dptr = dataPointer {
    let sampleArray = dptr.assumingMemoryBound(to: yourSampleType.self)
    for i in 0..<(count) {
        let x = sampleArray[i]
        let y = myModifySample( x )   
        sampleArray[i] =  y 
    }
}

我将 Int16 用于 mySampleType（大小 = 2 字节），但它也适用于 4 字节类型 Float。

直接使用原始内存指针可能会避免一些缓冲区 getter/setter 效率低下（以及验证，所以要小心！）。