GLSL 优化。什么更快？

Question

我正在使用 OpenGL ES。 并且有两种计算“dir”向量，哪种代码最快？

attribute vec2 order;

代码1：

  if( abs(sinA) < 0.2 ) {
    if(order.x == 1.0){
        dir = sNormalPrev;   
    } else {
        dir = sNormalNext;   
    }
  } else {
    dir *= order.x / sinA;
  }

代码 2：

float k = step(0.2, abs(sinA));
dir = k * dir * order.x / sinA - (k-1.0) * (step(1.0, order.x + 1.0) * sNormalPrev + step(1.0, -order.x + 1.0) * sNormalNext);

Answer 1

写一个测试我看不出有什么不同

var iterationsPerTiming = 40;

var gl = document.createElement("canvas").getContext("webgl");
gl.canvas.width = 1;
gl.canvas.height = 1;
var programInfo1 = twgl.createProgramInfo(gl, ["vs1", "fs"])
var programInfo2 = twgl.createProgramInfo(gl, ["vs2", "fs"]);

var count = new Float32Array(1000000);
for (var i = 0; i < count.length; ++i) {
  count[i] = i % 3 / 2;
}

var arrays = {
  vertexId: {
    data: count, numComponents: 1,
  },
};
var bufferInfo = twgl.createBufferInfoFromArrays(gl, arrays);

iterateTest(programInfo1, 10)  // prime this path
  .then(function() { return iterateTest(programInfo2, 10)})  // prime this path
  .then(function() { return iterateTest(programInfo1, 20)})
  .then(log)
  .then(function() { return iterateTest(programInfo2, 20)})
  .then(log);

function iterateTest(programInfo, times) {
  return new Promise(function(resolve, reject) {
    var timings = [];
    var totalTime = 0;

    function runNextIteration() {
      if (times) {
         --times;
         timings.push(test(programInfo, iterationsPerTiming));
         setTimeout(runNextIteration, 1);
      } else {
        var totalTime = 0;
        var msgs = timings.map(function(timing, ndx) {
          totalTime += timing;
          return "" + ndx + ": " + timing.toFixed(3);
        });
        msgs.push("average timing: " + (totalTime / timings.length).toFixed(3));
        resolve(msgs.join("\n"));
      }
    }
    runNextIteration();
  });
}

function test(programInfo, iterations) {
  gl.useProgram(programInfo.program);
  twgl.setBuffersAndAttributes(gl, programInfo, bufferInfo);
  var startTime = performance.now();
  for (var i = 0; i < iterations; ++i) {
    twgl.drawBufferInfo(gl, gl.TRIANGLES, bufferInfo, count.length);
  }
  
  // this effectively does a gl.finish. It's not useful for real timing
  // beacuse it stalls the pipeline but it should be useful for 
  // comparing times since the stalling would be included in both
  var temp = new Uint8Array(4);
  gl.readPixels(0, 0, 1, 1, gl.RGBA, gl.UNSIGNED_BYTE, temp);
  return performance.now() - startTime;
}

function log(msg) {
  var div = document.createElement("pre");
  div.appendChild(document.createTextNode(msg));
  document.body.appendChild(div);
  return Promise.resolve();
}

html, body { font-family: monospace; }

<script src="https://twgljs.org/dist/twgl.min.js"></script>

  <script id="vs1" type="notjs">
attribute float vertexId;
void main() {
  vec2 order = vec2(vertexId, 0);
  float sinA = vertexId;
  vec3 dir = vec3(0);
  vec3 sNormalPrev = vec3(1);
  vec3 sNormalNext = vec3(-1);
  if( abs(sinA) < 0.2 ) {
    if(order.x == 1.0){
        dir = sNormalPrev;   
    } else {
        dir = sNormalNext;   
    }
  } else {
    dir *= order.x / sinA;
  }
  gl_Position = vec4(dir, 1.0); // have to use dir
  gl_PointSize = 1.0;
}
  </script>
  <script id="vs2" type="notjs">
attribute float vertexId;

void main() {
  vec2 order = vec2(vertexId, 0);
  float sinA = vertexId;
  vec3 dir = vec3(0);
  vec3 sNormalPrev = vec3(1);
  vec3 sNormalNext = vec3(-1);
  
  float k = step(0.2, abs(sinA));
  dir = k * dir * order.x / sinA - (k-1.0) * (step(1.0, order.x + 1.0) * sNormalPrev + step(1.0,   -order.x + 1.0) * sNormalNext);
  
  gl_Position = vec4(dir, 1.0); // have to use dir
  gl_PointSize = 1.0;
}
  </script>
  <script id="fs" type="notjs">
precision mediump float;
void main() {
  gl_FragColor = vec4(1);
}
  </script>

也许我的测试很糟糕。在 2015 年初的 macbook pro 和 iPhone6s+ 上测试

Answer 2

GPU 核心大多是宽 SIMD 单元，它们通过掩码处理 if 语句。根据 GPU 架构，着色器编译器将控制语句转换为掩码操作，与您处理代码的方式几乎相同。

在 PC 上，GPU 驱动程序具有足够的处理能力来正确优化着色器，因此您的优化没有任何区别。根据 2010 年的 this blog post，您的优化在移动平台上是有意义的。我认为今天的现代智能手机并非如此，因为它们有足够的处理能力来正确优化着色器，而且驱动程序也会随着时间的推移而成熟。

您也可以试用之前的博文中提到的工具GLSL optimizer。一些 GPU 供应商还提供用于分析着色器的工具。