Packed slice (tensorflow#1497)

PERF

Packed input sampling and packed output coordinate calculation
for arbitrary tensor rank. Slice tests verify 4-D, 5-D and 6-D.

Introduced WEBGL_PACK_ARRAY_OPERATIONS flag to configure usage
of packed SpaceToBatchND, BatchToSpaceND, slice, pad, transpose.

Author: astojilj

src/kernels/backend_webgl.ts

@@ -96,6 +96,7 @@ import {ScatterProgram} from './webgl/scatter_gpu';
 import {SegmentOpProgram} from './webgl/segment_gpu';
 import {SelectProgram} from './webgl/select_gpu';
 import {SliceProgram} from './webgl/slice_gpu';
+import {SlicePackedProgram} from './webgl/slice_packed_gpu';
 import {StridedSliceProgram} from './webgl/strided_slice_gpu';
 import * as tex_util from './webgl/tex_util';
 import {TextureData, TextureUsage} from './webgl/tex_util';
@@ -641,7 +642,9 @@ export class MathBackendWebGL implements KernelBackend {
     const {isPacked} = this.texData.get(x.dataId);
     const isContinous = isSliceContinous(x.shape, begin, size);
     if (isPacked || !isContinous) {
-      const program = new SliceProgram(size);
+      const program = ENV.get('WEBGL_PACK_ARRAY_OPERATIONS') ?
+          new SlicePackedProgram(size) :
+          new SliceProgram(size);
       const customSetup = program.getCustomSetupFunc(begin);
       return this.compileAndRun(program, [x], null, customSetup);
     }

src/kernels/webgl/slice_packed_gpu.ts

@@ -0,0 +1,102 @@
+/**
+ * @license
+ * Copyright 2019 Google Inc. All Rights Reserved.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ * =============================================================================
+ */
+
+import {GPGPUContext} from './gpgpu_context';
+import {GPGPUProgram} from './gpgpu_math';
+import {getCoordsDataType} from './shader_compiler';
+import {getChannels} from '../packing_util';
+
+export class SlicePackedProgram implements GPGPUProgram {
+  variableNames = ['source'];
+  usesPackedTextures = true;
+  outputShape: number[];
+  userCode: string;
+  rank: number;
+
+  // Caching uniform location for speed.
+  startLoc: WebGLUniformLocation;
+
+  constructor(destSize: number[]) {
+    this.outputShape = destSize;
+    this.rank = destSize.length;
+
+    const dtype = getCoordsDataType(this.rank);
+    const coords = getChannels('coords', this.rank);
+    const sourceLoc = getChannels('sourceLoc', this.rank);
+
+    const innerDims =
+        this.rank === 1 ? 'sourceLoc' : `vec2(${sourceLoc.slice(-2).join()})`;
+    const getChannel = 
+        `getChannel(getSource(${sourceLoc.join()}), ${innerDims})`;
+    const upperRow = `
+      result.x = ${getChannel};
+      if (++${coords[this.rank - 1]} < ${destSize[this.rank - 1]}) {
+        ++${sourceLoc[this.rank - 1]};
+        result.y = ${getChannel};
+        --${sourceLoc[this.rank - 1]};
+      }
+    `;
+    const lowerRow = this.rank === 1 ? '' : `
+      --${coords[this.rank - 1]};
+      if (++${coords[this.rank - 2]} < ${destSize[this.rank - 2]}) {
+        ++${sourceLoc[this.rank - 2]};
+        result.z = ${getChannel};
+        if (++${coords[this.rank - 1]} < ${destSize[this.rank - 1]}) {
+          ++${sourceLoc[this.rank - 1]};
+          result.w = ${getChannel};
+        }
+      }
+    `;
+
+    const sourceLocSetup = this.rank <= 4 ?
+        `sourceLoc = coords +
+            ${dtype}(${destSize.map((_, i) => `start[${i}]`).join()});` :
+        destSize.map((_, i) => `${sourceLoc[i]} = ${coords[i]} + start[${i}];`)
+            .join('\n');
+    this.userCode = `
+      uniform int start[${this.rank}];
+      void main() {
+        ${dtype} coords = getOutputCoords();
+        ${dtype} sourceLoc;
+        ${sourceLocSetup} 
+        vec4 result = vec4(0.);
+        ${upperRow}
+        ${lowerRow}
+        setOutput(result);
+      }
+    `;
+  }
+
+  getCustomSetupFunc(start: number[]) {
+    if (start.length !== this.rank) {
+      throw Error(
+          `The rank (${this.rank}) of the program must match the ` +
+          `length of start (${start.length})`);
+    }
+    return (gpgpu: GPGPUContext, webGLProgram: WebGLProgram) => {
+      if (this.startLoc == null) {
+        this.startLoc = gpgpu.getUniformLocationNoThrow(webGLProgram, 'start');
+        if (this.startLoc == null) {
+          // This means the compiler has optimized and realized it doesn't need
+          // the uniform.
+          return;
+        }
+      }
+      gpgpu.gl.uniform1iv(this.startLoc, start);
+    };
+  }
+}