Optimize base64 decoding (#2623)

* Optimize base64 decoding (about 7x-10x faster and temporary garbage-free compared to the original version)

* new Uint8Array

* Reuse Uint8Array view

* Fix end handling

* Code format

* Update tests

1 files changed, 34 insertions, 30 deletions

diff --git a/test/wasm2js/emscripten.2asm.js.opt b/test/wasm2js/emscripten.2asm.js.opt
index 84e33bb6f..f554f869a 100644
--- a/test/wasm2js/emscripten.2asm.js.opt
+++ b/test/wasm2js/emscripten.2asm.js.opt
@@ -183,37 +183,41 @@ function asmFunc(global, env, buffer) {
  };
 }
 
-var writeSegment = (
-    function(mem) {
-      var _mem = new Uint8Array(mem);
-      return function(offset, s) {
-        var bytes, i;
-        if (typeof Buffer === 'undefined') {
-          bytes = atob(s);
-          for (i = 0; i < bytes.length; i++)
-            _mem[offset + i] = bytes.charCodeAt(i);
-        } else {
-          bytes = Buffer.from(s, 'base64');
-          for (i = 0; i < bytes.length; i++)
-            _mem[offset + i] = bytes[i];
-        }
-      }
+for (var base64ReverseLookup = new Uint8Array(123/*'z'+1*/), i = 25; i >= 0; --i) {
+    base64ReverseLookup[48+i] = 52+i; // '0-9'
+    base64ReverseLookup[65+i] = i; // 'A-Z'
+    base64ReverseLookup[97+i] = 26+i; // 'a-z'
+  }
+  base64ReverseLookup[43] = 62; // '+'
+  base64ReverseLookup[47] = 63; // '/'
+  /** @noinline Inlining this function would mean expanding the base64 string 4x times in the source code, which Closure seems to be happy to do. */
+  function base64DecodeToExistingUint8Array(uint8Array, offset, b64) {
+    var b1, b2, i = 0, j = offset, bLength = b64.length, end = offset + (bLength*3>>2);
+    if (b64[bLength-2] == '=') --end;
+    if (b64[bLength-1] == '=') --end;
+    for (; i < bLength; i += 4, j += 3) {
+      b1 = base64ReverseLookup[b64.charCodeAt(i+1)];
+      b2 = base64ReverseLookup[b64.charCodeAt(i+2)];
+      uint8Array[j] = base64ReverseLookup[b64.charCodeAt(i)] << 2 | b1 >> 4;
+      if (j+1 < end) uint8Array[j+1] = b1 << 4 | b2 >> 2;
+      if (j+2 < end) uint8Array[j+2] = b2 << 6 | base64ReverseLookup[b64.charCodeAt(i+3)];
     }
-  )(wasmMemory.buffer);
-writeSegment(1024, "aGVsbG8sIHdvcmxkIQoAAJwMAAAtKyAgIDBYMHgAKG51bGwp");
-writeSegment(1072, "EQAKABEREQAAAAAFAAAAAAAACQAAAAALAAAAAAAAAAARAA8KERERAwoHAAETCQsLAAAJBgsAAAsABhEAAAARERE=");
-writeSegment(1153, "CwAAAAAAAAAAEQAKChEREQAKAAACAAkLAAAACQALAAAL");
-writeSegment(1211, "DA==");
-writeSegment(1223, "DAAAAAAMAAAAAAkMAAAAAAAMAAAM");
-writeSegment(1269, "Dg==");
-writeSegment(1281, "DQAAAAQNAAAAAAkOAAAAAAAOAAAO");
-writeSegment(1327, "EA==");
-writeSegment(1339, "DwAAAAAPAAAAAAkQAAAAAAAQAAAQAAASAAAAEhIS");
-writeSegment(1394, "EgAAABISEgAAAAAAAAk=");
-writeSegment(1443, "Cw==");
-writeSegment(1455, "CgAAAAAKAAAAAAkLAAAAAAALAAAL");
-writeSegment(1501, "DA==");
-writeSegment(1513, "DAAAAAAMAAAAAAkMAAAAAAAMAAAMAAAwMTIzNDU2Nzg5QUJDREVGLTBYKzBYIDBYLTB4KzB4IDB4AGluZgBJTkYAbmFuAE5BTgAu");
+  }
+var bufferView = new Uint8Array(wasmMemory.buffer);
+base64DecodeToExistingUint8Array(bufferView, 1024, "aGVsbG8sIHdvcmxkIQoAAJwMAAAtKyAgIDBYMHgAKG51bGwp");
+base64DecodeToExistingUint8Array(bufferView, 1072, "EQAKABEREQAAAAAFAAAAAAAACQAAAAALAAAAAAAAAAARAA8KERERAwoHAAETCQsLAAAJBgsAAAsABhEAAAARERE=");
+base64DecodeToExistingUint8Array(bufferView, 1153, "CwAAAAAAAAAAEQAKChEREQAKAAACAAkLAAAACQALAAAL");
+base64DecodeToExistingUint8Array(bufferView, 1211, "DA==");
+base64DecodeToExistingUint8Array(bufferView, 1223, "DAAAAAAMAAAAAAkMAAAAAAAMAAAM");
+base64DecodeToExistingUint8Array(bufferView, 1269, "Dg==");
+base64DecodeToExistingUint8Array(bufferView, 1281, "DQAAAAQNAAAAAAkOAAAAAAAOAAAO");
+base64DecodeToExistingUint8Array(bufferView, 1327, "EA==");
+base64DecodeToExistingUint8Array(bufferView, 1339, "DwAAAAAPAAAAAAkQAAAAAAAQAAAQAAASAAAAEhIS");
+base64DecodeToExistingUint8Array(bufferView, 1394, "EgAAABISEgAAAAAAAAk=");
+base64DecodeToExistingUint8Array(bufferView, 1443, "Cw==");
+base64DecodeToExistingUint8Array(bufferView, 1455, "CgAAAAAKAAAAAAkLAAAAAAALAAAL");
+base64DecodeToExistingUint8Array(bufferView, 1501, "DA==");
+base64DecodeToExistingUint8Array(bufferView, 1513, "DAAAAAAMAAAAAAkMAAAAAAAMAAAMAAAwMTIzNDU2Nzg5QUJDREVGLTBYKzBYIDBYLTB4KzB4IDB4AGluZgBJTkYAbmFuAE5BTgAu");
 return asmFunc({
     'Int8Array': Int8Array,
     'Int16Array': Int16Array,