Speed up dropFunction by using a typed array instead of a sparse JS a…

…rray.

This improves firefox-devtools#4668 from 7.0 seconds to 1.6 seconds.

Before: https://share.firefox.dev/3qP8iB3
After: https://share.firefox.dev/43JVAC8

src/profile-logic/transforms.js

@@ -906,7 +906,9 @@ export function dropFunction(
   const { stackTable, frameTable } = thread;
 
   // Go through each stack, and label it as containing the function or not.
-  const stackContainsFunc: Array<void | true> = [];
+  // stackContainsFunc is a stackIndex => bool map, implemented as a U8 typed
+  // array for better performance. 0 means false, 1 means true.
+  const stackContainsFunc = new Uint8Array(stackTable.length);
   for (let stackIndex = 0; stackIndex < stackTable.length; stackIndex++) {
     const prefix = stackTable.prefix[stackIndex];
     const frameIndex = stackTable.frame[stackIndex];
@@ -915,15 +917,15 @@ export function dropFunction(
       // This is the function we want to remove.
       funcIndex === funcIndexToDrop ||
       // The parent of this stack contained the function.
-      (prefix !== null && stackContainsFunc[prefix])
+      (prefix !== null && stackContainsFunc[prefix] === 1)
     ) {
-      stackContainsFunc[stackIndex] = true;
+      stackContainsFunc[stackIndex] = 1;
     }
   }
 
   return updateThreadStacks(thread, stackTable, (stack) =>
     // Drop the stacks that contain that function.
-    stack !== null && stackContainsFunc[stack] ? null : stack
+    stack !== null && stackContainsFunc[stack] === 1 ? null : stack
   );
 }