Use Int32Array for the stackTable.prefix column.

The `prefix` column of both `RawStackTable` and `StackTable` is now an
`Int32Array`, with `-1` (instead of `null`) indicating that a stack node is a
root. The wire format changes accordingly, so the processed profile version is
bumped to 63 with an upgrader that converts the column on load.

Author: mstange

docs-developer/CHANGELOG-formats.md

@@ -6,6 +6,13 @@ Note that this is not an exhaustive list. Processed profile format upgraders can
 
 ## Processed profile format
 
+### Version 66
+
+The `prefix` column of `profile.shared.stackTable` now uses `-1` instead of `null`
+to indicate "this stack node is a root". In-memory it is now an `Int32Array`;
+on the JSON wire, the column is just an array of numbers (where `-1` marks a
+root).
+
 ### Version 65
 
 The stack table's `frame` column (stored at `profile.shared.stackTable.frame`) can now optionally be stored as an `Int32Array`, for profiles loaded from [JsonSlabs](https://github.com/mstange/json-slabs/) files (.jslb, .jslb.gz). Regular JS / JSON arrays are still accepted.

src/app-logic/constants.ts

@@ -12,7 +12,7 @@ export const GECKO_PROFILE_VERSION = 34;
 // The current version of the "processed" profile format.
 // Please don't forget to update the processed profile format changelog in
 // `docs-developer/CHANGELOG-formats.md`.
-export const PROCESSED_PROFILE_VERSION = 65;
+export const PROCESSED_PROFILE_VERSION = 66;
 
 // The following are the margin sizes for the left and right of the timeline. Independent
 // components need to share these values.

src/app-logic/url-handling.ts

@@ -1649,8 +1649,9 @@ function getStackIndexFromVersion3JSCallNodePath(
   oldCallNodePath: CallNodePath
 ): IndexIntoStackTable | null {
   const { stackTable, funcTable, frameTable } = shared;
-  const stackIndexDepth: Map<IndexIntoStackTable | null, number> = new Map();
-  stackIndexDepth.set(null, -1);
+  const stackIndexDepth: Map<IndexIntoStackTable, number> = new Map();
+  // Map key -1 represents the virtual root above all stack indexes.
+  stackIndexDepth.set(-1, -1);
 
   for (let stackIndex = 0; stackIndex < stackTable.length; stackIndex++) {
     const prefix = stackTable.prefix[stackIndex];
@@ -1659,9 +1660,8 @@ function getStackIndexFromVersion3JSCallNodePath(
     const isJS = funcTable.isJS[funcIndex];
     // We know that at this point stack table is sorted and the following
     // condition holds:
-    // assert(prefixStack === null || prefixStack < stackIndex);
-    const doesPrefixMatchCallNodePath =
-      prefix === null || stackIndexDepth.has(prefix);
+    // assert(prefixStack === -1 || prefixStack < stackIndex);
+    const doesPrefixMatchCallNodePath = stackIndexDepth.has(prefix);
 
     if (!doesPrefixMatchCallNodePath) {
       continue;
@@ -1696,8 +1696,8 @@ function getVersion4JSCallNodePathFromStackIndex(
 ): CallNodePath {
   const { funcTable, stackTable, frameTable } = shared;
   const callNodePath = [];
-  let nextStackIndex: IndexIntoStackTable | null = stackIndex;
-  while (nextStackIndex !== null) {
+  let nextStackIndex: IndexIntoStackTable = stackIndex;
+  while (nextStackIndex !== -1) {
     const frameIndex: IndexIntoFrameTable = stackTable.frame[nextStackIndex];
     const funcIndex = frameTable.func[frameIndex];
     if (funcTable.isJS[funcIndex] || funcTable.relevantForJS[funcIndex]) {

src/profile-logic/address-timings.ts

@@ -127,7 +127,7 @@ export function getStackAddressInfo(
   for (let stackIndex = 0; stackIndex < stackTable.length; stackIndex++) {
     const prefixStack = stackTable.prefix[stackIndex];
     const prefixAddressSet: IndexIntoAddressSetTable | -1 =
-      prefixStack !== null ? stackIndexToAddressSetIndex[prefixStack] : -1;
+      prefixStack !== -1 ? stackIndexToAddressSetIndex[prefixStack] : -1;
 
     const frame = stackTable.frame[stackIndex];
     const nativeSymbolOfThisStack = frameTable.nativeSymbol[frame];

src/profile-logic/data-structures.ts

@@ -70,9 +70,14 @@ export function getRawStackTableBuilder(): RawStackTableBuilder {
 export function getRawStackTableBuilderWithExistingContents(
   existing: RawStackTable
 ): RawStackTableBuilder {
+  const prefix = new Array<IndexIntoStackTable | null>(existing.length);
+  for (let i = 0; i < existing.length; i++) {
+    const p = existing.prefix[i];
+    prefix[i] = p === -1 ? null : p;
+  }
   return {
     frame: [...existing.frame],
-    prefix: [...existing.prefix],
+    prefix,
     length: existing.length,
   };
 }
@@ -81,9 +86,14 @@ export function finishRawStackTableBuilder(
   builder: RawStackTableBuilder
 ): RawStackTable {
   const { frame, prefix, length } = builder;
+  const prefixCol = new Int32Array(length);
+  for (let i = 0; i < length; i++) {
+    const p = prefix[i];
+    prefixCol[i] = p === null ? -1 : p;
+  }
   return {
     frame: new Int32Array(frame),
-    prefix,
+    prefix: prefixCol,
     length,
   };
 }

src/profile-logic/import/chrome.ts

@@ -5,12 +5,15 @@
 import type {
   Profile,
   RawThread,
-  RawStackTable,
   IndexIntoStackTable,
   MixedObject,
 } from 'firefox-profiler/types';
 
-import { getEmptyProfile, getEmptyThread } from '../data-structures';
+import {
+  getEmptyProfile,
+  getEmptyThread,
+  type RawStackTableBuilder,
+} from '../data-structures';
 import type { StringTable } from '../../utils/string-table';
 import { ensureExists, coerce } from '../../utils/types';
 import {
@@ -868,7 +871,7 @@ function getImageSize(
  * For sanity, check that stacks are ordered where the prefix stack
  * always preceeds the current stack index in the StackTable.
  */
-function assertStackOrdering(stackTable: RawStackTable) {
+function assertStackOrdering(stackTable: RawStackTableBuilder) {
   const visitedStacks = new Set<number | null>([null]);
   for (let i = 0; i < stackTable.length; i++) {
     if (!visitedStacks.has(stackTable.prefix[i])) {

src/profile-logic/insert-stack-labels.ts

@@ -4,7 +4,6 @@
 
 import type {
   IndexIntoFrameTable,
-  IndexIntoStackTable,
   RawStackTable,
   IndexIntoFuncTable,
   Profile,
@@ -199,7 +198,7 @@ export function insertStackLabels(
   for (let stackIndex = 0; stackIndex < oldStackTable.length; stackIndex++) {
     const parentStackIndex = oldStackTable.prefix[stackIndex];
     const inheritedLabelFrameIndex =
-      parentStackIndex !== null
+      parentStackIndex !== -1
         ? inheritedLabelFrameIndexAtStack[parentStackIndex]
         : null;
     const frameIndex = oldStackTable.frame[stackIndex];
@@ -218,7 +217,7 @@ export function insertStackLabels(
     ) {
       labelFrameIndexToInsertAtStack[stackIndex] = null;
       inheritedLabelFrameIndexAtStack[stackIndex] = null;
-    } else if (parentStackIndex === null) {
+    } else if (parentStackIndex === -1) {
       labelFrameIndexToInsertAtStack[stackIndex] = rootLabelFrameIndex;
       inheritedLabelFrameIndexAtStack[stackIndex] = rootLabelFrameIndex;
       stacksToInsertCount++;
@@ -230,7 +229,7 @@ export function insertStackLabels(
 
   // Now compute the new stack table.
   const newStackCount = oldStackTable.length + stacksToInsertCount;
-  const newPrefixCol = new Array<IndexIntoStackTable | null>(newStackCount);
+  const newPrefixCol = new Int32Array(newStackCount);
   const newFrameCol = new Array<IndexIntoFrameTable>(newStackCount);
   const oldStackToNewStackPlusOne = new Int32Array(oldStackTable.length);
   let nextNewStackIndex = 0;
@@ -243,7 +242,7 @@ export function insertStackLabels(
       labelFrameIndexToInsertAtStack[oldStackIndex];
     const oldPrefix = oldStackTable.prefix[oldStackIndex];
     let newPrefix =
-      oldPrefix !== null ? oldStackToNewStackPlusOne[oldPrefix] - 1 : null;
+      oldPrefix !== -1 ? oldStackToNewStackPlusOne[oldPrefix] - 1 : -1;
     const frameIndex = oldStackTable.frame[oldStackIndex];
     if (labelFrameIndexToInsert !== null) {
       const insertedStackIndex = nextNewStackIndex++;

src/profile-logic/line-timings.ts

@@ -57,7 +57,7 @@ export function getStackLineInfo(
   for (let stackIndex = 0; stackIndex < stackTable.length; stackIndex++) {
     const prefixStack = stackTable.prefix[stackIndex];
     const prefixLineSet: IndexIntoLineSetTable | -1 =
-      prefixStack !== null ? stackIndexToLineSetIndex[prefixStack] : -1;
+      prefixStack !== -1 ? stackIndexToLineSetIndex[prefixStack] : -1;
 
     const frame = stackTable.frame[stackIndex];
     const func = frameTable.func[frame];

src/profile-logic/merge-compare.ts

@@ -1132,7 +1132,7 @@ function mergeStackTables(
       );
       const prefix = stackTable.prefix[i];
       const newPrefix =
-        prefix === null ? null : oldStackToNewStackPlusOne[prefix] - 1;
+        prefix === -1 ? null : oldStackToNewStackPlusOne[prefix] - 1;
 
       newStackTable.frame.push(frameIndex);
       newStackTable.prefix.push(newPrefix);

src/profile-logic/process-profile.ts

@@ -2085,7 +2085,6 @@ export function serializeProfileToJsonSlabsFile(
     // JSON" for these tables will become much smaller and we won't need to split
     // out those tables anymore.
     profile.threads,
-    profile.shared.stackTable,
     profile.shared.frameTable,
     profile.shared.funcTable,
     profile.shared.stringArray,