In the assembly view state, refer to the current symbol by index

src/actions/profile-view.ts

@@ -1920,19 +1920,30 @@ export function updateBottomBoxContentsAndMaybeOpen(
   currentTab: TabSlug,
   { libIndex, sourceIndex, nativeSymbols, lineNumber }: BottomBoxInfo
 ): Action {
-  // TODO: If the set has more than one element, pick the native symbol with
-  // the highest total sample count
-  const nativeSymbol = nativeSymbols.length !== 0 ? nativeSymbols[0] : null;
+  const haveSource = sourceIndex !== null;
+  const haveAssembly = nativeSymbols.length !== 0;
+
+  const shouldOpenBottomBox = haveSource || haveAssembly;
+
+  // By default, only open the source view and keep the assembly
+  // view closed - unless the only thing we have is assembly.
+  const shouldOpenAssemblyView = !haveSource && haveAssembly;
+
+  // If we have at least one native symbol to show assembly for, pick
+  // the first one arbitrarily.
+  // TODO: If we have more than one native symbol, pick the one
+  // with the highest total sample count.
+  const currentNativeSymbol = nativeSymbols.length !== 0 ? 0 : null;
 
   return {
     type: 'UPDATE_BOTTOM_BOX',
     libIndex,
     sourceIndex,
-    nativeSymbol,
-    allNativeSymbolsForInitiatingCallNode: nativeSymbols,
+    nativeSymbols,
+    currentNativeSymbol,
     currentTab,
-    shouldOpenBottomBox: sourceIndex !== null || nativeSymbol !== null,
-    shouldOpenAssemblyView: sourceIndex === null && nativeSymbol !== null,
+    shouldOpenBottomBox,
+    shouldOpenAssemblyView,
     lineNumber,
   };
 }

src/app-logic/url-handling.ts

@@ -353,9 +353,10 @@ export function getQueryStringFromUrlState(urlState: UrlState): string {
         if (sourceView.sourceIndex !== null) {
           query.sourceViewIndex = sourceView.sourceIndex;
         }
-        if (assemblyView.isOpen && assemblyView.nativeSymbol !== null) {
+        if (assemblyView.isOpen && assemblyView.currentNativeSymbol !== null) {
+          const { currentNativeSymbol, nativeSymbols } = assemblyView;
           query.assemblyView = stringifyAssemblyViewSymbol(
-            assemblyView.nativeSymbol
+            nativeSymbols[currentNativeSymbol]
           );
         }
       }
@@ -514,8 +515,8 @@ export function stateFromLocation(
   const assemblyView: AssemblyViewState = {
     isOpen: false,
     scrollGeneration: 0,
-    nativeSymbol: null,
-    allNativeSymbolsForInitiatingCallNode: [],
+    nativeSymbols: [],
+    currentNativeSymbol: null,
   };
   const isBottomBoxOpenPerPanel: any = {};
   tabSlugs.forEach((tabSlug) => (isBottomBoxOpenPerPanel[tabSlug] = false));
@@ -526,8 +527,8 @@ export function stateFromLocation(
   if (query.assemblyView) {
     const symbol = parseAssemblyViewSymbol(query.assemblyView);
     if (symbol !== null) {
-      assemblyView.nativeSymbol = symbol;
-      assemblyView.allNativeSymbolsForInitiatingCallNode = [symbol];
+      assemblyView.nativeSymbols = [symbol];
+      assemblyView.currentNativeSymbol = 0;
       assemblyView.isOpen = true;
       isBottomBoxOpenPerPanel[selectedTab] = true;
     }

src/reducers/url-state.ts

@@ -585,20 +585,21 @@ const sourceView: Reducer<SourceViewState> = (
 const assemblyView: Reducer<AssemblyViewState> = (
   state = {
     scrollGeneration: 0,
-    nativeSymbol: null,
-    allNativeSymbolsForInitiatingCallNode: [],
+    nativeSymbols: [],
+    currentNativeSymbol: null,
     isOpen: false,
   },
   action
 ) => {
   switch (action.type) {
     case 'UPDATE_BOTTOM_BOX': {
+      const { nativeSymbols, currentNativeSymbol, shouldOpenAssemblyView } =
+        action;
       return {
         scrollGeneration: state.scrollGeneration + 1,
-        nativeSymbol: action.nativeSymbol,
-        allNativeSymbolsForInitiatingCallNode:
-          action.allNativeSymbolsForInitiatingCallNode,
-        isOpen: state.isOpen || action.shouldOpenAssemblyView,
+        nativeSymbols,
+        currentNativeSymbol,
+        isOpen: state.isOpen || shouldOpenAssemblyView,
       };
     }
     case 'OPEN_ASSEMBLY_VIEW': {

src/selectors/url-state.ts

@@ -81,7 +81,13 @@ export const getAssemblyViewIsOpen: Selector<boolean> = (state) =>
   getProfileSpecificState(state).assemblyView.isOpen;
 export const getAssemblyViewNativeSymbol: Selector<NativeSymbolInfo | null> = (
   state
-) => getProfileSpecificState(state).assemblyView.nativeSymbol;
+) => {
+  const { nativeSymbols, currentNativeSymbol } =
+    getProfileSpecificState(state).assemblyView;
+  return currentNativeSymbol !== null
+    ? nativeSymbols[currentNativeSymbol]
+    : null;
+};
 export const getAssemblyViewScrollGeneration: Selector<number> = (state) =>
   getProfileSpecificState(state).assemblyView.scrollGeneration;
 export const getShowJsTracerSummary: Selector<boolean> = (state) =>

src/types/actions.ts

@@ -338,8 +338,8 @@ type ProfileAction =
       readonly type: 'UPDATE_BOTTOM_BOX';
       readonly libIndex: IndexIntoLibs | null;
       readonly sourceIndex: IndexIntoSourceTable | null;
-      readonly nativeSymbol: NativeSymbolInfo | null;
-      readonly allNativeSymbolsForInitiatingCallNode: NativeSymbolInfo[];
+      readonly nativeSymbols: NativeSymbolInfo[];
+      readonly currentNativeSymbol: number | null;
       readonly currentTab: TabSlug;
       readonly shouldOpenBottomBox: boolean;
       readonly shouldOpenAssemblyView: boolean;

src/types/state.ts

@@ -262,15 +262,13 @@ export type AssemblyViewState = {
   isOpen: boolean;
   // When this is incremented, the assembly view scrolls to the "hotspot" line.
   scrollGeneration: number;
-  // The native symbol for which the assembly code is being shown at the moment.
-  // Null if the initiating call node did not have a native symbol.
-  nativeSymbol: NativeSymbolInfo | null;
-  // The set of native symbols which contributed samples to the initiating call
-  // node. Often, this will just be one element (the same as `nativeSymbol`),
-  // but it can also be multiple elements, for example when double-clicking a
-  // function like `Vec::push` in an inverted call tree, if that function has
-  // been inlined into multiple different callers.
-  allNativeSymbolsForInitiatingCallNode: NativeSymbolInfo[];
+  // The list of native symbols whose assembly code should be accessible in the
+  // assembly view. Often empty or just one element, but sometimes this can have
+  // multiple elements, for example the different JIT compilations of one JS function.
+  nativeSymbols: NativeSymbolInfo[];
+  // The index in `nativeSymbols` for the symbol whose assembly code is being shown at the moment.
+  // Null if nativeSymbols is empty.
+  currentNativeSymbol: number | null;
 };
 
 export type DecodedInstruction = {