[JSC] Stack positions: keep the divot of an instruction, and let a provider find the line and column

JSTests/stress/stack-position-is-the-same-on-every-read.js

@@ -0,0 +1,25 @@
+function shouldBe(actual, expected) {
+    if (actual !== expected)
+        throw new Error(`expected ${expected} but got ${actual}`);
+}
+
+function makeStack(which) {
+    if (which === 0) return new Error("a").stack;
+    if (which === 1)
+        return new Error("b").stack;
+    const error = (
+        new Error("c")
+    );
+    return error.stack;
+}
+
+function positionOfFirstFrame(stack) {
+    const match = /^makeStack@.*:(\d+):(\d+)$/m.exec(stack);
+    return `${match[1]}:${match[2]}`;
+}
+
+const expected = ["7:38", "9:25", "11:18"];
+for (let i = 0; i < 100; i++) {
+    for (let which = 0; which < 3; which++)
+        shouldBe(positionOfFirstFrame(makeStack(which)), expected[which]);
+}

Source/JavaScriptCore/bytecode/CodeBlock.cpp

@@ -2576,8 +2576,12 @@ void CodeBlock::removeExceptionHandlerForCallSite(DisposableCallSiteIndex callSi
 LineColumn CodeBlock::lineColumnForBytecodeIndex(BytecodeIndex bytecodeIndex) const
 {
     RELEASE_ASSERT(bytecodeIndex.offset() < instructions().size());
+#if USE(BUN_JSC_ADDITIONS)
+    unsigned divotInProvider = sourceOffset() + m_unlinkedCode->divotForBytecodeIndex(bytecodeIndex);
+#else
     auto entry = m_unlinkedCode->expressionInfoForBytecodeIndex(bytecodeIndex);
     unsigned divotInProvider = sourceOffset() + entry.divot;
+#endif
     return source().provider()->documentLineColumnForOffset(divotInProvider);
 }
 

Source/JavaScriptCore/bytecode/ExpressionInfo.cpp

@@ -890,6 +890,19 @@ auto ExpressionInfo::entryForInstPC(InstPC instPC) -> Entry
     return decoder.entry();
 }
 
+#if USE(BUN_JSC_ADDITIONS)
+unsigned ExpressionInfo::divotForInstPC(InstPC instPC)
+{
+    auto iter = m_cachedDivots.find(instPC);
+    if (iter != m_cachedDivots.end())
+        return iter->value;
+
+    unsigned divot = entryForInstPC(instPC).divot;
+    m_cachedDivots.add(instPC, divot);
+    return divot;
+}
+#endif
+
 template<unsigned bitCount>
 void ExpressionInfo::print(PrintStream& out, FieldID fieldID, unsigned value)
 {

Source/JavaScriptCore/bytecode/ExpressionInfo.h

@@ -183,6 +183,12 @@ class ExpressionInfo {
 
     Entry NODELETE entryForInstPC(InstPC);
 
+#if USE(BUN_JSC_ADDITIONS)
+    // entryForInstPC() decodes from the start of the chapter on every call, and a stack trace
+    // asks for the same instructions again. This keeps the divot of each instruction it was asked for.
+    unsigned divotForInstPC(InstPC);
+#endif
+
     bool isEmpty() const { return !m_numberOfEncodedInfo; };
     size_t NODELETE byteSize() const; // owned by this object
     size_t NODELETE byteSizeForGCPacing() const; // what a generated (non-borrowed) one this size would own
@@ -312,7 +318,11 @@ class ExpressionInfo {
 
     static constexpr unsigned numberOfWordsBetweenChapters = 10000;
 
+#if USE(BUN_JSC_ADDITIONS)
+    using DivotMap = UncheckedKeyHashMap<InstPC, unsigned, WTF::IntHash<InstPC>, WTF::UnsignedWithZeroKeyHashTraits<InstPC>>;
 
+    DivotMap m_cachedDivots;
+#endif
     unsigned m_numberOfChapters;
     unsigned m_numberOfEncodedInfo;
     unsigned m_numberOfEncodedInfoExtensions;

Source/JavaScriptCore/bytecode/UnlinkedCodeBlock.cpp

@@ -187,6 +187,13 @@ ExpressionInfo::Entry UnlinkedCodeBlock::expressionInfoForBytecodeIndex(Bytecode
     return expressionInfo().entryForInstPC(bytecodeIndex.offset());
 }
 
+#if USE(BUN_JSC_ADDITIONS)
+unsigned UnlinkedCodeBlock::divotForBytecodeIndex(BytecodeIndex bytecodeIndex)
+{
+    return expressionInfo().divotForInstPC(bytecodeIndex.offset());
+}
+#endif
+
 #ifndef NDEBUG
 static void dumpExpressionInfoDetails(size_t index, const JSInstructionStream& instructionStream, unsigned instructionOffset, unsigned divot, unsigned startOffset, unsigned endOffset)
 {

Source/JavaScriptCore/bytecode/UnlinkedCodeBlock.h

@@ -183,7 +183,7 @@ class UnlinkedCodeBlock : public JSCell {
     bool hasExpressionInfo() { return !expressionInfo().isEmpty(); }
     // Null while the expression info is still in the cache payload. For a caller that cannot take m_lock or allocate
     // (a sampling hook inside malloc): unlike expressionInfo() it never decodes. On the result use entryForInstPC(),
-    // which does neither; lineColumnForInstPC() fills a cache.
+    // which does neither; divotForInstPC() fills a cache.
     ExpressionInfo* expressionInfoIfDecoded() const { return m_expressionInfo.get(); }
 
     bool hasCheckpoints() const { return m_hasCheckpoints; }
@@ -291,6 +291,9 @@ class UnlinkedCodeBlock : public JSCell {
     bool hasRareData() const { return m_rareData.get(); }
 
     ExpressionInfo::Entry expressionInfoForBytecodeIndex(BytecodeIndex);
+#if USE(BUN_JSC_ADDITIONS)
+    unsigned divotForBytecodeIndex(BytecodeIndex);
+#endif
 
     bool typeProfilerExpressionInfoForBytecodeOffset(unsigned bytecodeOffset, unsigned& startDivot, unsigned& endDivot);
 

Source/JavaScriptCore/parser/SourceProvider.h

@@ -124,6 +124,9 @@ class JS_EXPORT_PRIVATE SourceProvider : public ThreadSafeRefCounted<SourceProvi
 #if USE(BUN_JSC_ADDITIONS)
     JS_EXPORT_PRIVATE virtual size_t memoryCost() const { return 0; }
     JS_EXPORT_PRIVATE virtual void didGenerateUnlinkedCodeBlock(VM&, const SourceCodeKey&, UnlinkedCodeBlock*) const { }
+    // A provider that can find a line and a column without the line start table overrides this.
+    // Both are zero-based and in the provider's own text. Any thread can call it. False means: use the table.
+    JS_EXPORT_PRIVATE virtual bool lineAndColumnForOffset(unsigned /* offset */, unsigned& /* line0Based */, unsigned& /* column0Based */) { return false; }
 #endif
 
     StringView getRange(int start, int end) const LIFETIME_BOUND
@@ -189,11 +192,24 @@ class JS_EXPORT_PRIVATE SourceProvider : public ThreadSafeRefCounted<SourceProvi
         return m_lineStartTable.offsetForPosition(source(), line0Based, column0Based);
     }
 
+#if USE(BUN_JSC_ADDITIONS)
+    // positionInfoForOffset() for a caller that reads the line and the column only.
+    LineStartTable::PositionInfo lineAndColumnInfoForOffset(unsigned offset)
+    {
+        LineStartTable::PositionInfo info;
+        if (lineAndColumnForOffset(offset, info.line0Based, info.column0Based))
+            return info;
+        return positionInfoForOffset(offset);
+    }
+#else
+    LineStartTable::PositionInfo lineAndColumnInfoForOffset(unsigned offset) { return positionInfoForOffset(offset); }
+#endif
+
     // An inline <script> shifts every line of its document, but shifts the column only on its first
     // line, since later lines begin where their own line begins.
     LineColumn documentLineColumnForOffset(unsigned offset)
     {
-        auto info = positionInfoForOffset(offset);
+        auto info = lineAndColumnInfoForOffset(offset);
         return {
             m_startPosition.m_line.oneBasedInt() + info.line0Based,
             info.line0Based ? info.column0Based + 1 : m_startPosition.m_column.oneBasedInt() + info.column0Based,
@@ -202,7 +218,7 @@ class JS_EXPORT_PRIVATE SourceProvider : public ThreadSafeRefCounted<SourceProvi
 
     LineColumn documentZeroBasedLineColumnForOffset(unsigned offset)
     {
-        auto info = positionInfoForOffset(offset);
+        auto info = lineAndColumnInfoForOffset(offset);
         return {
             m_startPosition.m_line.zeroBasedInt() + info.line0Based,
             info.line0Based ? info.column0Based : m_startPosition.m_column.zeroBasedInt() + info.column0Based,