JSC/WTF: startup snapshot support (immortal snapshot heap, restore hooks, deterministic placement)

CLAUDE.md

@@ -149,6 +149,10 @@ High-performance memory allocator with:
    - Stack trace improvements
    - Better error reporting for development
 
+5. **Startup Snapshots**
+   - `heap/Heap.cpp` (`freezeCurrentHeapAsImmortalStartupSnapshot`, `resetPacingAfterSnapshotRestore`), `runtime/VM.cpp` (`didRestoreFromStartupSnapshot`)
+   - Freezes the live heap as immortal so an embedder can write the process's memory out and resume later launches from it
+
 ### USE_BUN_EVENT_LOOP
 Custom event loop implementation for Bun's runtime requirements
 

Source/JavaScriptCore/bytecode/BytecodeRewriter.cpp

@@ -49,6 +49,7 @@ void BytecodeRewriter::applyModification()
             m_writer.m_instructions.insertVector(insertion.index.bytecodeOffset, insertion.instructions.m_instructions);
         }
     }
+    m_writer.didMutateBuffer();
     m_insertions.clear();
 }
 

Source/JavaScriptCore/bytecode/ExpressionInfo.cpp

@@ -898,6 +898,14 @@ std::unique_ptr<ExpressionInfo> ExpressionInfo::createUninitialized(unsigned num
     return std::unique_ptr<ExpressionInfo>(new (allocation) ExpressionInfo(numberOfChapters, numberOfEncodedInfo, numberOfEncodedInfoExtensions));
 }
 
+std::unique_ptr<ExpressionInfo> ExpressionInfo::createBorrowed(unsigned numberOfChapters, unsigned numberOfEncodedInfo, unsigned numberOfEncodedInfoExtensions, const unsigned* payload)
+{
+    void* allocation = FastMalloc::malloc(sizeof(ExpressionInfo));
+    auto info = std::unique_ptr<ExpressionInfo>(new (allocation) ExpressionInfo(numberOfChapters, numberOfEncodedInfo, numberOfEncodedInfoExtensions));
+    info->m_borrowedPayload = const_cast<unsigned*>(payload);
+    return info;
+}
+
 ExpressionInfo::ExpressionInfo(unsigned numberOfChapters, unsigned numberOfEncodedInfo, unsigned numberOfEncodedInfoExtensions)
     : m_numberOfChapters(numberOfChapters)
     , m_numberOfEncodedInfo(numberOfEncodedInfo)
@@ -914,6 +922,8 @@ ExpressionInfo::ExpressionInfo(Vector<Chapter>&& chapters, Vector<EncodedInfo>&&
 
 size_t ExpressionInfo::byteSize() const
 {
+    if (m_borrowedPayload) [[unlikely]]
+        return sizeof(ExpressionInfo); // the payload belongs to the cache mapping, not to this object
     return totalSizeInBytes(m_numberOfChapters, m_numberOfEncodedInfo, m_numberOfEncodedInfoExtensions);
 }
 

Source/JavaScriptCore/bytecode/ExpressionInfo.h

@@ -214,7 +214,7 @@ class ExpressionInfo {
 
     Chapter* chapters() const
     {
-        return std::bit_cast<Chapter*>(this + 1);
+        return std::bit_cast<Chapter*>(payload());
     }
 
     EncodedInfo* encodedInfo() const
@@ -239,10 +239,14 @@ class ExpressionInfo {
 
     unsigned* payload() const
     {
+        if (m_borrowedPayload) [[unlikely]]
+            return m_borrowedPayload;
         return std::bit_cast<unsigned*>(this + 1);
     }
 
     static std::unique_ptr<ExpressionInfo> createUninitialized(unsigned numberOfChapters, unsigned numberOfEncodedInfo, unsigned numberOfEncodedInfoExtensions);
+    // Header-only object whose (immutable) payload lives elsewhere, e.g. inside an mmap'd bytecode cache.
+    static std::unique_ptr<ExpressionInfo> createBorrowed(unsigned numberOfChapters, unsigned numberOfEncodedInfo, unsigned numberOfEncodedInfoExtensions, const unsigned* payload);
 
     static constexpr unsigned bitsPerWord = sizeof(unsigned) * CHAR_BIT;
 
@@ -323,6 +327,7 @@ class ExpressionInfo {
     unsigned m_numberOfChapters;
     unsigned m_numberOfEncodedInfo;
     unsigned m_numberOfEncodedInfoExtensions;
+    unsigned* m_borrowedPayload { nullptr };
     // Followed by the following which are allocated but are dynamically sized.
     //   Chapter chapters[numberOfChapters];
     //   EncodedInfo encodedInfo[numberOfEncodedInfo + numberOfEncodedInfoExtensions];