Blob: stop reporting a shared store's bytes to the GC as newly allocated for every view

.claude/skills/javascriptcore-garbage-collector/SKILL.md

@@ -276,7 +276,7 @@ visitor.reportExtraMemoryVisited(thisObject->wrapped().byteSize());
 - If the size changes over time, report the delta on growth (`reportExtraMemoryAllocated(cell, newSize - oldSize)`) and report the current size in `visitChildren`.
 - `deprecatedReportExtraMemory` exists for callers that can't satisfy the visit-side half — avoid it.
 
-In `.classes.ts`, `estimatedSize: true` generates the `reportExtraMemoryVisited` side; you implement `estimated_size()` in Rust. You still call `reportExtraMemoryAllocated` (or the binding's helper) at allocation time.
+In `.classes.ts`, `estimatedSize: true` generates both halves from one Rust method: the generated constructor and `${T}__create` call the generated `JS${T}::reportExtraMemoryAllocated(vm)` once the wrapper exists, and `visitChildren` re-reports `estimated_size()` as visited. Hand-written C++ that creates one of these wrappers itself (`JSBunRequest`, `JSBakeResponse`, the shell interpreter; `JSDOMFile` still needs it) calls the same member rather than reporting a size by hand. If the object may merely take a reference to memory another wrapper already reported (a `Blob` sharing its store with the blob it was sliced from), add `newlyAllocatedSize: true` and implement `newly_allocated_size()`: the member then reports that instead, while visits keep re-reporting `estimated_size()`. Otherwise every view reports the whole payload as a fresh allocation and JSC collects after every few views.
 
 ## `HeapAnalyzer` — heap snapshots and labelling
 

bench/snippets/blob.mjs

@@ -23,6 +23,13 @@ bench("blob.slice()", function () {
   return small.slice();
 });
 
+// slice() shares the source's bytes, so this only differs from the small case
+// by what each new Blob tells the GC.
+var large = new Blob([new Uint8Array(8 * 1024 * 1024)]);
+bench("blob.slice() (8 MiB blob)", function () {
+  return large.slice();
+});
+
 if ((await small.text()) !== JSON.stringify("hello world ")) {
   throw new Error("blob.text() failed");
 }

src/codegen/class-definitions.ts

@@ -229,8 +229,20 @@ export class ClassDefinition {
    * ```
    *
    * Report `size_of::<Self>()` as well as any external allocations.
+   *
+   * Reported once per wrapper by the generated `JS${name}::reportExtraMemoryAllocated(vm)`
+   * and again from `visitChildren` on every GC.
    */
   estimatedSize?: boolean;
+  /**
+   * Requires `estimatedSize`. `JS${name}::reportExtraMemoryAllocated(vm)` reports this
+   * instead of `estimated_size()`, for objects that may only take a reference to memory
+   * another wrapper already reported (a Blob sharing its store). Called on the JS thread.
+   * ```rust
+   * pub fn newly_allocated_size(&self) -> usize;
+   * ```
+   */
+  newlyAllocatedSize?: boolean;
   /**
    * Used in heap snapshots.
    *

src/codegen/generate-classes.ts

@@ -59,6 +59,11 @@ function DOMJITName(fnName) {
   return `${fnName}WithoutTypeChecks`;
 }
 
+// Emitted once `instance->m_ctx` is set at every generated wrapper creation site.
+function reportExtraMemoryAllocated(obj: ClassDefinition) {
+  return obj.estimatedSize ? `instance->reportExtraMemoryAllocated(vm);` : "";
+}
+
 function argTypeName(arg) {
   return {
     ["bool"]: "bool",
@@ -652,13 +657,7 @@ ${
 ${
   obj.call
     ? `    RETURN_IF_EXCEPTION(scope, {});
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
 
     RELEASE_AND_RETURN(scope, JSValue::encode(instance));`
     : ""
@@ -708,13 +707,7 @@ JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES ${name}::construct(JSC::JSGlobalObj
     instance->m_ctx = ptr;
     `) +
     `
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
 
     auto value = JSValue::encode(instance);
     RELEASE_AND_RETURN(scope, value);
@@ -1294,6 +1287,13 @@ function generateClassHeader(typeName, obj: ClassDefinition) {
 
   if (obj.estimatedSize) {
     externs += `extern JSC_CALLCONV size_t ${symbolName(typeName, "estimatedSize")}(void* ptr);` + "\n";
+    if (obj.newlyAllocatedSize) {
+      externs += `extern JSC_CALLCONV size_t ${symbolName(typeName, "newlyAllocatedSize")}(void* ptr);` + "\n";
+    }
+  } else if (obj.newlyAllocatedSize) {
+    throw new Error(
+      `${typeName}: 'newlyAllocatedSize' only replaces the allocation-time half of 'estimatedSize'; set 'estimatedSize: true' as well.`,
+    );
   }
 
   for (const a of [...Object.values(klass), ...Object.values(proto)]) {
@@ -1415,7 +1415,17 @@ function generateClassHeader(typeName, obj: ClassDefinition) {
          * Memory cost of the object from Zig, without necessarily having a JS wrapper alive.
          */
         static size_t memoryCost(void* ptr);
-
+${
+  obj.estimatedSize
+    ? `
+        /**
+         * Call once per wrapper, after m_ctx is set. Reports estimatedSize, or
+         * newlyAllocatedSize when the class definition sets it.
+         */
+        void reportExtraMemoryAllocated(JSC::VM& vm);
+`
+    : ""
+}
         void* m_ctx { nullptr };
 
         ${name}(JSC::VM& vm, JSC::Structure* structure, void* sinkPtr${obj.valuesArray ? ", WTF::FixedVector<JSC::WriteBarrier<JSC::Unknown>>&& jsvalueArray_" : ""})
@@ -1657,6 +1667,15 @@ size_t ${name}::memoryCost(void* ptr) {
   `;
   }
 
+  if (obj.estimatedSize) {
+    const sizeFn = symbolName(typeName, obj.newlyAllocatedSize ? "newlyAllocatedSize" : "estimatedSize");
+    output += `
+void ${name}::reportExtraMemoryAllocated(JSC::VM& vm) {
+  vm.heap.reportExtraMemoryAllocated(this, ${sizeFn}(m_ctx));
+}
+`;
+  }
+
   output += `
 
 size_t ${name}::estimatedSize(JSC::JSCell* cell, JSC::VM& vm) {
@@ -1809,13 +1828,7 @@ extern JSC_CALLCONV JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES ${typeName}__cr
   auto &vm = globalObject->vm();
   JSC::Structure* structure = globalObject->${className(typeName)}Structure();
   ${className(typeName)}* instance = ${className(typeName)}::create(vm, globalObject, structure, ptr);
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
   return JSValue::encode(instance);
 }
 
@@ -1830,13 +1843,7 @@ ${
     jsvalueArray[i].setWithoutWriteBarrier(args->at(i));
   }
   ${className(typeName)}* instance = ${className(typeName)}::create(vm, globalObject, structure, ptr, WTF::move(jsvalueArray));
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
   return JSValue::encode(instance);
 }`
     : ""
@@ -1848,13 +1855,7 @@ ${
   auto &vm = globalObject->vm();
   JSC::Structure* structure = globalObject->${className(typeName)}Structure();
   ${className(typeName)}* instance = ${className(typeName)}::create(vm, globalObject, structure, ptr${obj.values.map(v => `, JSC::JSValue::decode(${v})`).join("")});
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
   return JSValue::encode(instance);
 }`
     : ""
@@ -1871,13 +1872,7 @@ ${
     jsvalueArray[i].setWithoutWriteBarrier(args->at(i));
   }
   ${className(typeName)}* instance = ${className(typeName)}::create(vm, globalObject, structure, ptr, WTF::move(jsvalueArray)${obj.values.map(v => `, JSC::JSValue::decode(${v})`).join("")});
-  ${
-    obj.estimatedSize
-      ? `
-      auto size = ${symbolName(typeName, "estimatedSize")}(ptr);
-      vm.heap.reportExtraMemoryAllocated(instance, size);`
-      : ""
-  }
+  ${reportExtraMemoryAllocated(obj)}
   return JSValue::encode(instance);
 }`
     : ""
@@ -2184,6 +2179,7 @@ function generateRust(
     noConstructor = false,
     overridesToJS = false,
     estimatedSize,
+    newlyAllocatedSize = false,
     call = false,
     memoryCost,
     values = [],
@@ -2248,6 +2244,13 @@ function generateRust(
   }
   if (estimatedSize) {
     thunk(symbolName(typeName, "estimatedSize"), `(this: &${T}) -> usize`, `    ${T}::estimated_size(this)`);
+    if (newlyAllocatedSize) {
+      thunk(
+        symbolName(typeName, "newlyAllocatedSize"),
+        `(this: &${T}) -> usize`,
+        `    ${T}::newly_allocated_size(this)`,
+      );
+    }
   }
   if (!memoryCost && !estimatedSize) {
     symbols.push(symbolName(typeName, "ZigStructSize"));

src/jsc/bindings/JSBakeResponse.cpp

@@ -38,7 +38,6 @@ extern "C" SYSV_ABI JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES ResponseClass__
 extern "C" SYSV_ABI JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES ResponseClass__constructJSON(JSC::JSGlobalObject*, JSC::CallFrame*);
 extern "C" SYSV_ABI JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES BakeResponseClass__constructRender(JSC::JSGlobalObject*, JSC::CallFrame*);
 extern "C" SYSV_ABI JSC::EncodedJSValue JSC_HOST_CALL_ATTRIBUTES BakeResponseClass__constructRedirect(JSC::JSGlobalObject*, JSC::CallFrame*);
-extern JSC_CALLCONV size_t Response__estimatedSize(void* ptr);
 
 bool isJSXElement(JSC::EncodedJSValue JSValue0, JSC::JSGlobalObject* globalObject)
 {
@@ -224,8 +223,7 @@ class JSBakeResponseConstructor final : public JSC::InternalFunction {
             instance->wrapInnerComponent(globalObject, vm, arg, responseOptions);
         }
 
-        auto size = Response__estimatedSize(ptr);
-        vm.heap.reportExtraMemoryAllocated(instance, size);
+        instance->reportExtraMemoryAllocated(vm);
 
         auto value = JSValue::encode(instance);
         RELEASE_AND_RETURN(scope, value);
@@ -250,8 +248,7 @@ class JSBakeResponseConstructor final : public JSC::InternalFunction {
 
         RETURN_IF_EXCEPTION(scope, {});
 
-        auto size = Response__estimatedSize(ptr);
-        vm.heap.reportExtraMemoryAllocated(instance, size);
+        instance->reportExtraMemoryAllocated(vm);
 
         RELEASE_AND_RETURN(scope, JSValue::encode(instance));
     }

src/jsc/bindings/JSBunRequest.cpp

@@ -156,14 +156,10 @@ JSBunRequest::JSBunRequest(JSC::VM& vm, JSC::Structure* structure, void* sinkPtr
     , m_cookies(nullptr, JSC::WriteBarrierEarlyInit)
 {
 }
-extern SYSV_ABI "C" size_t Request__estimatedSize(void* requestPtr);
-extern "C" void Bun__JSRequest__calculateEstimatedByteSize(void* requestPtr);
 void JSBunRequest::finishCreation(JSC::VM& vm)
 {
     Base::finishCreation(vm);
-
-    auto size = Request__estimatedSize(this->wrapped());
-    vm.heap.reportExtraMemoryAllocated(this, size);
+    reportExtraMemoryAllocated(vm);
 }
 
 template<typename Visitor>

src/jsc/bindings/ShellBindings.cpp

@@ -2,8 +2,6 @@
 
 #include "ZigGeneratedClasses.h"
 
-extern "C" SYSV_ABI size_t ShellInterpreter__estimatedSize(void* ptr);
-
 namespace Bun {
 
 using namespace JSC;
@@ -23,9 +21,7 @@ extern "C" SYSV_ABI EncodedJSValue Bun__createShellInterpreter(Zig::GlobalObject
     ASSERT(structure);
 
     auto* result = WebCore::JSShellInterpreter::create(vm, globalObject, structure, ptr, WTF::move(args), resolveFn, rejectFn);
-
-    size_t size = ShellInterpreter__estimatedSize(ptr);
-    vm.heap.reportExtraMemoryAllocated(result, size);
+    result->reportExtraMemoryAllocated(vm);
     return JSValue::encode(result);
 }
 

src/runtime/webcore/Blob.rs

@@ -387,6 +387,7 @@ pub trait BlobExt {
         Self: Sized;
     fn calculate_estimated_byte_size(&self);
     fn estimated_size(&self) -> usize;
+    fn newly_allocated_size(&self) -> usize;
     fn to_js(&self, global_object: &JSGlobalObject) -> JSValue;
     fn find_or_create_file_from_path(
         path_or_fd: &mut PathOrFileDescriptor,
@@ -3521,36 +3522,20 @@ impl BlobExt for Blob {
     // is_detached: defined once above; duplicate removed to fix E0034.
 
     fn calculate_estimated_byte_size(&self) {
-        // in-memory size. not the size on disk.
-        let mut size: usize = core::mem::size_of::<Blob>();
-
-        if let Some(store) = self.store.get() {
-            size += core::mem::size_of::<Store>();
-            match &store.data {
-                store::Data::Bytes(bytes) => {
-                    size += bytes.stored_name.len();
-                    size += if self.size.get() != MAX_SIZE {
-                        self.size.get() as usize
-                    } else {
-                        bytes.len() as usize
-                    };
-                }
-                store::Data::File(file) => size += file.pathlike.estimated_size(),
-                store::Data::S3(s3) => size += s3.estimated_size(),
-            }
-        }
-
-        let ct = self.content_type.get();
-        self.reported_estimated_size.set(
-            size + (ct.as_slice().len() * (ct.is_owned() as usize))
-                + self.name.get().byte_slice().len(),
-        );
+        self.reported_estimated_size
+            .set(estimate_in_memory_size(self, true));
     }
 
     fn estimated_size(&self) -> usize {
         self.reported_estimated_size.get()
     }
 
+    /// A store shared with other Blobs (`slice()`, `dupe()`) is already accounted
+    /// for through them; re-reporting it per view made JSC collect every few views.
+    fn newly_allocated_size(&self) -> usize {
+        estimate_in_memory_size(self, self.store().is_none_or(|store| store.has_one_ref()))
+    }
+
     fn to_js(&self, global_object: &JSGlobalObject) -> JSValue {
         // if cfg!(debug_assertions) { debug_assert!(self.is_heap_allocated()); }
         self.calculate_estimated_byte_size();
@@ -3700,6 +3685,32 @@ impl BlobExt for Blob {
     }
 }
 
+/// In-memory size, not the size on disk.
+fn estimate_in_memory_size(blob: &Blob, include_store: bool) -> usize {
+    let mut size: usize = core::mem::size_of::<Blob>();
+
+    if include_store {
+        if let Some(store) = blob.store() {
+            size += core::mem::size_of::<Store>();
+            match &store.data {
+                store::Data::Bytes(bytes) => {
+                    size += bytes.stored_name.len();
+                    size += if blob.size.get() != MAX_SIZE {
+                        blob.size.get() as usize
+                    } else {
+                        bytes.len() as usize
+                    };
+                }
+                store::Data::File(file) => size += file.pathlike.estimated_size(),
+                store::Data::S3(s3) => size += s3.estimated_size(),
+            }
+        }
+    }
+
+    let ct = blob.content_type.get();
+    size + (ct.as_slice().len() * (ct.is_owned() as usize)) + blob.name.get().byte_slice().len()
+}
+
 // ──────────────────────────────────────────────────────────────────────────
 // Basic accessors
 // ──────────────────────────────────────────────────────────────────────────

src/runtime/webcore/response.classes.ts

@@ -155,6 +155,8 @@ export default [
       storable: true,
     },
     estimatedSize: true,
+    // slice() / dupe() share the source's store; see Blob::newly_allocated_size.
+    newlyAllocatedSize: true,
     values: ["stream"],
     overridesToJS: true,
     proto: {