Review comments.

crates/runtime/src/instance.rs

@@ -460,6 +460,7 @@ impl Instance {
     /// (imported or defined in this module) and store into the given
     /// location. Used during lazy initialization the first time the
     /// VMCallerCheckedAnyfunc in the VMContext is referenced.
+    #[cold]
     fn construct_anyfunc(&mut self, index: FuncIndex, into: *mut VMCallerCheckedAnyfunc) {
         let sig = self.module.functions[index];
         let type_index = self.runtime_info.shared_signatures().lookup(sig);

crates/runtime/src/instance/allocator.rs

@@ -481,12 +481,17 @@ fn initialize_instance(
 
 /// Initialize the VMContext data associated with an Instance.
 ///
-/// Precondition: the VMContext memory must be zeroed. We omit writes
-/// here to fields that should be initialized to zero. This allows the
-/// caller to use an efficient means of zeroing memory (such as using
-/// anonymous-mmap'd zero pages) if available, rather than falling
-/// back onto a memset (or the manual equivalent) here.
-unsafe fn initialize_vmcontext(instance: &mut Instance, req: InstanceAllocationRequest) {
+/// Optimization: if `pre_zeroed` is set, then the VMContext memory
+/// (following the `Instance`) is assumed to be zeroed. If so, we omit
+/// writes here to fields that should be initialized to zero. This
+/// allows the caller to use an efficient means of zeroing memory
+/// (such as using anonymous-mmap'd zero pages) if available, rather
+/// than falling back onto a memset (or the manual equivalent) here.
+unsafe fn initialize_vmcontext(
+    instance: &mut Instance,
+    req: InstanceAllocationRequest,
+    pre_zeroed: bool,
+) {
     if let Some(store) = req.store.as_raw() {
         *instance.interrupts() = (*store).vminterrupts();
         *instance.epoch_ptr() = (*store).epoch_ptr();
@@ -536,9 +541,27 @@ unsafe fn initialize_vmcontext(instance: &mut Instance, req: InstanceAllocationR
         req.imports.globals.len(),
     );
 
-    // N.B.: no need to initialize the anyfuncs array; it is zeroed
-    // already (by the precondition for this function) and the lazy
-    // init will fill in entries as we take pointers to them.
+    // N.B.: if `pre_zeroed`, then there is no need to initialize the
+    // anyfuncs array; it is zeroed already (by the precondition for
+    // this function) and the lazy init will fill in entries as we
+    // take pointers to them.
+    //
+    // If so, we'll debug_assert that it's all zeroes; if not, we'll
+    // fill it.
+    let anyfuncs_bytes = std::slice::from_raw_parts_mut(
+        instance.anyfunc_base() as *mut u8,
+        instance
+            .module
+            .functions
+            .len()
+            .checked_mul(std::mem::size_of::<VMCallerCheckedAnyfunc>())
+            .unwrap(),
+    );
+    if pre_zeroed {
+        debug_assert!(anyfuncs_bytes.iter().all(|b| *b == 0));
+    } else {
+        anyfuncs_bytes.fill(0);
+    }
 
     // Initialize the defined tables
     let mut ptr = instance.vmctx_plus_offset(instance.offsets.vmctx_tables_begin());
@@ -705,7 +728,7 @@ unsafe impl InstanceAllocator for OnDemandInstanceAllocator {
 
         let host_state = std::mem::replace(&mut req.host_state, Box::new(()));
 
-        let (mut handle, vmctx_data) = {
+        let mut handle = {
             let instance = Instance::create_raw(
                 &req.module,
                 req.runtime_info.clone(),
@@ -724,25 +747,10 @@ unsafe impl InstanceAllocator for OnDemandInstanceAllocator {
 
             ptr::write(instance_ptr, instance);
 
-            let vmctx_data = {
-                let vmctx_ptr = (*instance_ptr).vmctx_ptr() as *mut u8;
-                let vmctx_len = (*instance_ptr).offsets.size_of_vmctx() as usize;
-                std::slice::from_raw_parts_mut(vmctx_ptr, vmctx_len)
-            };
-
-            (handle, vmctx_data)
+            handle
         };
 
-        // Zero the VMContext memory first; `initialize_vmcontext()`
-        // requires this.
-        vmctx_data.fill(0);
-        // Drop the &mut slice over the VMContext data before writing
-        // to it via `initialize_vmcontext()` below; we should not let
-        // its lifetime extend over the function call that initializes
-        // the VMContext.
-        drop(vmctx_data);
-
-        initialize_vmcontext(handle.instance_mut(), req);
+        initialize_vmcontext(handle.instance_mut(), req, /* pre_zeroed = */ false);
 
         Ok(handle)
     }

crates/runtime/src/instance/allocator/pooling.rs

@@ -399,7 +399,7 @@ impl InstancePool {
         // will use either madvise(MADV_DONTNEED) (on Linux) or
         // mmap-of-fresh-anonymous-memory to efficiently zero the
         // instance state.
-        initialize_vmcontext(instance, req);
+        initialize_vmcontext(instance, req, /* pre_zeroed = */ true);
 
         Ok(InstanceHandle {
             instance: instance as _,