Eliminate size redundancy in the Object header

dfinity · Dec 20, 2023 · 0fe3926 · 0fe3926
1 parent c77355e
commit 0fe3926
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Showing 3 changed files with 8 additions and 6 deletions.
diff --git a/rts/motoko-rts/src/debug.rs b/rts/motoko-rts/src/debug.rs
@@ -142,7 +142,7 @@ pub(crate) unsafe fn print_boxed_object(buf: &mut WriteBuf, p: usize) {
             let _ = write!(
                 buf,
                 "<Object size={:#x} hash_ptr={:#x} field=[",
-                (*object).size,
+                object.size(),
                 (*object).hash_blob.get_raw()
             );
             for i in 0..object.size() {
@@ -155,7 +155,7 @@ pub(crate) unsafe fn print_boxed_object(buf: &mut WriteBuf, p: usize) {
                     let _ = write!(buf, ")");
                 }
 
-                if i != (*object).size - 1 {
+                if i != object.size() - 1 {
                     let _ = write!(buf, ",");
                 }
             }

diff --git a/rts/motoko-rts/src/stabilization/layout/stable_object.rs b/rts/motoko-rts/src/stabilization/layout/stable_object.rs
@@ -11,6 +11,7 @@ use super::{Serializer, StableToSpace, StableValue, StaticScanner};
 
 #[repr(C)]
 pub struct StableObject {
+    // The size information is redundant to the `hash_blob` word size but storing it simplifies and optimizes deserialization.
     size: u32, // Number of fields.
     hash_blob: StableValue, // Pointer to a blob containing the `u32` hashes of the field labels.
                // Dynamically sized body with `size` fields, each of `StableValue`, ordered according to the hashes in the blob.
@@ -30,7 +31,7 @@ impl StaticScanner<StableValue> for StableObject {
 impl Serializer<Object> for StableObject {
     unsafe fn serialize_static_part(main_object: *mut Object) -> Self {
         StableObject {
-            size: (*main_object).size,
+            size: main_object.size(),
             hash_blob: StableValue::serialize((*main_object).hash_blob),
         }
     }
@@ -70,7 +71,6 @@ impl Serializer<Object> for StableObject {
         (*target_object)
             .header
             .init_forward(Value::from_ptr(target_object as usize));
-        (*target_object).size = self.size;
         (*target_object).hash_blob = self.hash_blob.deserialize();
     }
 

diff --git a/rts/motoko-rts/src/types.rs b/rts/motoko-rts/src/types.rs
@@ -612,7 +612,6 @@ impl Region {
 #[repr(C)] // See the note at the beginning of this module
 pub struct Object {
     pub header: Obj,
-    pub size: u32,        // Number of elements
     pub hash_blob: Value, // Pointer to a blob containing the hashes of the object field labels.
 }
 
@@ -625,8 +624,11 @@ impl Object {
         self.add(1) as *mut Value // skip object header
     }
 
+    /// Number of fields in the object.
     pub(crate) unsafe fn size(self: *mut Self) -> u32 {
-        (*self).size
+        let hash_blob_length = (*self).hash_blob.as_blob().len().as_u32();
+        debug_assert_eq!(hash_blob_length % WORD_SIZE, 0);
+        hash_blob_length / WORD_SIZE
     }
 
     pub(crate) unsafe fn get(self: *mut Self, idx: u32) -> Value {