Track memory usage (#153)

* Track memory usage

* Track lowest_used in copy, etc.

* Extra comment for used_size

src/memory.rs

@@ -58,6 +58,7 @@ pub struct MemoryInstance {
     initial: Pages,
     current_size: Cell<usize>,
     maximum: Option<Pages>,
+    lowest_used: Cell<u32>,
 }
 
 impl fmt::Debug for MemoryInstance {
@@ -127,6 +128,7 @@ impl MemoryInstance {
             initial: initial,
             current_size: Cell::new(initial_size.0),
             maximum: maximum,
+            lowest_used: Cell::new(u32::max_value()),
         }
     }
 
@@ -148,6 +150,16 @@ impl MemoryInstance {
         self.maximum
     }
 
+    /// Returns lowest offset ever written or `u32::max_value()` if none.
+    pub fn lowest_used(&self) -> u32 {
+        self.lowest_used.get()
+    }
+
+    /// Resets tracked lowest offset.
+    pub fn reset_lowest_used(&self, addr: u32) {
+        self.lowest_used.set(addr)
+    }
+
     /// Returns current linear memory size.
     ///
     /// Maximum memory size cannot exceed `65536` pages or 4GiB.
@@ -171,6 +183,12 @@ impl MemoryInstance {
         Bytes(self.current_size.get()).round_up_to()
     }
 
+    /// Returns current used memory size in bytes.
+	/// This is the highest memory address that had been written to.
+    pub fn used_size(&self) -> Bytes {
+        Bytes(self.buffer.borrow().len())
+    }
+
     /// Get value from memory at given offset.
     pub fn get_value<T: LittleEndianConvert>(&self, offset: u32) -> Result<T, Error> {
         let mut buffer = self.buffer.borrow_mut();
@@ -213,6 +231,9 @@ impl MemoryInstance {
             .checked_region(&mut buffer, offset as usize, value.len())?
             .range();
 
+        if offset < self.lowest_used.get() {
+            self.lowest_used.set(offset);
+        }
         buffer[range].copy_from_slice(value);
 
         Ok(())
@@ -224,6 +245,9 @@ impl MemoryInstance {
         let range = self
             .checked_region(&mut buffer, offset as usize, ::core::mem::size_of::<T>())?
             .range();
+        if offset < self.lowest_used.get() {
+            self.lowest_used.set(offset);
+        }
         value.into_little_endian(&mut buffer[range]);
         Ok(())
     }
@@ -368,6 +392,10 @@ impl MemoryInstance {
         let (read_region, write_region) =
             self.checked_region_pair(&mut buffer, src_offset, len, dst_offset, len)?;
 
+		if dst_offset < self.lowest_used.get() as usize {
+			self.lowest_used.set(dst_offset as u32);
+		}
+
         unsafe {
             ::core::ptr::copy(
                 buffer[read_region.range()].as_ptr(),
@@ -407,6 +435,10 @@ impl MemoryInstance {
             )));
         }
 
+		if dst_offset < self.lowest_used.get() as usize {
+			self.lowest_used.set(dst_offset as u32);
+		}
+
         unsafe {
             ::core::ptr::copy_nonoverlapping(
                 buffer[read_region.range()].as_ptr(),
@@ -446,6 +478,10 @@ impl MemoryInstance {
             .checked_region(&mut dst_buffer, dst_offset, len)?
             .range();
 
+		if dst_offset < dst.lowest_used.get() as usize {
+			dst.lowest_used.set(dst_offset as u32);
+		}
+
         dst_buffer[dst_range].copy_from_slice(&src_buffer[src_range]);
 
         Ok(())
@@ -462,6 +498,11 @@ impl MemoryInstance {
         let mut buffer = self.buffer.borrow_mut();
 
         let range = self.checked_region(&mut buffer, offset, len)?.range();
+
+		if offset < self.lowest_used.get() as usize {
+			self.lowest_used.set(offset as u32);
+		}
+
         for val in &mut buffer[range] {
             *val = new_val
         }