YJIT: Skip adding past_page_bytes for past pages (#8433)

YJIT: Skip adding past_pages_bytes for past pages

yjit/src/asm/mod.rs

@@ -59,6 +59,9 @@ pub struct CodeBlock {
     // Current writing position
     write_pos: usize,
 
+    // The index of the last page with written bytes
+    last_page_idx: usize,
+
     // Total number of bytes written to past pages
     past_page_bytes: usize,
 
@@ -119,6 +122,7 @@ impl CodeBlock {
             mem_size,
             page_size,
             write_pos: 0,
+            last_page_idx: 0,
             past_page_bytes: 0,
             page_end_reserve: 0,
             label_addrs: Vec::new(),
@@ -203,11 +207,15 @@ impl CodeBlock {
                 assert!(!cb.has_dropped_bytes());
             });
 
-            // Update past_page_bytes for code_size()
-            self.past_page_bytes += self.current_page_bytes();
+            // Update past_page_bytes for code_size() if this is a new page
+            if self.last_page_idx < page_idx {
+                self.past_page_bytes += self.current_page_bytes();
+            }
 
             // Start the next code from dst_pos
             self.write_pos = dst_pos;
+            // Update the last_page_idx if page_idx points to the furthest page
+            self.last_page_idx = usize::max(self.last_page_idx, page_idx);
         }
         !self.dropped_bytes
     }
@@ -805,6 +813,7 @@ mod tests
 
     #[test]
     fn test_code_size() {
+        // Write 4 bytes in the first page
         let mut cb = CodeBlock::new_dummy(CodeBlock::PREFERRED_CODE_PAGE_SIZE * 2);
         cb.write_bytes(&[0, 0, 0, 0]);
         assert_eq!(cb.code_size(), 4);
@@ -813,7 +822,18 @@ mod tests
         cb.next_page(cb.get_write_ptr(), |_, _| {});
         assert_eq!(cb.code_size(), 4);
 
+        // Write 4 bytes in the second page
         cb.write_bytes(&[0, 0, 0, 0]);
         assert_eq!(cb.code_size(), 8);
+
+        // Rewrite 4 bytes in the first page
+        let old_write_pos = cb.get_write_pos();
+        cb.set_pos(0);
+        cb.write_bytes(&[1, 1, 1, 1]);
+
+        // Moving from an old page to the next page should not increase code_size
+        cb.next_page(cb.get_write_ptr(), |_, _| {});
+        cb.set_pos(old_write_pos);
+        assert_eq!(cb.code_size(), 8);
     }
 }