Improve arena allocator a bit

Author: dcodeIO

std/assembly/memory/arena.ts

@@ -1,23 +1,25 @@
-// An simple arena allocator that provides a `clear_memory` function to reset
-// everything to the initial state. A user must have to make sure that there
-// a no more references to cleared memory.
+// A simple arena allocator that provides a `clear_memory` function to reset
+// the heap to its initial state. A user has to make sure that there are no
+// more references to cleared memory afterwards. Always aligns to 8 bytes.
 
 const ALIGN_LOG2: usize = 3;
 const ALIGN_SIZE: usize = 1 << ALIGN_LOG2;
 const ALIGN_MASK: usize = ALIGN_SIZE - 1;
 
-var HEAP_OFFSET: usize = HEAP_BASE; // generated by the compiler
+var HEAP_OFFSET: usize = HEAP_BASE;
 
 export function allocate_memory(size: usize): usize {
   if (!size) return 0;
-  var len: i32 = current_memory();
-  if (HEAP_OFFSET + size > <usize>len << 16)
-    if(grow_memory(max<i32>(<i32>ceil<f64>(<f64>size / 65536), len * 2 - len)) < 0)
-      unreachable();
-  var ptr: usize = HEAP_OFFSET;
-  if ((HEAP_OFFSET += size) & ALIGN_MASK) // align next offset
-    HEAP_OFFSET = (HEAP_OFFSET | ALIGN_MASK) + 1;
-  set_memory(ptr, 0, size);
+  var ptr = HEAP_OFFSET;
+  var off = (ptr + size + ALIGN_MASK) & ~ALIGN_MASK;
+  var avail = <usize>current_memory() << 16;
+  if (off > avail && grow_memory(
+    max(
+      (((off + 0xffff) & ~0xffff) - avail) >> 16, // minimum required pages
+      avail                                >> 16  // at least double memory
+    )
+  ) < 0) unreachable(); // out of memory
+  HEAP_OFFSET = off;
   return ptr;
 }