diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index c1f498b6ac..919c86e35e 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -108,20 +108,18 @@ pub fn malloc(n int) byteptr {
 	if n < 0 {
 		panic('malloc(<0)')
 	}
-	/*
-	$if debug {
+	$if prealloc {
 		res := g_m2_ptr
 		g_m2_ptr += n
 		nr_mallocs++
 		return res
 	} $else {
-		*/
 		ptr := C.malloc(n)
 		if ptr == 0 {
 			panic('malloc($n) failed')
 		}
 		return ptr
-	//}
+	}
 /*
 TODO
 #ifdef VPLAY
diff --git a/vlib/compiler/main.v b/vlib/compiler/main.v
index d176a791f9..79a5734ec1 100644
--- a/vlib/compiler/main.v
+++ b/vlib/compiler/main.v
@@ -230,6 +230,9 @@ pub fn (v mut V) compile() {
 			cgen.genln('#define VDEBUG (1)')
 		}
 	}
+	if v.pref.prealloc {
+		cgen.genln('#define VPREALLOC (1)')
+	}
 	if v.os == .js {
 		cgen.genln('#define _VJS (1) ')
 	}