From 083dc23db8ce409a41f27e668ee54f99e158ade8 Mon Sep 17 00:00:00 2001
From: Delyan Angelov <delian66@gmail.com>
Date: Fri, 8 Jan 2021 14:56:55 +0200
Subject: [PATCH] gen: implement a `[manualfree]` tag, for functions, that want
 to do their own memory management

---
 CHANGELOG.md                  |  1 +
 cmd/v/help/build-c.txt        |  7 ++++
 doc/docs.md                   |  3 +-
 vlib/v/ast/ast.v              |  1 +
 vlib/v/gen/auto_str_methods.v |  3 +-
 vlib/v/gen/cgen.v             | 70 +++++++++++++++++------------------
 vlib/v/gen/cmain.v            | 10 ++---
 vlib/v/gen/fn.v               | 25 +++++++++----
 vlib/v/parser/fn.v            |  2 +
 vlib/v/pref/pref.v            |  8 +++-
 10 files changed, 77 insertions(+), 53 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 98a24e2b92..39540f10ee 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,7 @@
 - `byte.str()` has been fixed and works like with all other numbers. `byte.ascii_str()` has been added.
 - Smart cast in for loops: `for mut x is string {}`.
 - `[noinit]` struct attribute to disallow direct struct initialization with `Foo{}`.
+- `[manualfree]` attribute for functions, that want to do their own memory management.
 
 ## V 0.2.1
 *30 Dec 2020*
diff --git a/cmd/v/help/build-c.txt b/cmd/v/help/build-c.txt
index 94a103efb8..fffc8f944c 100644
--- a/cmd/v/help/build-c.txt
+++ b/cmd/v/help/build-c.txt
@@ -67,3 +67,10 @@ These build flags are enabled on `build` and `run` as long as the backend is set
 
    -keepc
       Do not remove the temporary .tmp.c and .tmp.c.rsp files. Also do not use a random prefix for them, so they would be fixed and predictable.
+
+   -autofree
+      Free memory used in functions automatically.
+      
+   -manualfree
+      Do not free memory used in functions (the developer has to put x.free() and unsafe{free(x)} calls manually in this mode).
+      Some short lived applications, like compilers and other CLI tools are more performant without autofree.
diff --git a/doc/docs.md b/doc/docs.md
index 2e531a2b8c..c33533e151 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -2517,7 +2517,8 @@ Python, Go, or Java, except there's no heavy GC tracing everything or expensive
 each object.
 
 For developers willing to have more low level control, autofree can be disabled with
-`-noautofree`.
+`-manualfree`, or by adding a `[manualfree]` on each function that wants manage its 
+memory manually.
 
 Note: right now autofree is hidden behind the -autofree flag. It will be enabled by
 default in V 0.3.
diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index e92618e894..0309d64615 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -296,6 +296,7 @@ pub:
 	is_pub          bool
 	is_variadic     bool
 	is_anon         bool
+	is_manualfree   bool // true, when [manualfree] is used on a fn
 	receiver        Field
 	receiver_pos    token.Position // `(u User)` in `fn (u User) name()` position
 	is_method       bool
diff --git a/vlib/v/gen/auto_str_methods.v b/vlib/v/gen/auto_str_methods.v
index 05fdd8696f..84df234f85 100644
--- a/vlib/v/gen/auto_str_methods.v
+++ b/vlib/v/gen/auto_str_methods.v
@@ -3,7 +3,6 @@
 module gen
 
 import v.table
-import v.pref
 import v.util
 
 fn (mut g Gen) gen_str_default(sym table.TypeSymbol, styp string, str_fn_name string) {
@@ -230,7 +229,7 @@ fn (mut g Gen) gen_str_for_array(info table.Array, styp string, str_fn_name stri
 		}
 	}
 	g.auto_str_funcs.writeln('\t\tstrings__Builder_write(&sb, x);')
-	if g.pref.autofree && typ != table.bool_type {
+	if g.is_autofree && typ != table.bool_type {
 		// no need to free "true"/"false" literals
 		g.auto_str_funcs.writeln('\t\tstring_free(&x);')
 	}
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 0b61ef6776..ffa766bddc 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -78,7 +78,7 @@ mut:
 	stmt_path_pos                    []int // positions of each statement start, for inserting C statements before the current statement
 	skip_stmt_pos                    bool  // for handling if expressions + autofree (since both prepend C statements)
 	right_is_opt                     bool
-	autofree                         bool
+	is_autofree                      bool // false, inside the bodies of fns marked with [manualfree], otherwise === g.pref.autofree    
 	indent                           int
 	empty_line                       bool
 	is_test                          bool
@@ -186,7 +186,7 @@ pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 		table: table
 		pref: pref
 		fn_decl: 0
-		autofree: true
+		is_autofree: true
 		indent: -1
 		module_built: module_built
 		timers: util.new_timers(timers_should_print)
@@ -216,9 +216,9 @@ pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 		if g.file.path == '' || !g.pref.autofree {
 			// cgen test or building V
 			// println('autofree=false')
-			g.autofree = false
+			g.is_autofree = false
 		} else {
-			g.autofree = true
+			g.is_autofree = true
 			autofree_used = true
 		}
 		// anon fn may include assert and thus this needs
@@ -232,7 +232,7 @@ pub fn cgen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 	}
 	g.timers.start('cgen common')
 	if autofree_used {
-		g.autofree = true // so that void _vcleanup is generated
+		g.is_autofree = true // so that void _vcleanup is generated
 	}
 	// to make sure type idx's are the same in cached mods
 	if g.pref.build_mode == .build_module {
@@ -799,7 +799,7 @@ fn (mut g Gen) stmts_with_tmp_var(stmts []ast.Stmt, tmp_var string) {
 		g.write('')
 		g.write(')')
 	}
-	if g.pref.autofree && !g.inside_vweb_tmpl && stmts.len > 0 {
+	if g.is_autofree && !g.inside_vweb_tmpl && stmts.len > 0 {
 		// use the first stmt to get the scope
 		stmt := stmts[0]
 		// stmt := stmts[stmts.len-1]
@@ -878,7 +878,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 				}
 			} else {
 				// continue or break
-				if g.pref.autofree && !g.is_builtin_mod {
+				if g.is_autofree && !g.is_builtin_mod {
 					g.writeln('// free before continue/break')
 					g.autofree_scope_vars_stop(node.pos.pos - 1, node.pos.line_nr, true,
 						g.branch_parent_pos)
@@ -935,7 +935,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 		}
 		ast.ExprStmt {
 			g.write_v_source_line_info(node.pos)
-			// af := g.pref.autofree && node.expr is ast.CallExpr && !g.is_builtin_mod
+			// af := g.autofree && node.expr is ast.CallExpr && !g.is_builtin_mod
 			// if af {
 			// g.autofree_call_pregen(node.expr as ast.CallExpr)
 			// }
@@ -1129,9 +1129,9 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 		}
 		ast.Return {
 			g.write_defer_stmts_when_needed()
-			// af := g.pref.autofree && node.exprs.len > 0 && node.exprs[0] is ast.CallExpr && !g.is_builtin_mod
+			// af := g.autofree && node.exprs.len > 0 && node.exprs[0] is ast.CallExpr && !g.is_builtin_mod
 			/*
-			af := g.pref.autofree && !g.is_builtin_mod
+			af := g.autofree && !g.is_builtin_mod
 			if false && af {
 				g.writeln('// ast.Return free')
 				g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
@@ -1172,7 +1172,7 @@ fn (mut g Gen) stmt(node ast.Stmt) {
 	}
 	// If we have temporary string exprs to free after this statement, do it. e.g.:
 	// `foo('a' + 'b')` => `tmp := 'a' + 'b'; foo(tmp); string_free(&tmp);`
-	if g.pref.autofree {
+	if g.is_autofree {
 		// if node is ast.ExprStmt {&& node.expr is ast.CallExpr {
 		if node !is ast.FnDecl {
 			// p := node.position()
@@ -1584,7 +1584,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 	}
 	// Free the old value assigned to this string var (only if it's `str = [new value]`
 	// or `x.str = [new value]` )
-	mut af := g.pref.autofree && !g.is_builtin_mod && assign_stmt.op == .assign && assign_stmt.left_types.len ==
+	mut af := g.is_autofree && !g.is_builtin_mod && assign_stmt.op == .assign && assign_stmt.left_types.len ==
 		1 &&
 		(assign_stmt.left[0] is ast.Ident || assign_stmt.left[0] is ast.SelectorExpr)
 	// assign_stmt.left_types[0] in [table.string_type, table.array_type] &&
@@ -1623,7 +1623,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 	}
 	// Autofree tmp arg vars
 	// first_right := assign_stmt.right[0]
-	// af := g.pref.autofree && first_right is ast.CallExpr && !g.is_builtin_mod
+	// af := g.autofree && first_right is ast.CallExpr && !g.is_builtin_mod
 	// if af {
 	// g.autofree_call_pregen(first_right as ast.CallExpr)
 	// }
@@ -1641,7 +1641,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 	// }
 	// int pos = *(int*)_t190.data;
 	mut tmp_opt := ''
-	is_optional := g.pref.autofree &&
+	is_optional := g.is_autofree &&
 		(assign_stmt.op in [.decl_assign, .assign]) && assign_stmt.left_types.len == 1 && assign_stmt.right[0] is
 		ast.CallExpr
 	if is_optional {
@@ -1727,7 +1727,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 						if left.left_type.is_ptr() {
 							g.write('*')
 						}
-						needs_clone := info.elem_type == table.string_type && g.pref.autofree
+						needs_clone := info.elem_type == table.string_type && g.is_autofree
 						if needs_clone {
 							g.write('/*1*/string_clone(')
 						}
@@ -1969,7 +1969,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 				g.write(', ')
 			}
 			mut cloned := false
-			if g.autofree && right_sym.kind in [.array, .string] {
+			if g.is_autofree && right_sym.kind in [.array, .string] {
 				if g.gen_clone_assignment(val, right_sym, false) {
 					cloned = true
 				}
@@ -1980,7 +1980,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 				// `pos := s.index(...
 				// `int pos = *(int)_t10.data;`
 				g.write('*($styp*)')
-				if g.pref.autofree {
+				if g.is_autofree {
 					g.write(tmp_opt + '.data/*FFz*/')
 					g.right_is_opt = false
 					g.is_assign_rhs = false
@@ -2037,7 +2037,7 @@ fn (mut g Gen) gen_assign_stmt(assign_stmt ast.AssignStmt) {
 				}
 			}
 			if unwrap_optional {
-				if g.pref.autofree {
+				if g.is_autofree {
 					// g.write(tmp_opt + '/*FF*/')
 				} else {
 					g.write('.data')
@@ -2150,7 +2150,7 @@ fn (mut g Gen) gen_clone_assignment(val ast.Expr, right_sym table.TypeSymbol, ad
 	if val !is ast.Ident && val !is ast.SelectorExpr {
 		return false
 	}
-	if g.autofree && right_sym.kind == .array {
+	if g.is_autofree && right_sym.kind == .array {
 		// `arr1 = arr2` => `arr1 = arr2.clone()`
 		if add_eq {
 			g.write('=')
@@ -2158,7 +2158,7 @@ fn (mut g Gen) gen_clone_assignment(val ast.Expr, right_sym table.TypeSymbol, ad
 		g.write(' array_clone_static(')
 		g.expr(val)
 		g.write(')')
-	} else if g.autofree && right_sym.kind == .string {
+	} else if g.is_autofree && right_sym.kind == .string {
 		if add_eq {
 			g.write('=')
 		}
@@ -2407,8 +2407,8 @@ fn (mut g Gen) expr(node ast.Expr) {
 			// if g.fileis('1.strings') {
 			// println('before:' + node.autofree_pregen)
 			// }
-			if g.pref.autofree && !g.is_builtin_mod && !g.is_js_call && g.strs_to_free0.len ==
-				0 && !g.inside_lambda { // && g.inside_ternary ==
+			if g.is_autofree && !g.is_builtin_mod && !g.is_js_call && g.strs_to_free0.len == 0 &&
+				!g.inside_lambda { // && g.inside_ternary ==
 				// if len != 0, that means we are handling call expr inside call expr (arg)
 				// and it'll get messed up here, since it's handled recursively in autofree_call_pregen()
 				// so just skip it
@@ -2419,9 +2419,9 @@ fn (mut g Gen) expr(node ast.Expr) {
 				g.strs_to_free0 = []
 				// println('pos=$node.pos.pos')
 			}
-			// if g.pref.autofree && node.autofree_pregen != '' { // g.strs_to_free0.len != 0 {
+			// if g.autofree && node.autofree_pregen != '' { // g.strs_to_free0.len != 0 {
 			/*
-			if g.pref.autofree {
+			if g.autofree {
 				s := g.autofree_pregen[node.pos.pos.str()]
 				if s != '' {
 					// g.insert_before_stmt('/*START2*/' + g.strs_to_free0.join('\n') + '/*END*/')
@@ -3167,7 +3167,7 @@ fn (mut g Gen) infix_expr(node ast.InfixExpr) {
 			if elem_sym.kind == .interface_ && node.right_type != info.elem_type {
 				g.interface_call(node.right_type, info.elem_type)
 			}
-			// if g.pref.autofree
+			// if g.autofree
 			needs_clone := info.elem_type == table.string_type && !g.is_builtin_mod
 			if needs_clone {
 				g.write('string_clone(')
@@ -3713,12 +3713,12 @@ fn (mut g Gen) if_expr(node ast.IfExpr) {
 	// Always use this in -autofree, since ?: can have tmp expressions that have to be freed.
 	first_branch := node.branches[0]
 	needs_tmp_var := node.is_expr &&
-		(g.pref.autofree || (g.pref.experimental &&
+		(g.is_autofree || (g.pref.experimental &&
 		(first_branch.stmts.len > 1 || (first_branch.stmts[0] is ast.ExprStmt &&
 		(first_branch.stmts[0] as ast.ExprStmt).expr is ast.IfExpr))))
 	/*
 	needs_tmp_var := node.is_expr &&
-		(g.pref.autofree || g.pref.experimental) &&
+		(g.autofree || g.pref.experimental) &&
 		(node.branches[0].stmts.len > 1 || node.branches[0].stmts[0] is ast.IfExpr)
 	*/
 	tmp := if needs_tmp_var { g.new_tmp_var() } else { '' }
@@ -3957,7 +3957,7 @@ fn (mut g Gen) index_expr(node ast.IndexExpr) {
 						.function { 'voidptr*' }
 						else { '$elem_type_str*' }
 					}
-					needs_clone := info.elem_type == table.string_type_idx && g.pref.autofree &&
+					needs_clone := info.elem_type == table.string_type_idx && g.is_autofree &&
 						!g.is_assign_lhs
 					if needs_clone {
 						g.write('/*2*/string_clone(')
@@ -4141,7 +4141,7 @@ fn (mut g Gen) return_statement(node ast.Return) {
 			g.writeln('$styp $tmp = {.ok = true};')
 			g.writeln('return $tmp;')
 		} else {
-			if g.pref.autofree && !g.is_builtin_mod {
+			if g.is_autofree && !g.is_builtin_mod {
 				g.writeln('// free before return (no values returned)')
 				g.autofree_scope_vars(node.pos.pos - 1, node.pos.line_nr, true)
 			}
@@ -4275,7 +4275,7 @@ fn (mut g Gen) return_statement(node ast.Return) {
 			g.writeln('return $opt_tmp;')
 			return
 		}
-		free := g.pref.autofree && !g.is_builtin_mod // node.exprs[0] is ast.CallExpr
+		free := g.is_autofree && !g.is_builtin_mod // node.exprs[0] is ast.CallExpr
 		mut tmp := ''
 		if free {
 			// `return foo(a, b, c)`
@@ -4404,7 +4404,7 @@ fn (mut g Gen) const_decl_init_later(mod string, name string, val string, typ ta
 	cname := '_const_$name'
 	g.definitions.writeln('$styp $cname; // inited later')
 	g.inits[mod].writeln('\t$cname = $val;')
-	if g.pref.autofree {
+	if g.is_autofree {
 		if styp.starts_with('array_') {
 			g.cleanups[mod].writeln('\tarray_free(&$cname);')
 		}
@@ -4491,7 +4491,7 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 			}
 			field_type_sym := g.table.get_type_symbol(field.typ)
 			mut cloned := false
-			if g.autofree && !field.typ.is_ptr() && field_type_sym.kind in [.array, .string] {
+			if g.is_autofree && !field.typ.is_ptr() && field_type_sym.kind in [.array, .string] {
 				g.write('/*clone1*/')
 				if g.gen_clone_assignment(field.expr, field_type_sym, false) {
 					cloned = true
@@ -4560,7 +4560,7 @@ fn (mut g Gen) struct_init(struct_init ast.StructInit) {
 				mut cloned := false
 				is_interface := expected_field_type_sym.kind == .interface_ &&
 					field_type_sym.kind != .interface_
-				if g.autofree && !sfield.typ.is_ptr() && field_type_sym.kind in [.array, .string] {
+				if g.is_autofree && !sfield.typ.is_ptr() && field_type_sym.kind in [.array, .string] {
 					g.write('/*clone1*/')
 					if g.gen_clone_assignment(sfield.expr, field_type_sym, false) {
 						cloned = true
@@ -4705,7 +4705,7 @@ fn (mut g Gen) write_init_function() {
 	} else {
 		g.writeln('void _vinit() {')
 	}
-	if g.pref.autofree {
+	if g.is_autofree {
 		// Pre-allocate the string buffer
 		// s_str_buf_size := os.getenv('V_STRBUF_MB')
 		// mb_size := if s_str_buf_size == '' { 1 } else { s_str_buf_size.int() }
@@ -4735,7 +4735,7 @@ fn (mut g Gen) write_init_function() {
 	if g.pref.printfn_list.len > 0 && '_vinit' in g.pref.printfn_list {
 		println(g.out.after(fn_vinit_start_pos))
 	}
-	if g.autofree {
+	if g.is_autofree {
 		fn_vcleanup_start_pos := g.out.len
 		g.writeln('void _vcleanup() {')
 		// g.writeln('puts("cleaning up...");')
diff --git a/vlib/v/gen/cmain.v b/vlib/v/gen/cmain.v
index 0b6d764290..ad9b7ae9df 100644
--- a/vlib/v/gen/cmain.v
+++ b/vlib/v/gen/cmain.v
@@ -75,7 +75,7 @@ fn (mut g Gen) gen_c_main_header() {
 		g.writeln('')
 	}
 	if g.is_importing_os() {
-		if g.autofree {
+		if g.is_autofree {
 			g.writeln('free(_const_os__args.data); // empty, inited in _vinit()')
 		}
 		if g.pref.os == .windows {
@@ -90,7 +90,7 @@ fn (mut g Gen) gen_c_main_header() {
 }
 
 pub fn (mut g Gen) gen_c_main_footer() {
-	if g.autofree {
+	if g.is_autofree {
 		g.writeln('\t_vcleanup();')
 	}
 	g.writeln('\treturn 0;')
@@ -99,7 +99,7 @@ pub fn (mut g Gen) gen_c_main_footer() {
 
 pub fn (mut g Gen) gen_c_android_sokol_main() {
 	// Weave autofree into sokol lifecycle callback(s)
-	if g.autofree {
+	if g.is_autofree {
 		g.writeln('// Wrapping cleanup/free callbacks for sokol to include _vcleanup()
 void (*_vsokol_user_cleanup_ptr)(void);
 void (*_vsokol_user_cleanup_cb_ptr)(void *);
@@ -126,7 +126,7 @@ sapp_desc sokol_main(int argc, char* argv[]) {
 	_vinit();
 	main__main();
 ')
-	if g.autofree {
+	if g.is_autofree {
 		g.writeln('	// Wrap user provided cleanup/free functions for sokol to be able to call _vcleanup()
 	if (g_desc.cleanup_cb) {
 		_vsokol_user_cleanup_ptr = g_desc.cleanup_cb;
@@ -170,7 +170,7 @@ pub fn (mut g Gen) write_tests_main() {
 		g.writeln('\tmain__BenchedTests_end_testing(&bt);')
 	}
 	g.writeln('')
-	if g.autofree {
+	if g.is_autofree {
 		g.writeln('\t_vcleanup();')
 	}
 	g.writeln('\treturn g_test_fails > 0;')
diff --git a/vlib/v/gen/fn.v b/vlib/v/gen/fn.v
index b63e522f58..078364ecd3 100644
--- a/vlib/v/gen/fn.v
+++ b/vlib/v/gen/fn.v
@@ -16,6 +16,15 @@ fn (mut g Gen) gen_fn_decl(it ast.FnDecl, skip bool) {
 		return
 	}
 	g.returned_var_name = ''
+	//
+	old_g_autofree := g.is_autofree
+	if it.is_manualfree {
+		g.is_autofree = false
+	}
+	defer {
+		g.is_autofree = old_g_autofree
+	}
+	//
 	// if g.fileis('vweb.v') {
 	// println('\ngen_fn_decl() $it.name $it.is_generic $g.cur_generic_type')
 	// }
@@ -263,12 +272,12 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 	defer {
 		g.inside_call = false
 	}
-	gen_or := node.or_block.kind != .absent && !g.pref.autofree
+	gen_or := node.or_block.kind != .absent && !g.is_autofree
 	// if gen_or {
 	// g.writeln('/*start*/')
 	// }
 	is_gen_or_and_assign_rhs := gen_or && g.is_assign_rhs
-	cur_line := if is_gen_or_and_assign_rhs && !g.pref.autofree {
+	cur_line := if is_gen_or_and_assign_rhs && !g.is_autofree {
 		line := g.go_before_stmt(0)
 		g.out.write(tabs[g.indent])
 		line
@@ -290,8 +299,8 @@ fn (mut g Gen) call_expr(node ast.CallExpr) {
 	} else {
 		g.fn_call(node)
 	}
-	if gen_or { // && !g.pref.autofree {
-		if !g.pref.autofree {
+	if gen_or { // && !g.autofree {
+		if !g.is_autofree {
 			g.or_block(tmp_opt, node.or_block, node.return_type)
 		}
 		if is_gen_or_and_assign_rhs {
@@ -557,7 +566,7 @@ fn (mut g Gen) fn_call(node ast.CallExpr) {
 			tmp2 = g.new_tmp_var()
 			g.writeln('Option_$typ $tmp2 = $fn_name ($json_obj);')
 		}
-		if !g.pref.autofree {
+		if !g.is_autofree {
 			g.write('cJSON_Delete($json_obj); //del')
 		}
 		g.write('\n$cur_line')
@@ -594,7 +603,7 @@ fn (mut g Gen) fn_call(node ast.CallExpr) {
 		// check if alias parent also not a string
 		if typ != table.string_type {
 			expr := node.args[0].expr
-			if g.autofree && !typ.has_flag(.optional) {
+			if g.is_autofree && !typ.has_flag(.optional) {
 				// Create a temporary variable so that the value can be freed
 				tmp := g.new_tmp_var()
 				// tmps << tmp
@@ -640,7 +649,7 @@ fn (mut g Gen) autofree_call_pregen(node ast.CallExpr) {
 	// g.writeln('// autofree_call_pregen()')
 	// Create a temporary var before fn call for each argument in order to free it (only if it's a complex expression,
 	// like `foo(get_string())` or `foo(a + b)`
-	mut free_tmp_arg_vars := g.autofree && !g.is_builtin_mod && node.args.len > 0 && !node.args[0].typ.has_flag(.optional) // TODO copy pasta checker.v
+	mut free_tmp_arg_vars := g.is_autofree && !g.is_builtin_mod && node.args.len > 0 && !node.args[0].typ.has_flag(.optional) // TODO copy pasta checker.v
 	if !free_tmp_arg_vars {
 		return
 	}
@@ -775,7 +784,7 @@ fn (mut g Gen) call_args(node ast.CallExpr) {
 		if is_variadic && i == expected_types.len - 1 {
 			break
 		}
-		use_tmp_var_autofree := g.autofree && arg.typ == table.string_type && arg.is_tmp_autofree &&
+		use_tmp_var_autofree := g.is_autofree && arg.typ == table.string_type && arg.is_tmp_autofree &&
 			!g.inside_const && !g.is_builtin_mod
 		// g.write('/* af=$arg.is_tmp_autofree */')
 		mut is_interface := false
diff --git a/vlib/v/parser/fn.v b/vlib/v/parser/fn.v
index b1fedca941..738aa9a65e 100644
--- a/vlib/v/parser/fn.v
+++ b/vlib/v/parser/fn.v
@@ -156,6 +156,7 @@ pub fn (mut p Parser) call_args() []ast.CallArg {
 fn (mut p Parser) fn_decl() ast.FnDecl {
 	p.top_level_statement_start()
 	start_pos := p.tok.position()
+	is_manualfree := p.attrs.contains('manualfree')
 	is_deprecated := p.attrs.contains('deprecated')
 	is_direct_arr := p.attrs.contains('direct_array_access')
 	mut is_unsafe := p.attrs.contains('unsafe')
@@ -395,6 +396,7 @@ fn (mut p Parser) fn_decl() ast.FnDecl {
 		stmts: stmts
 		return_type: return_type
 		params: params
+		is_manualfree: is_manualfree
 		is_deprecated: is_deprecated
 		is_direct_arr: is_direct_arr
 		is_pub: is_pub
diff --git a/vlib/v/pref/pref.v b/vlib/v/pref/pref.v
index ca86733b2b..014da6f765 100644
--- a/vlib/v/pref/pref.v
+++ b/vlib/v/pref/pref.v
@@ -81,7 +81,6 @@ pub mut:
 	use_cache           bool // = true
 	retry_compilation   bool = true
 	is_stats            bool // `v -stats file_test.v` will produce more detailed statistics for the tests that were run
-	no_auto_free        bool // `v -nofree` disable automatic `free()` insertion for better performance in some applications  (e.g. compilers)
 	// TODO Convert this into a []string
 	cflags              string // Additional options which will be passed to the C compiler.
 	// For example, passing -cflags -Os will cause the C compiler to optimize the generated binaries for size.
@@ -92,7 +91,8 @@ pub mut:
 	ccompiler_type      CompilerType // the type of the C compiler used
 	third_party_option  string
 	building_v          bool
-	autofree            bool
+	autofree            bool // `v -manualfree` => false, `v -autofree` => true; false by default for now.
+	// Disabling `free()` insertion results in better performance in some applications (e.g. compilers)
 	compress            bool
 	// skip_builtin     bool   // Skips re-compilation of the builtin module
 	// to increase compilation time.
@@ -213,6 +213,10 @@ pub fn parse_args(args []string) (&Preferences, string) {
 				res.autofree = true
 				res.build_options << arg
 			}
+			'-manualfree' {
+				res.autofree = false
+				res.build_options << arg
+			}
 			'-compress' {
 				res.compress = true
 			}