diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b8b1a1ad56..a7be8fd29d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -254,7 +254,7 @@ jobs:
           ## sudo apt-get install --quiet -y libsdl2-dev libsdl2-ttf-dev libsdl2-mixer-dev libsdl2-image-dev
       - name: Build V
         run: make -j4 && ./v -cc gcc -cg -cflags -Werror -o v cmd/v
-      - name: Valgrind
+      - name: Valgrind v.c
         run: valgrind --error-exitcode=1 ./v -o v.c cmd/v
       - name: Run sanitizers
         run: |
diff --git a/doc/docs.md b/doc/docs.md
index 7187fdfc7c..881d84cc82 100644
--- a/doc/docs.md
+++ b/doc/docs.md
@@ -757,7 +757,7 @@ println(array_2) // [0, 1, 3, 5, 4]
 
 ### Fixed size arrays
 
-V also supports arrays with fixed size. Unlike ordinary arrays, their 
+V also supports arrays with fixed size. Unlike ordinary arrays, their
 length is constant. You cannot append elements to them, nor shrink them.
 You can only modify their elements in place.
 
@@ -1661,11 +1661,16 @@ fn sqr(n int) int {
 	return n * n
 }
 
+fn cube(n int) int {
+	return n * n * n
+}
+
 fn run(value int, op fn (int) int) int {
 	return op(value)
 }
 
 fn main() {
+	// Functions can be passed to other functions
 	println(run(5, sqr)) // "25"
 	// Anonymous functions can be declared inside other functions:
 	double_fn := fn (n int) int {
@@ -1676,6 +1681,11 @@ fn main() {
 	res := run(5, fn (n int) int {
 		return n + n
 	})
+	// You can even have an array/map of functions:
+	fns := [sqr, cube]
+	println(fns[0](10)) // "100"
+	fns_map := { 'sqr': sqr, 'cube': cube}
+	println(fns_map['cube'](2)) // "8"
 }
 ```
 
@@ -3867,8 +3877,8 @@ if x {
 }
 my_label:
 ```
-`goto` should be avoided when `for` can be used instead. In particular, 
-[labelled break](#labelled-break--continue) can be used to break out of 
+`goto` should be avoided when `for` can be used instead. In particular,
+[labelled break](#labelled-break--continue) can be used to break out of
 a nested loop.
 
 # Appendices
diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 3ce8b17505..cafb872151 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -329,6 +329,7 @@ pub:
 	generic_params  []GenericParam
 	is_direct_arr   bool // direct array access
 	attrs           []table.Attr
+	skip_gen        bool // this function doesn't need to be generated (for example [if foo])
 pub mut:
 	stmts         []Stmt
 	return_type   table.Type
@@ -1518,3 +1519,9 @@ pub fn ex2fe(x Expr) table.FExpr {
 	unsafe { C.memcpy(&res, &x, sizeof(table.FExpr)) }
 	return res
 }
+
+// experimental ast.Table
+pub struct Table {
+	// pub mut:
+	// main_fn_decl_node FnDecl
+}
diff --git a/vlib/v/builder/builder.v b/vlib/v/builder/builder.v
index 8c8b5d9b4a..9fd9cfdd16 100644
--- a/vlib/v/builder/builder.v
+++ b/vlib/v/builder/builder.v
@@ -27,6 +27,7 @@ pub mut:
 	parsed_files        []ast.File
 	cached_msvc         MsvcResult
 	table               &table.Table
+	table2              &ast.Table
 	timers              &util.Timers = util.new_timers(false)
 	ccoptions           CcompilerOptions
 }
@@ -36,6 +37,7 @@ pub fn new_builder(pref &pref.Preferences) Builder {
 	compiled_dir := if os.is_dir(rdir) { rdir } else { os.dir(rdir) }
 	mut table := table.new_table()
 	table.is_fmt = false
+	table2 := &ast.Table{}
 	if pref.use_color == .always {
 		util.emanager.set_support_color(true)
 	}
@@ -53,7 +55,8 @@ pub fn new_builder(pref &pref.Preferences) Builder {
 	return Builder{
 		pref: pref
 		table: table
-		checker: checker.new_checker(table, pref)
+		table2: table2
+		checker: checker.new_checker(table, table2, pref)
 		global_scope: &ast.Scope{
 			parent: 0
 		}
@@ -235,7 +238,7 @@ fn module_path(mod string) string {
 	return mod.replace('.', os.path_separator)
 }
 
-// TODO: try to merge this & util.module functions to create a 
+// TODO: try to merge this & util.module functions to create a
 // reliable multi use function. see comments in util/module.v
 pub fn (b &Builder) find_module_path(mod string, fpath string) ?string {
 	// support @VROOT/v.mod relative paths:
diff --git a/vlib/v/builder/c.v b/vlib/v/builder/c.v
index 8e26bb8d9c..2518c3ea79 100644
--- a/vlib/v/builder/c.v
+++ b/vlib/v/builder/c.v
@@ -22,7 +22,7 @@ pub fn (mut b Builder) gen_c(v_files []string) string {
 	b.print_warnings_and_errors()
 	// TODO: move gen.cgen() to c.gen()
 	b.timing_start('C GEN')
-	res := c.gen(b.parsed_files, b.table, b.pref)
+	res := c.gen(b.parsed_files, b.table, b.table2, b.pref)
 	b.timing_measure('C GEN')
 	// println('cgen done')
 	// println(res)
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index 402daf733f..9596d4fb63 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -12,6 +12,8 @@ import v.pref
 import v.util
 import v.errors
 import v.pkgconfig
+import v.walker
+import time
 
 const (
 	max_nr_errors                 = 300
@@ -36,6 +38,7 @@ pub struct Checker {
 	pref &pref.Preferences // Preferences shared from V struct
 pub mut:
 	table            &table.Table
+	table2           &ast.Table
 	file             &ast.File = 0
 	nr_errors        int
 	nr_warnings      int
@@ -63,6 +66,7 @@ pub mut:
 	skip_flags        bool // should `#flag` and `#include` be skipped
 	cur_generic_types []table.Type
 mut:
+	files                            []ast.File
 	expr_level                       int  // to avoid infinite recursion segfaults due to compiler bugs
 	inside_sql                       bool // to handle sql table fields pseudo variables
 	cur_orm_ts                       table.TypeSymbol
@@ -76,15 +80,18 @@ mut:
 	timers                           &util.Timers = util.new_timers(false)
 	comptime_fields_type             map[string]table.Type
 	fn_scope                         &ast.Scope = voidptr(0)
+	used_fns                         map[string]bool // used_fns['println'] == true
+	main_fn_decl_node                ast.FnDecl
 }
 
-pub fn new_checker(table &table.Table, pref &pref.Preferences) Checker {
+pub fn new_checker(table &table.Table, table2 &ast.Table, pref &pref.Preferences) Checker {
 	mut timers_should_print := false
 	$if time_checking ? {
 		timers_should_print = true
 	}
 	return Checker{
 		table: table
+		table2: table2
 		pref: pref
 		cur_fn: 0
 		timers: util.new_timers(timers_should_print)
@@ -140,6 +147,7 @@ pub fn (mut c Checker) check2(ast_file &ast.File) []errors.Error {
 }
 
 pub fn (mut c Checker) check_files(ast_files []ast.File) {
+	// c.files = ast_files
 	mut has_main_mod_file := false
 	mut has_main_fn := false
 	mut files_from_main_module := []&ast.File{}
@@ -223,6 +231,27 @@ pub fn (mut c Checker) check_files(ast_files []ast.File) {
 	} else if !has_main_fn {
 		c.error('function `main` must be declared in the main module', token.Position{})
 	}
+	// Walk the tree starting at main() and mark all used fns.
+	if c.pref.experimental {
+		println('walking the ast')
+		// c.is_recursive = true
+		// c.fn_decl(mut c.table2.main_fn_decl_node)
+		t := time.ticks()
+		mut walker := walker.Walker{
+			files: ast_files
+		}
+		for stmt in c.main_fn_decl_node.stmts {
+			walker.stmt(stmt)
+		}
+		// walker.fn_decl(mut c.table2.main_fn_decl_node)
+		println('time = ${time.ticks() - t}ms, nr used fns=$walker.used_fns.len')
+
+		for key, _ in walker.used_fns {
+			println(key)
+		}
+		// println(walker.used_fns)
+		// c.walk(ast_files)
+	}
 }
 
 // do checks specific to files in main module
@@ -1303,7 +1332,7 @@ fn (mut c Checker) check_map_and_filter(is_map bool, elem_typ table.Type, call_e
 		ast.Ident {
 			if arg_expr.kind == .function {
 				func := c.table.find_fn(arg_expr.name) or {
-					c.error('$arg_expr.name is not exist', arg_expr.pos)
+					c.error('$arg_expr.name does not exist', arg_expr.pos)
 					return
 				}
 				if func.params.len > 1 {
@@ -1772,6 +1801,7 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			call_expr.name = name_prefixed
 			found = true
 			f = f1
+			c.table.fns[name_prefixed].is_used = true
 		}
 	}
 	if !found && call_expr.left is ast.IndexExpr {
@@ -1805,6 +1835,7 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 		if f1 := c.table.find_fn(fn_name) {
 			found = true
 			f = f1
+			c.table.fns[fn_name].is_used = true
 		}
 	}
 	if c.pref.is_script && !found {
@@ -1816,6 +1847,7 @@ pub fn (mut c Checker) call_fn(mut call_expr ast.CallExpr) table.Type {
 			call_expr.name = os_name
 			found = true
 			f = f1
+			c.table.fns[os_name].is_used = true
 		}
 	}
 	// check for arg (var) of fn type
@@ -3827,7 +3859,7 @@ fn (mut c Checker) comptime_call(mut node ast.ComptimeCall) table.Type {
 			...pref_
 			is_vweb: true
 		}
-		mut c2 := new_checker(c.table, pref2)
+		mut c2 := new_checker(c.table, c.table2, pref2)
 		c2.check(node.vweb_tmpl)
 		mut i := 0 // tmp counter var for skipping first three tmpl vars
 		for k, _ in c2.file.scope.children[0].objects {
@@ -5655,9 +5687,12 @@ fn (mut c Checker) fn_decl(mut node ast.FnDecl) {
 	if node.language == .v && !c.is_builtin_mod {
 		c.check_valid_snake_case(node.name, 'function name', node.pos)
 	}
+	if node.name == 'main.main' {
+		c.main_fn_decl_node = node
+	}
 	if node.return_type != table.void_type {
 		for attr in node.attrs {
-			if attr.is_ctdefine {
+			if attr.is_comptime_define {
 				c.error('only functions that do NOT return values can have `[if $attr.name]` tags',
 					node.pos)
 				break
diff --git a/vlib/v/gen/c/cgen.v b/vlib/v/gen/c/cgen.v
index 93f0da6437..b118aab103 100644
--- a/vlib/v/gen/c/cgen.v
+++ b/vlib/v/gen/c/cgen.v
@@ -32,6 +32,7 @@ struct Gen {
 	module_built string
 mut:
 	table               &table.Table
+	table2              &ast.Table
 	out                 strings.Builder
 	cheaders            strings.Builder
 	includes            strings.Builder // all C #includes required by V modules
@@ -150,9 +151,10 @@ mut:
 	force_main_console bool // true when [console] used on fn main()
 	as_cast_type_names map[string]string // table for type name lookup in runtime (for __as_cast)
 	obf_table          map[string]string
+	// main_fn_decl_node  ast.FnDecl
 }
 
-pub fn gen(files []ast.File, table &table.Table, pref &pref.Preferences) string {
+pub fn gen(files []ast.File, table &table.Table, table2 &ast.Table, pref &pref.Preferences) string {
 	// println('start cgen2')
 	mut module_built := ''
 	if pref.build_mode == .build_module {
@@ -191,6 +193,7 @@ pub fn gen(files []ast.File, table &table.Table, pref &pref.Preferences) string
 		enum_typedefs: strings.new_builder(100)
 		sql_buf: strings.new_builder(100)
 		table: table
+		table2: table2
 		pref: pref
 		fn_decl: 0
 		is_autofree: true
diff --git a/vlib/v/gen/c/fn.v b/vlib/v/gen/c/fn.v
index be6910b4c7..cbef70c4ed 100644
--- a/vlib/v/gen/c/fn.v
+++ b/vlib/v/gen/c/fn.v
@@ -15,6 +15,23 @@ fn (mut g Gen) gen_fn_decl(node ast.FnDecl, skip bool) {
 		// || node.no_body {
 		return
 	}
+	// Skip [if xxx] if xxx is not defined
+	for attr in node.attrs {
+		if !attr.is_comptime_define {
+			continue
+		}
+		if attr.name !in g.pref.compile_defines_all {
+			// println('skipping [if]')
+			return
+		}
+	}
+	if f := g.table.find_fn(node.name) {
+		if !f.is_used {
+			g.writeln('// fn $node.name UNUSED')
+			// return
+		}
+	}
+
 	g.returned_var_name = ''
 	//
 	old_g_autofree := g.is_autofree
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index 73bebe724e..a261e40542 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -828,7 +828,7 @@ fn (mut p Parser) attributes() {
 			p.error_with_pos('duplicate attribute `$attr.name`', start_pos.extend(p.prev_tok.position()))
 			return
 		}
-		if attr.is_ctdefine {
+		if attr.is_comptime_define {
 			if has_ctdefine {
 				p.error_with_pos('only one `[if flag]` may be applied at a time `$attr.name`',
 					start_pos.extend(p.prev_tok.position()))
@@ -863,10 +863,9 @@ fn (mut p Parser) parse_attr() table.Attr {
 			pos: apos.extend(p.tok.position())
 		}
 	}
-	mut is_ctdefine := false
-	if p.tok.kind == .key_if {
+	is_comptime_define := p.tok.kind == .key_if
+	if is_comptime_define {
 		p.next()
-		is_ctdefine = true
 	}
 	mut name := ''
 	mut arg := ''
@@ -899,7 +898,7 @@ fn (mut p Parser) parse_attr() table.Attr {
 	return table.Attr{
 		name: name
 		is_string: is_string
-		is_ctdefine: is_ctdefine
+		is_comptime_define: is_comptime_define
 		arg: arg
 		is_string_arg: is_string_arg
 		pos: apos.extend(p.tok.position())
diff --git a/vlib/v/pref/pref.v b/vlib/v/pref/pref.v
index 8a5af4d7fd..cbc22d4263 100644
--- a/vlib/v/pref/pref.v
+++ b/vlib/v/pref/pref.v
@@ -3,6 +3,7 @@
 // that can be found in the LICENSE file.
 module pref
 
+// import v.ast // TODO this results in a compiler bug
 import os.cmdline
 import os
 import v.vcache
diff --git a/vlib/v/table/attr.v b/vlib/v/table/attr.v
index 1aa540e874..b66572e2e4 100644
--- a/vlib/v/table/attr.v
+++ b/vlib/v/table/attr.v
@@ -8,18 +8,18 @@ import v.token
 // e.g. `[unsafe]`
 pub struct Attr {
 pub:
-	name          string // [name]
-	is_string     bool   // ['name']
-	is_ctdefine   bool   // [if name]
-	arg           string // [name: arg]
-	is_string_arg bool   // [name: 'arg']
-	pos           token.Position
+	name               string // [name]
+	is_string          bool   // ['name']
+	is_comptime_define bool   // [if name]
+	arg                string // [name: arg]
+	is_string_arg      bool   // [name: 'arg']
+	pos                token.Position
 }
 
 // no square brackets
 pub fn (attr Attr) str() string {
 	mut s := ''
-	if attr.is_ctdefine {
+	if attr.is_comptime_define {
 		s += 'if '
 	}
 	if attr.is_string {
diff --git a/vlib/v/table/table.v b/vlib/v/table/table.v
index 885a7a2036..915218938b 100644
--- a/vlib/v/table/table.v
+++ b/vlib/v/table/table.v
@@ -34,11 +34,12 @@ pub:
 	is_placeholder bool
 	no_body        bool
 	mod            string
-	ctdefine       string // compile time define. myflag, when [if myflag] tag
+	ctdefine       string // compile time define. "myflag", when [if myflag] tag
 	attrs          []Attr
 pub mut:
 	name      string
 	source_fn voidptr // set in the checker, while processing fn declarations
+	is_used   bool
 }
 
 fn (f &Fn) method_equals(o &Fn) bool {
diff --git a/vlib/v/walker/walker.v b/vlib/v/walker/walker.v
new file mode 100644
index 0000000000..86f4eb2d44
--- /dev/null
+++ b/vlib/v/walker/walker.v
@@ -0,0 +1,129 @@
+// Copyright (c) 2019-2021 Alexander Medvednikov. All rights reserved.
+// Use of this source code is governed by an MIT license that can be found in the LICENSE file.
+module walker
+
+// This module walks the entire program starting at fn main and marks used (called)
+// functions.
+// Unused functions can be safely skipped by the backends to save CPU time and space.
+import v.ast
+
+pub struct Walker {
+pub mut:
+	used_fns map[string]bool // used_fns['println'] == true
+mut:
+	files []ast.File
+}
+
+/*
+fn (mut w Walker) walk_files(ast_files []ast.File) {
+		t := time.ticks()
+*/
+
+pub fn (mut w Walker) stmt(node ast.Stmt) {
+	match mut node {
+		ast.AssignStmt {
+			for l in node.left {
+				w.expr(l)
+			}
+			for r in node.right {
+				w.expr(r)
+			}
+		}
+		ast.ExprStmt {
+			w.expr(node.expr)
+		}
+		// ast.FnDecl {
+		// w.fn_decl(mut node)
+		//}
+		ast.ForStmt {
+			w.expr(node.cond)
+			for stmt in node.stmts {
+				w.stmt(stmt)
+			}
+		}
+		else {}
+	}
+}
+
+fn (mut w Walker) expr(node ast.Expr) {
+	match mut node {
+		ast.CallExpr {
+			w.call_expr(mut node)
+		}
+		ast.GoExpr {
+			w.expr(node.go_stmt.call_expr)
+		}
+		ast.IndexExpr {
+			w.expr(node.left)
+			w.expr(node.index)
+		}
+		ast.IfExpr {
+			for b in node.branches {
+				w.expr(b.cond)
+				for stmt in b.stmts {
+					w.stmt(stmt)
+				}
+			}
+		}
+		ast.MatchExpr {
+			w.expr(node.cond)
+			for b in node.branches {
+				for expr in b.exprs {
+					w.expr(expr)
+				}
+				for stmt in b.stmts {
+					w.stmt(stmt)
+				}
+			}
+		}
+		else {}
+	}
+}
+
+/*
+pub fn (mut w Walker) fn_decl(mut node ast.FnDecl) {
+	fn_name := if node.is_method { node.receiver.typ.str() + '.' + node.name } else { node.name }
+	if w.used_fns[fn_name] {
+		// This function is already known to be called, meaning it has been processed already.
+		// Save CPU time and do nothing.
+		return
+	}
+	if node.language == .c {
+		return
+	}
+	println('fn decl $fn_name')
+	w.used_fns[fn_name] = true
+	for stmt in node.stmts {
+		w.stmt(stmt)
+	}
+}
+*/
+
+pub fn (mut w Walker) call_expr(mut node ast.CallExpr) {
+	fn_name := if node.is_method { node.receiver_type.str() + '.' + node.name } else { node.name }
+	// fn_name := node.name
+	println('call_expr $fn_name')
+	// if node.is_method {
+	// println('M $node.name $node.receiver_type')
+	//}
+	if w.used_fns[fn_name] {
+		return
+	}
+	// w.used_fns[fn_name] = true
+	// Find the FnDecl for this CallExpr, mark the function as used, and process
+	// all its statements.
+	loop: for file in w.files {
+		for stmt in file.stmts {
+			if stmt is ast.FnDecl {
+				if stmt.name == node.name
+					&& (!node.is_method || (node.receiver_type == stmt.receiver.typ)) {
+					w.used_fns[fn_name] = true
+					for fn_stmt in stmt.stmts {
+						w.stmt(fn_stmt)
+					}
+					break loop
+				}
+			}
+		}
+	}
+}