diff --git a/vlib/v/builder/builder.v b/vlib/v/builder/builder.v
index 9aa5d79626..aecad70c50 100644
--- a/vlib/v/builder/builder.v
+++ b/vlib/v/builder/builder.v
@@ -36,7 +36,7 @@ pub fn new_builder(pref &pref.Preferences) Builder {
 
 pub fn (b mut Builder) gen_c(v_files []string) string {
 	t0 := time.ticks()
-	b.parsed_files = parser.parse_files(v_files, b.table)
+	b.parsed_files = parser.parse_files(v_files, b.table, b.pref)
 	b.parse_imports()
 	t1 := time.ticks()
 	parse_time := t1 - t0
@@ -71,7 +71,7 @@ pub fn (b mut Builder) build_c(v_files []string, out_file string) {
 
 pub fn (b mut Builder) build_x64(v_files []string, out_file string) {
 	t0 := time.ticks()
-	b.parsed_files = parser.parse_files(v_files, b.table)
+	b.parsed_files = parser.parse_files(v_files, b.table, b.pref)
 	b.parse_imports()
 	t1 := time.ticks()
 	parse_time := t1 - t0
@@ -109,7 +109,7 @@ pub fn (b mut Builder) parse_imports() {
 				panic('cannot import module "$mod" (no .v files in "$import_path")')
 			}
 			// Add all imports referenced by these libs
-			parsed_files := parser.parse_files(v_files, b.table)
+			parsed_files := parser.parse_files(v_files, b.table, b.pref)
 			for file in parsed_files {
 				if file.mod.name != mod {
 					// v.parsers[pidx].error_with_token_index('bad module definition: ${v.parsers[pidx].file_path} imports module "$mod" but $file is defined as module `$p_mod`', 1
diff --git a/vlib/v/doc/doc.v b/vlib/v/doc/doc.v
index 6eb0f419c0..348add4abe 100644
--- a/vlib/v/doc/doc.v
+++ b/vlib/v/doc/doc.v
@@ -45,7 +45,7 @@ pub fn doc(mod string, table &table.Table) string {
 		if file.ends_with('_test.v') || file.ends_with('_windows.v') || file.ends_with('_macos.v') {
 			continue
 		}
-		file_ast := parser.parse_file(os.join_path(path,file), table, .skip_comments)
+		file_ast := parser.parse_file(os.join_path(path,file), table, .skip_comments,	&pref.Preferences{})
 		d.stmts << file_ast.stmts
 	}
 	if d.stmts.len == 0 {
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index 79a4fb87c0..fa6bff07d3 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -295,28 +295,7 @@ fn (g mut Gen) stmt(node ast.Stmt) {
 			g.const_decl(it)
 		}
 		ast.CompIf {
-			ifdef := comp_if_to_ifdef(it.val)
-			if it.is_not {
-				g.writeln('\n#ifndef ' + ifdef)
-				g.writeln('// #if not $it.val')
-			}
-			else {
-				g.writeln('\n#ifdef ' + ifdef)
-				g.writeln('// #if $it.val')
-			}
-			// NOTE: g.defer_ifdef is needed for defers called witin an ifdef
-			// in v1 this code would be completely excluded
-			g.defer_ifdef = if it.is_not { '#ifndef ' + ifdef } else { '#ifdef ' + ifdef }
-			// println('comp if stmts $g.file.path:$it.pos.line_nr')
-			g.stmts(it.stmts)
-			g.defer_ifdef = ''
-			if it.has_else {
-				g.writeln('#else')
-				g.defer_ifdef = if it.is_not { '#ifdef ' + ifdef } else { '#ifndef ' + ifdef }
-				g.stmts(it.else_stmts)
-				g.defer_ifdef = ''
-			}
-			g.writeln('#endif')
+			g.comp_if(it)
 		}
 		ast.DeferStmt {
 			mut defer_stmt := *it
@@ -2494,3 +2473,28 @@ pub fn (g mut Gen) write_tests_main() {
 fn (g &Gen) is_importing_os() bool {
 	return 'os' in g.table.imports
 }
+
+fn (g mut Gen) comp_if(it ast.CompIf) {
+	ifdef := comp_if_to_ifdef(it.val)
+	if it.is_not {
+		g.writeln('\n#ifndef ' + ifdef)
+		g.writeln('// #if not $it.val')
+	}
+	else {
+		g.writeln('\n#ifdef ' + ifdef)
+		g.writeln('// #if $it.val')
+	}
+	// NOTE: g.defer_ifdef is needed for defers called witin an ifdef
+	// in v1 this code would be completely excluded
+	g.defer_ifdef = if it.is_not { '#ifndef ' + ifdef } else { '#ifdef ' + ifdef }
+	// println('comp if stmts $g.file.path:$it.pos.line_nr')
+	g.stmts(it.stmts)
+	g.defer_ifdef = ''
+	if it.has_else {
+		g.writeln('#else')
+		g.defer_ifdef = if it.is_not { '#ifdef ' + ifdef } else { '#ifndef ' + ifdef }
+		g.stmts(it.else_stmts)
+		g.defer_ifdef = ''
+	}
+	g.writeln('#endif')
+}
diff --git a/vlib/v/parser/comptime.v b/vlib/v/parser/comptime.v
index b7fb521a68..68da656cd9 100644
--- a/vlib/v/parser/comptime.v
+++ b/vlib/v/parser/comptime.v
@@ -2,9 +2,15 @@ module parser
 
 import (
 	v.ast
+	v.pref
 )
 
-pub fn (p mut Parser) comp_if() ast.CompIf {
+const (
+	supported_platforms = ['windows', 'mac', 'macos', 'linux', 'freebsd', 'openbsd', 'netbsd',
+	'dragonfly', 'android', 'js', 'solaris', 'haiku', 'linux_or_macos']
+)
+
+fn (p mut Parser) comp_if() ast.CompIf {
 	pos := p.tok.position()
 	p.next()
 	p.check(.key_if)
@@ -13,14 +19,51 @@ pub fn (p mut Parser) comp_if() ast.CompIf {
 		p.next()
 	}
 	val := p.check_name()
+	mut stmts := []ast.Stmt
+	mut skip_os := false
+	if val in supported_platforms {
+		os := os_from_string(val)
+		// `$if os {` for a different target, skip everything inside
+		// to avoid compilation errors (like including <windows.h> or calling WinAPI fns
+		// on non-Windows systems)
+		if ((!is_not && os != p.pref.os) || (is_not && os == p.pref.os)) && !p.pref.output_cross_c {
+			skip_os = true
+			p.check(.lcbr)
+			p.warn('SKIPPING $val os=$os p.pref.os=$p.pref.os')
+			mut stack := 1
+			for {
+				if p.tok.kind == .key_return {
+					p.returns = true
+				}
+				if p.tok.kind == .lcbr {
+					stack++
+				}
+				else if p.tok.kind == .rcbr {
+					stack--
+				}
+				if p.tok.kind == .eof {
+					break
+				}
+				if stack <= 0 && p.tok.kind == .rcbr {
+					// p.warn('exiting $stack')
+					p.next()
+					break
+				}
+				p.next()
+			}
+		}
+	}
 	if p.tok.kind == .question {
 		p.next()
 	}
+	if !skip_os {
+		stmts = p.parse_block()
+	}
 	mut node := ast.CompIf{
 		is_not: is_not
-		stmts: p.parse_block()
 		pos: pos
 		val: val
+		stmts: stmts
 	}
 	if p.tok.kind == .dollar && p.peek_tok.kind == .key_else {
 		p.next()
@@ -30,3 +73,56 @@ pub fn (p mut Parser) comp_if() ast.CompIf {
 	}
 	return node
 }
+
+fn os_from_string(os string) pref.OS {
+	match os {
+		'linux' {
+			return .linux
+		}
+		'windows' {
+			return .windows
+		}
+		'mac' {
+			return .mac
+		}
+		'macos' {
+			return .mac
+		}
+		'freebsd' {
+			return .freebsd
+		}
+		'openbsd' {
+			return .openbsd
+		}
+		'netbsd' {
+			return .netbsd
+		}
+		'dragonfly' {
+			return .dragonfly
+		}
+		'js' {
+			return .js
+		}
+		'solaris' {
+			return .solaris
+		}
+		'android' {
+			return .android
+		}
+		'msvc' {
+			// notice that `-os msvc` became `-cc msvc`
+			verror('use the flag `-cc msvc` to build using msvc')
+		}
+		'haiku' {
+			return .haiku
+		}
+		'linux_or_macos' {
+			return .linux
+		}
+		else {
+			panic('bad os $os')
+		}
+	}
+	// println('bad os $os') // todo panic?
+	return .linux
+}
diff --git a/vlib/v/parser/parser.v b/vlib/v/parser/parser.v
index a8245a6a8d..9bb7362c68 100644
--- a/vlib/v/parser/parser.v
+++ b/vlib/v/parser/parser.v
@@ -40,6 +40,7 @@ mut:
 	imports     map[string]string
 	ast_imports []ast.Import
 	is_amp      bool
+	returns     bool
 }
 
 // for tests
@@ -51,14 +52,14 @@ pub fn parse_stmt(text string, table &table.Table, scope &ast.Scope) ast.Stmt {
 		pref: &pref.Preferences{}
 		scope: scope
 		// scope: &ast.Scope{start_pos: 0, parent: 0}
-		
+
 	}
 	p.init_parse_fns()
 	p.read_first_token()
 	return p.stmt()
 }
 
-pub fn parse_file(path string, table &table.Table, comments_mode scanner.CommentsMode) ast.File {
+pub fn parse_file(path string, table &table.Table, comments_mode scanner.CommentsMode, pref &pref.Preferences) ast.File {
 	// println('parse_file("$path")')
 	// text := os.read_file(path) or {
 	// panic(err)
@@ -69,13 +70,13 @@ pub fn parse_file(path string, table &table.Table, comments_mode scanner.Comment
 		scanner: scanner.new_scanner_file(path, comments_mode)
 		table: table
 		file_name: path
-		pref: &pref.Preferences{}
+		pref: pref //&pref.Preferences{}
 		scope: &ast.Scope{
 			start_pos: 0
 			parent: 0
 		}
 		// comments_mode: comments_mode
-		
+
 	}
 	p.read_first_token()
 	// p.scope = &ast.Scope{start_pos: p.tok.position(), parent: 0}
@@ -141,7 +142,7 @@ fn (q mut Queue) run() {
 */
 
 
-pub fn parse_files(paths []string, table &table.Table) []ast.File {
+pub fn parse_files(paths []string, table &table.Table, pref &pref.Preferences) []ast.File {
 	/*
 	println('\n\n\nparse_files()')
 	println(paths)
@@ -162,7 +163,7 @@ pub fn parse_files(paths []string, table &table.Table) []ast.File {
 	mut files := []ast.File
 	for path in paths {
 		// println('parse_files $path')
-		files << parse_file(path, table, .skip_comments)
+		files << parse_file(path, table, .skip_comments, pref)
 	}
 	return files
 }
@@ -686,7 +687,7 @@ pub fn (p mut Parser) name_expr() ast.Expr {
 		p.expr_mod = ''
 		return ast.EnumVal{
 			enum_name: enum_name // lp.prepend_mod(enum_name)
-			
+
 			val: val
 			pos: p.tok.position()
 			mod: mod
@@ -776,7 +777,7 @@ pub fn (p mut Parser) expr(precedence int) ast.Expr {
 			node = ast.SizeOf{
 				typ: sizeof_type
 				// type_name: type_name
-				
+
 			}
 		}
 		.key_typeof {
@@ -1044,7 +1045,7 @@ fn (p mut Parser) infix_expr(left ast.Expr) ast.Expr {
 		left: left
 		right: right
 		// right_type: typ
-		
+
 		op: op
 		pos: pos
 	}
@@ -1449,7 +1450,7 @@ fn (p mut Parser) const_decl() ast.ConstDecl {
 		fields << ast.Field{
 			name: name
 			// typ: typ
-			
+
 		}
 		exprs << expr
 		// TODO: once consts are fixed reg here & update in checker