diff --git a/vlib/compiler/cc.v b/vlib/compiler/cc.v
index b9eebf0224..d17d19b490 100644
--- a/vlib/compiler/cc.v
+++ b/vlib/compiler/cc.v
@@ -486,24 +486,47 @@ fn find_c_compiler_default() string {
 
 fn find_c_compiler_thirdparty_options() string {
 	fullargs := env_vflags_and_os_args()
-	mut cflags := get_cmdline_cflags(fullargs)
+	mut cflags := get_cmdline_multiple_values(fullargs,'-cflags')
 	$if !windows {
-		cflags += ' -fPIC'
+		cflags << '-fPIC'
 	}
 	if '-m32' in fullargs {
-		cflags += ' -m32'
+		cflags << '-m32'
 	}
-	return cflags
+	return cflags.join(' ')
 }
 
-fn get_cmdline_cflags(args []string) string {
-	mut cflags := ''
+fn get_cmdline_multiple_values(args []string, optname string) []string {
+	mut flags := []string
 	for ci, cv in args {
-		if cv == '-cflags' {
-			cflags += args[ci + 1] + ' '
+		if cv == optname {
+			flags << args[ci + 1]
 		}
 	}
-	return cflags
+	return flags
+}
+
+fn parse_defines(defines []string) ([]string,[]string) {
+	// '-d abc -d xyz=1 -d qwe=0' should produce:
+	// compile_defines:      ['abc','xyz']
+	// compile_defines_all   ['abc','xyz','qwe']
+	mut compile_defines := []string
+	mut compile_defines_all := []string
+	for dfn in defines {
+		dfn_parts := dfn.split('=')
+		if dfn_parts.len == 1 {
+			compile_defines << dfn
+			compile_defines_all << dfn
+			continue
+		}
+		if dfn_parts.len == 2 {
+			compile_defines_all << dfn_parts[0]
+			if dfn_parts[1] == '1' {
+				compile_defines << dfn_parts[0]
+			}
+		}
+	}
+	return compile_defines, compile_defines_all
 }
 
 fn missing_compiler_info() string {
diff --git a/vlib/compiler/cflags.v b/vlib/compiler/cflags.v
index 8648e803bf..ce51e4e186 100644
--- a/vlib/compiler/cflags.v
+++ b/vlib/compiler/cflags.v
@@ -19,10 +19,18 @@ pub fn (c &CFlag) str() string {
 // get flags for current os
 fn (v &V) get_os_cflags() []CFlag {
 	mut flags := []CFlag
+	mut ctimedefines := []string
+	if v.compile_defines.len > 0 {
+		ctimedefines << v.compile_defines
+	}
+	
 	for flag in v.table.cflags {
 		if flag.os == '' || (flag.os == 'linux' && v.os == .linux) || (flag.os == 'darwin' && v.os == .mac) || (flag.os == 'freebsd' && v.os == .freebsd) || (flag.os == 'windows' && v.os == .windows) {
 			flags << flag
 		}
+		if flag.os in ctimedefines {
+			flags << flag
+		}
 	}
 	return flags
 }
@@ -66,7 +74,7 @@ fn (table &Table) has_cflag(cflag CFlag) bool {
 
 // parse the flags to (table.cflags) []CFlag
 // Note: clean up big time (joe-c)
-fn (table mut Table) parse_cflag(cflag string, mod string) ?bool {
+fn (table mut Table) parse_cflag(cflag string, mod string, ctimedefines []string) ?bool {
 	allowed_flags := ['framework', 'library', 'Wa', 'Wl', 'Wp', 'I', 'l', 'L', ]
 	flag_orig := cflag.trim_space()
 	mut flag := flag_orig
@@ -74,7 +82,10 @@ fn (table mut Table) parse_cflag(cflag string, mod string) ?bool {
 		return true
 	}
 	mut fos := ''
-	if flag.starts_with('linux') || flag.starts_with('darwin') || flag.starts_with('freebsd') || flag.starts_with('windows') {
+	mut allowed_os_overrides := ['linux','darwin','freebsd','windows']
+	allowed_os_overrides << ctimedefines
+	for os_override in allowed_os_overrides {
+		if !flag.starts_with( os_override ) { continue }
 		pos := flag.index(' ') or {
 			return none
 		}
diff --git a/vlib/compiler/cgen.v b/vlib/compiler/cgen.v
index e0d53579f2..be15bc386d 100644
--- a/vlib/compiler/cgen.v
+++ b/vlib/compiler/cgen.v
@@ -351,7 +351,11 @@ fn os_name_to_ifdef(name string) string {
 	return ''
 }
 
-fn platform_postfix_to_ifdefguard(name string) string {
+fn (v &V) platform_postfix_to_ifdefguard(name string) string {
+	if name.starts_with('custom '){
+		cdefine := name.replace('custom ','')
+		return '#ifdef CUSTOM_DEFINE_${cdefine}'
+	}
 	s := match name {
 		'.v'{
 			''
diff --git a/vlib/compiler/comptime.v b/vlib/compiler/comptime.v
index b8c8377c6f..a1d66bcc6a 100644
--- a/vlib/compiler/comptime.v
+++ b/vlib/compiler/comptime.v
@@ -20,6 +20,7 @@ fn (p mut Parser) comp_time() {
 		}
 		name := p.check_name()
 		p.fspace()
+
 		if name in supported_platforms {
 			ifdef_name := os_name_to_ifdef(name)
 			if name == 'mac' {
@@ -99,6 +100,9 @@ fn (p mut Parser) comp_time() {
 		else if name == 'clang' {
 			p.comptime_if_block('__clang__')
 		}
+		else if p.v.compile_defines_all.len > 0 && name in p.v.compile_defines_all {
+			p.comptime_if_block('CUSTOM_DEFINE_${name}')
+		}
 		else {
 			println('Supported platforms:')
 			println(supported_platforms)
@@ -209,7 +213,7 @@ fn (p mut Parser) chash() {
 			flag = flag.replace('@VLIB_PATH', p.pref.vlib_path)
 			flag = flag.replace('@VMOD', v_modules_path)
 			// p.log('adding flag "$flag"')
-			_ = p.table.parse_cflag(flag, p.mod) or {
+			_ = p.table.parse_cflag(flag, p.mod, p.v.compile_defines_all ) or {
 				p.error_with_token_index(err, p.cur_tok_index() - 1)
 				return
 			}
diff --git a/vlib/compiler/fn.v b/vlib/compiler/fn.v
index d16cb9f781..abf32dc210 100644
--- a/vlib/compiler/fn.v
+++ b/vlib/compiler/fn.v
@@ -749,7 +749,7 @@ fn (p mut Parser) verify_fn_before_call(f &Fn) {
 // p.tok == fn_name
 fn (p mut Parser) fn_call(f mut Fn, method_ph int, receiver_var, receiver_type string) {
 	p.verify_fn_before_call(f)
-	is_comptime_define := f.comptime_define != '' && f.comptime_define != p.pref.comptime_define
+	is_comptime_define := f.comptime_define != '' && !(f.comptime_define in p.v.compile_defines )
 	if is_comptime_define {
 		p.cgen.nogen = true
 	}
diff --git a/vlib/compiler/main.v b/vlib/compiler/main.v
index 96c8ef9b1f..76d2e959dd 100644
--- a/vlib/compiler/main.v
+++ b/vlib/compiler/main.v
@@ -76,6 +76,10 @@ pub mut:
 	gen_parser_idx      map[string]int
 	cached_mods         []string
 	module_lookup_paths []string
+	
+	// -d vfmt and -d another=0 for `$if vfmt { will execute }` and `$if another { will NOT get here }`
+	compile_defines     []string // just ['vfmt']
+	compile_defines_all []string // contains both: ['vfmt','another']
 }
 
 struct Preferences {
@@ -117,7 +121,6 @@ pub mut:
 	// This is on by default, since a vast majority of users do not
 	// work on the builtin module itself.
 	// generating_vh bool
-	comptime_define string // -D vfmt for `if $vfmt {`
 	fast            bool // use tcc/x64 codegen
 	enable_globals  bool // allow __global for low level code
 	// is_fmt bool
@@ -251,7 +254,20 @@ pub fn (v mut V) compile() {
 		else {
 			cgen.genln('#include <stdint.h>')
 		}
+
+		if v.compile_defines_all.len > 0 {
+			cgen.genln('')
+			cgen.genln('// All custom defines      : ' + v.compile_defines_all.join(','))
+			cgen.genln('// Turned ON custom defines: ' + v.compile_defines.join(','))
+			for cdefine in v.compile_defines {
+				cgen.genln('#define CUSTOM_DEFINE_${cdefine}')
+			}
+			cgen.genln('//')
+			cgen.genln('')
+		}
+		
 		cgen.genln(c_builtin_types)
+		
 		if !v.pref.is_bare {
 			cgen.genln(c_headers)
 		}
@@ -638,6 +654,19 @@ pub fn (v &V) v_files_from_dir(dir string) []string {
 		if file.ends_with('_c.v') && v.os == .js {
 			continue
 		}
+		if v.compile_defines_all.len > 0 && file.contains('_d_') {
+			mut allowed := false
+			for cdefine in v.compile_defines {
+				file_postfix := '_d_${cdefine}.v'
+				if file.ends_with(file_postfix) {
+					allowed = true
+					break
+				}
+			}
+			if !allowed {
+				continue
+			}
+		}
 		res << filepath.join(dir,file)
 	}
 	return res
@@ -905,8 +934,6 @@ pub fn new_v(args []string) &V {
 	vgen_buf.writeln('module vgen\nimport strings')
 	joined_args := args.join(' ')
 	target_os := get_arg(joined_args, 'os', '')
-	comptime_define := get_arg(joined_args, 'd', '')
-	// println('comptimedefine=$comptime_define')
 	mut out_name := get_arg(joined_args, 'o', 'a.out')
 	mut dir := args.last()
 	if 'run' in args {
@@ -1031,7 +1058,11 @@ pub fn new_v(args []string) &V {
 		exit(1)
 	}
 	mut out_name_c := get_vtmp_filename(out_name, '.tmp.c')
-	cflags := get_cmdline_cflags(args)
+	cflags := get_cmdline_multiple_values(args, '-cflags').join(' ')
+  
+	defines := get_cmdline_multiple_values(args, '-d')
+	compile_defines, compile_defines_all := parse_defines( defines )
+	
 	rdir := os.realpath(dir)
 	rdir_name := os.filename(rdir)
 	if '-bare' in args {
@@ -1073,7 +1104,6 @@ pub fn new_v(args []string) &V {
 		cflags: cflags
 		ccompiler: find_c_compiler()
 		building_v: !is_repl && (rdir_name == 'compiler' || rdir_name == 'v.v' || dir.contains('vlib'))
-		comptime_define: comptime_define
 		// is_fmt: comptime_define == 'vfmt'
 		
 		user_mod_path: user_mod_path
@@ -1104,7 +1134,9 @@ pub fn new_v(args []string) &V {
 		vroot: vroot
 		pref: pref
 		mod: mod
-		vgen_buf: vgen_buf
+		vgen_buf: vgen_buf		
+		compile_defines: compile_defines
+		compile_defines_all: compile_defines_all
 	}
 }
 
diff --git a/vlib/compiler/parser.v b/vlib/compiler/parser.v
index 638018eb5e..760de61058 100644
--- a/vlib/compiler/parser.v
+++ b/vlib/compiler/parser.v
@@ -155,10 +155,22 @@ fn (v mut V) new_parser_from_file(path string) Parser {
 				println('warning: use "$p" file name instead of "$path"')
 			}
 			path_platform = path_ending
-			path_pcguard = platform_postfix_to_ifdefguard(path_ending)
+			path_pcguard = v.platform_postfix_to_ifdefguard(path_ending)
 			break
 		}
 	}
+	
+	if v.compile_defines.len > 0 {
+		for cdefine in v.compile_defines {
+			custom_path_ending := '_d_${cdefine}.v'
+			if path.ends_with(custom_path_ending){
+				path_platform = custom_path_ending
+				path_pcguard = v.platform_postfix_to_ifdefguard('custom $cdefine')
+				break
+			}
+		}
+	}
+	
 	mut p := v.new_parser(new_scanner_file(path))
 	p = {
 		p |