parser: allow lock prefix instructions and numbered reg in asm inline

vlib/v/parser/parser.v

@@ -1253,6 +1253,19 @@ fn (mut p Parser) asm_stmt(is_top_level bool) ast.AsmStmt {
 		}
 		if p.tok.kind in [.key_in, .key_lock, .key_orelse, .key_select, .key_return] { // `in`, `lock`, `or`, `select`, `return` are v keywords that are also x86/arm/riscv/wasm instructions.
 			name += p.tok.kind.str()
+			if p.tok.kind == .key_lock && arch in [.i386, .amd64] {
+				p.next()
+				allowed_ins := ['add', 'adc', 'and', 'btc', 'btr', 'bts', 'cmpxchg', 'cmpxch8b',
+					'cmpxchg16b', 'dec', 'inc', 'neg', 'not', 'or', 'sbb', 'sub', 'xor', 'xadd',
+					'xchg']
+				has_suffix := p.tok.lit[p.tok.lit.len - 1] in [`b`, `w`, `l`, `q`]
+				if !(p.tok.lit in allowed_ins
+					|| (has_suffix && p.tok.lit[0..p.tok.lit.len - 1] in allowed_ins)) {
+					p.error('The lock prefix cannot be used on this instruction')
+				}
+				name += ' '
+				name += p.tok.lit
+			}
 			p.next()
 		} else if p.tok.kind == .number {
 			name += p.tok.lit
@@ -1764,7 +1777,14 @@ fn (mut p Parser) asm_ios(output bool) []ast.AsmIO {
 			if p.tok.kind == .at {
 				p.next()
 			} else {
-				p.check(.name)
+				if p.tok.kind == .number {
+					if p.tok.lit.int() >= 10 {
+						p.error_with_pos('The digit must be between 0 and 9 only', pos)
+					}
+					p.check(.number)
+				} else {
+					p.check(.name)
+				}
 			}
 		}
 		mut expr := p.expr(0)

vlib/v/slow_tests/assembly/asm_test.amd64.v

@@ -235,3 +235,21 @@ fn generic_asm[T](var &T) T {
 	}
 	return ret
 }
+
+fn test_lock_prefix() {
+	mut rv := u64(123)
+	mut atom := u64(0)
+	cmp := u64(1)
+	xchg := u64(1)
+
+	asm amd64 {
+		lock cmpxchgq '%1', '%2'
+		; =a (rv)
+		  +m (atom)
+		; q (xchg)
+		  0 (cmp)
+		; memory
+	}
+	assert rv == 0
+	assert atom == 0
+}