cgen: initialize globals directly for simple cases (#11002)

vlib/v/ast/ast.v

@@ -1989,3 +1989,32 @@ fn gen_all_registers(mut t Table, without_numbers []string, with_numbers map[str
 	}
 	return res
 }
+
+// is `expr` a literal, i.e. it does not depend on any other declarations (C compile time constant)
+pub fn (expr Expr) is_literal() bool {
+	match expr {
+		BoolLiteral, CharLiteral, FloatLiteral, IntegerLiteral {
+			return true
+		}
+		PrefixExpr {
+			return expr.right.is_literal()
+		}
+		InfixExpr {
+			return expr.left.is_literal() && expr.right.is_literal()
+		}
+		ParExpr {
+			return expr.expr.is_literal()
+		}
+		CastExpr {
+			return !expr.has_arg && expr.expr.is_literal()
+				&& (expr.typ.is_ptr() || expr.typ.is_pointer()
+				|| expr.typ in [i8_type, i16_type, int_type, i64_type, byte_type, u8_type, u16_type, u32_type, u64_type, f32_type, f64_type, char_type, bool_type, rune_type])
+		}
+		SizeOf, IsRefType {
+			return expr.is_type || expr.expr.is_literal()
+		}
+		else {
+			return false
+		}
+	}
+}

vlib/v/gen/c/cgen.v

@@ -5156,8 +5156,13 @@ fn (mut g Gen) global_decl(node ast.GlobalDecl) {
 		}
 		styp := g.typ(field.typ)
 		if field.has_expr {
-			g.definitions.writeln('$mod$styp $field.name;')
-			g.global_inits[key].writeln('\t$field.name = ${g.expr_string(field.expr)}; // global')
+			g.definitions.write_string('$mod$styp $field.name')
+			if field.expr.is_literal() {
+				g.definitions.writeln(' = ${g.expr_string(field.expr)}; // global')
+			} else {
+				g.definitions.writeln(';')
+				g.global_inits[key].writeln('\t$field.name = ${g.expr_string(field.expr)}; // global')
+			}
 		} else {
 			default_initializer := g.type_default(field.typ)
 			if default_initializer == '{0}' {

vlib/v/tests/init_global_test.v

@@ -36,7 +36,7 @@ __global (
 	sem       sync.Semaphore
 	shmap     shared map[string]f64
 	mtx       sync.RwMutex
-	f1        = f64(34.0625)
+	f1        = f64(545 / (sizeof(f64) + f32(8))) // directly initialized
 	f2        f64
 )
 
@@ -116,6 +116,7 @@ fn switch2() u64 {
 }
 
 fn test_global_mutex() {
+	assert f1 == 34.0625
 	t := go switch2()
 	for _ in 0 .. 2500000 {
 		mtx.@lock()