v.gen.c: add tests to ensure the same calling convention is used on exported functions for -m32/-m64 (#11977)

vlib/v/gen/c/fn.v

@@ -401,7 +401,8 @@ fn (mut g Gen) gen_fn_decl(node &ast.FnDecl, skip bool) {
 	for attr in node.attrs {
 		if attr.name == 'export' {
 			g.writeln('// export alias: $attr.arg -> $name')
-			export_alias := '$type_name ${attr.arg}($arg_str)'
+			calling_conv := if msvc_attrs.len > 0 { '$msvc_attrs ' } else { '' }
+			export_alias := '$type_name $calling_conv${attr.arg}($arg_str)'
 			g.definitions.writeln('VV_EXPORTED_SYMBOL $export_alias; // exported fn $node.name')
 			g.writeln('$export_alias {')
 			g.write('\treturn ${name}(')

vlib/v/tests/create_dll/create_win_dll.v

@@ -0,0 +1,34 @@
+module test
+
+fn C._vinit(int, voidptr)
+fn C.GC_INIT()
+
+const (
+	foo = 1
+	bar = (foo << 5) + 9
+)
+
+[export: Tatltuae]
+pub fn test_tatltuae() int {
+	return test.foo + test.bar
+}
+
+[export: DllMain]
+[windows_stdcall]
+fn main(hinst voidptr, fdw_reason int, lp_reserved voidptr) bool {
+	match fdw_reason {
+		C.DLL_PROCESS_ATTACH {
+			$if static_boehm ? {
+				C.GC_INIT()
+			}
+			C._vinit(0, 0)
+		}
+		C.DLL_THREAD_ATTACH {}
+		C.DLL_THREAD_DETACH {}
+		C.DLL_PROCESS_DETACH {}
+		else {
+			return false
+		}
+	}
+	return true
+}

vlib/v/tests/create_dll/create_win_dll_test.v

@@ -0,0 +1,61 @@
+import dl
+import os
+
+type Func = fn () int
+
+fn run_test(cc_used string) {
+	cc_flag := match cc_used {
+		'tcc64' { '-cc tcc' }
+		'gcc64' { '-cc gcc' }
+		'clang64' { '-cc clang' }
+		'msvc64' { '-cc msvc' }
+		'tcc32' { '-cc tcc -m32 -d no_backtrace' }
+		'gcc32' { '-cc gcc -m32' }
+		'clang32' { '-cc clang -m32' }
+		'msvc32' { '-cc msvc -m32' }
+		else { '' }
+	}
+
+	assert os.system('"${@VEXE}" $cc_flag -o create_win_${cc_used}.dll -shared create_win_dll.v') == 0
+	assert os.exists('create_win_${cc_used}.dll')
+	handle := dl.open('create_win_${cc_used}.dll', 0)
+	assert handle != 0
+	test := Func(dl.sym(handle, 'Tatltuae'))
+	assert test() == 42
+	assert test() != 666
+	// dl.close(handle)  // works for gcc, clang and msvc but crashes with tcc
+}
+
+fn test_create_and_dllmain() {
+	os.chdir(os.dir(@FILE)) or {}
+	$if windows {
+		$if x64 {
+			$if tinyc {
+				run_test('tcc64')
+			}
+			$if gcc {
+				run_test('gcc64')
+			}
+			$if clang {
+				run_test('clang64')
+			}
+			$if msvc {
+				run_test('msvc64')
+			}
+		}
+		$if x32 {
+			$if tinyc {
+				run_test('tcc32')
+			}
+			$if gcc {
+				run_test('gcc32')
+			}
+			$if clang {
+				run_test('clang32')
+			}
+			$if msvc {
+				// run_test('msvc32')  // something wrong as it passes when it should fail
+			}
+		}
+	}
+}