Re: [Tinycc-devel] _Generic or __builtin_choose_expr

[uso ewin]

On Tue, Jul 4, 2017 at 6:04 PM, uso ewin <address@hidden> wrote:
> On Tue, Jul 4, 2017 at 5:59 PM, uso ewin <address@hidden> wrote:
>> Hi
>>
>> I've push a new version that use skip_or_save_block from mob:
>> https://github.com/cosmo-ray/tcc/commits/mob
>> I've let the patch that change ex1.c that I used as testing so you can
>> see my ugly tests.
>>
>> On Tue, Jul 4, 2017 at 4:46 PM, Michael Matz <address@hidden> wrote:
>>> Hi,
>>>
>>> On Tue, 4 Jul 2017, Michael Matz wrote:
>>>
>>>> Ugh, you're right.  Indeed the standard doesn't mandate a const char[]
>>>> type for string literals (I was confused because it does say that
>>>> modification of string literals is undefined, i.e. that's an extension).
>>>> Let me work on this a bit.
>>>
>>> Actually, not much need for work.  I've checked GCC sources and, even
>>> though it wonders in a comment if it's a good idea to change expression
>>> types with warning flags, it indeed does so; so if for nothing else than
>>> compatibility I've reverted the patch.
>>>
>>> The hack in the current patch for _Generic (to regard string literals as
>>> const char* always) isn't needed.  GCC indeed differs in behaviour
>>> depending on the warning flag, also in _Generic:
>>>
>>> -----------------------
>>> extern int printf(const char *,...);
>>> int main()
>>> {
>>>   const char *msg;
>>>   msg = _Generic("foo", const char *:"const char*", char*:"char *",
>>> default:"something else");
>>>   printf ("type \"foo\" = %s\n", msg);
>>>   return 0;
>>> }
>>> -----------------------
>>> % gcc-6 -Wwrite-strings x.c && ./a.out
>>> type "foo" = const char*
>>> % gcc-6 -Wno-write-strings x.c && ./a.out
>>> type "foo" = char*
>>>
>>> So, thanks for checking after me ;)
>>>
>>
>> Well, I've just test
>> int main()
>>  {
>>    const char *msg;
>>
>>    msg = _Generic("foo", const char *:"const char*", char*:"char *",
>>  default:"something else");
>>    printf ("type \"foo\" = %s\n", msg);
>>    return 0;
>> }
> oops, I've just type a bad shortcut on google,
> and apparently unexpectedly send the mail before finish typing... sorry
>
> So it appear than gcc remove const from variable:
> so
> const int a;
>
> i = _Generic(a, int: 10, const int: 20);
> printf("%d\n", i);
>
> will print 10...
>
> But as your example show early with  -Wwrite-strings gcc let the const....
>
>
> I've made some code that remove each const,
> and change array to pointer on my branch,
> but I'm not sure of the idea

the problem is describe here:
http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1930.htm

I've made more test, so it appear that gcc unconst variable
but not variable pointed by the variable so:

const int, will be see as a int,
const char * will be see as a char *
int * const will be see as an int *,

I've just force push the change on my github, so now it should have
the same behaviour as gcc