Is this program well-defined?

Is this program well-defined?

[Haoxin Tu]

Hi all.

I hope this is the right place to ask the question.

I found a test case that makes almost all (GCC-5.1 above) versions
(may) produce the wrong code at -Os.

So, I am confused here whether this test case is well-defined or not.

Here is the program (small.c):
```
#include <stdint.h>
#include <stdio.h>
int16_t a;
uint8_t func_2();
static int32_t func_1() { func_2();}
uint8_t func_2(int p) {
int32_t b[4][9] = {{0}, {0}, {0}, 1L};
for (;; a ++) {
for (;;) {
if (p)
break;
return p;
}
p = b[3][1];
}
return 0;
}
int main() {
func_1();
printf("%d\n", a);
return 0;
}
```
```
$gcc -O1 small.c ; ./a.out
0
$gcc -Os small.c ; ./a.out
1
```

I tested it on today's trunk version of GCC, please also check here for
more results for different versions: https://godbolt.org/z/3nfv9vaY8

My confusion is that whether the called function "func_2()" inside the
"func_1" is defined or not.

Can anyone help me to figure it out?

If it's a well-defined program, I will open a bug report.

Thank you very much and looking forward to hearing from you!


Best regards,
Haoxin

Re: Is this program well-defined?

[Jonathan Wakely]

On Sat, 13 Nov 2021, 09:21 Haoxin Tu wrote:
>
> Hi all.
>
> I hope this is the right place to ask the question.


It's not IMHO. Questions like "is this a bug?" generally belong on the
gcc-help list, or just straight to bugzilla.

>
> My confusion is that whether the called function "func_2()" inside the
> "func_1" is defined or not.
>
> Can anyone help me to figure it out?


I think it's undefined, due to C17 6.5.2.2 p6:


If the expression that denotes the called function has a type that
does not include a prototype, the
integer promotions are performed on each argument, and arguments that
have type float are
promoted to double. These are called the default argument promotions.
If the number of arguments
does not equal the number of parameters, the behavior is undefined.