[Bug c/107285] New: Incorrect code generation when we use __builtin_constant_p built-in function.

["alex.curiou at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107285

            Bug ID: 107285
           Summary: Incorrect code generation when we use
                    __builtin_constant_p built-in function.
           Product: gcc
           Version: 12.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alex.curiou at gmail dot com
  Target Milestone: ---

All versions of GCC are affected (I've checked versions from 5 till 12.2).

The minimized example of code:

~~~
#define test(val1) \
  __builtin_constant_p(val1) ? (val1 + 10) : (val1 * 20);


unsigned int bar(unsigned int value1) {
    return test(value1);
}

int main(void) {
    unsigned int a = bar(3);

    return a;
}
~~~

The __builtin_constant_p considers the val1 value to be NOT known to be
constant at compile time. So the generated code is
val1 * 20

But optimizer considers that the value is WELL known and can WELL be
calculated.
So, the return value of func bar is calculated at compile time and the call to
the bar func is dropped away.

And the final return value is WRONG:
val1 + 10

As a result we have different return values with -O2 and -Og optimization
flags, 13 and 60 accordingly.

Here is the compiler output for version 12.2:

-O2 will return 13
https://godbolt.org/z/Y1Thdcfch

~~~
bar:
        lea     eax, [rdi+rdi*4]
        sal     eax, 2
        ret
main:
        mov     eax, 13
        ret
~~~

-Og will return 60
https://godbolt.org/z/sbcer78rP

~~~
bar:
        lea     eax, [rdi+rdi*4]
        sal     eax, 2
        ret
main:
        mov     edi, 3
        call    bar
        ret
~~~