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From: "alex.curiou at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/107285] New: Incorrect code generation when we use
 __builtin_constant_p built-in function.
Date: Mon, 17 Oct 2022 06:54:40 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107285

            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 =3D 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
~~~=