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From: "guminb at ajou dot ac.kr" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/112758] New: Inconsistent Bitwise AND Operation Result
 between int and long long int on Different Optimization Levels in GCC Trunk
Date: Wed, 29 Nov 2023 06:05:24 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112758

            Bug ID: 112758
           Summary: Inconsistent Bitwise AND Operation Result between int
                    and long long int on Different Optimization Levels in
                    GCC Trunk
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: guminb at ajou dot ac.kr
  Target Milestone: ---

Dear GCC Development Team,

I would like to report an inconsistency observed in the GCC RISC-V 64 versi=
on
14.0.0 when compiling code involving bitwise AND operations between an `int`
and a `long long int` variable under different optimization levels. The iss=
ue
appears when both operands are negative, with results varying significantly
between non-optimized and optimized compilations.

The proof of concept (PoC) code provided below demonstrates this issue. The
code performs a bitwise AND operation between a 32-bit integer with its high
bit set (`globalVar`) and a 64-bit long long integer (`localVar`), both
containing negative values. The expected result of the operation seems to
differ based on the optimization level used during compilation.

PoC Code:

```c
#include <stdio.h>

int globalVar =3D 0x80000000; // 32-bit int with high bit set

int main () {
    long long int localVar =3D 0xffFF00ffffffffff; // 64-bit long long int
    printf("localVar: 0x%llx\\n", localVar);
    printf("globalVar: 0x%llx\\n", (long long int)globalVar);
    printf("Result: 0x%llx\\n", ((localVar) & ((long long int) (globalVar))=
));
    return 0;
}
```

Observed Results:

- With **`O0`** optimization, the result of the bitwise AND operation is as
expected (**`0xffff00ff80000000`**).
- With **`O1`**, **`O2`**, **`O3`**, **`Os`**, **`Oz`** optimizations, the
result changes to **`0x80000000`**.

Assembly Output:
The assembly output for -O0 and -O1 can be viewed at the following Compiler
Explorer link:

https://godbolt.org/z/fb33vWT7o

- The **`O0`** output shows the expected behavior with explicit casting and=
 AND
operation.
- The **`O1`** output, however, omits the casting and AND operation, leadin=
g to
an unexpected result.

I suspect this might be related to how the compiler handles casting or the
bitwise operation under different optimization levels. This inconsistency c=
ould
potentially lead to unintended behavior in applications that rely on such
operations, especially when negative values are involved.

I appreciate your attention to this matter and look forward to any insights=
 or
potential solutions you might provide.

Best regards,
[Gyumin Baek]=