• d-bugmail puremagic.com (34/34) Jan 28 2022 https://issues.dlang.org/show_bug.cgi?id=22710

d-bugmail puremagic.com writes:

https://issues.dlang.org/show_bug.cgi?id=22710

          Issue ID: 22710
           Summary: Different behaviour of bitfields between CTFE and
                    runtime
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: major
          Priority: P1
         Component: dmd
          Assignee: nobody puremagic.com
          Reporter: moonlightsentinel disroot.org

The CTFE engine ins't aware of the subtle differences of bitfields vs normal
variables. This manifests e.g. in incorrect overflow behaviour:

--- app.d
import bitfields;

extern(C) int main()
{
    HasBitfield bf;
    bf.a += 4;
    assert(bf.a == 0); // Fails during CTFE
    assert(bf.b == 0);
    return 0;
}

enum test = main();

--- bitfields.c

struct HasBitfield
{
        int a : 2;
    int b : 2;
};

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